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Basic BGP Routing Policies

Understanding Routing Policies

Each routing policy is identified by a policy name. The name can contain letters, numbers, and hyphens (-) and can be up to 255 characters long. To include spaces in the name, enclose the entire name in double quotation marks. Each routing policy name must be unique within a configuration.

Once a policy is created and named, it must be applied before it is active. You apply routing policies using the import and export statements at the protocols protocol-name level in the configuration hierarchy.

In the import statement, you list the name of the routing policy to be evaluated when routes are imported into the routing table from the routing protocol.

In the export statement, you list the name of the routing policy to be evaluated when routes are being exported from the routing table into a dynamic routing protocol. Only active routes are exported from the routing table.

To specify more than one policy and create a policy chain, you list the policies using a space as a separator. If multiple policies are specified, the policies are evaluated in the order in which they are specified. As soon as an accept or reject action is executed, the policy chain evaluation ends.

See Also

Example: Applying Routing Policies at Different Levels of the BGP Hierarchy

This example shows BGP configured in a simple network topology and explains how routing polices take effect when they are applied at different levels of the BGP configuration.

Requirements

No special configuration beyond device initialization is required before configuring this example.

Overview

For BGP, you can apply policies as follows:

  • BGP global import and export statements—Include these statements at the [edit protocols bgp] hierarchy level (for routing instances, include these statements at the [edit routing-instances routing-instance-name protocols bgp] hierarchy level).

  • Group import and export statements—Include these statements at the [edit protocols bgp group group-name] hierarchy level (for routing instances, include these statements at the [edit routing-instances routing-instance-name protocols bgp group group-name] hierarchy level).

  • Peer import and export statements—Include these statements at the [edit protocols bgp group group-name neighbor address] hierarchy level (for routing instances, include these statements at the [edit routing-instances routing-instance-name protocols bgp group group-name neighbor address] hierarchy level).

A peer-level import or export statement overrides a group import or export statement. A group-level import or export statement overrides a global BGP import or export statement.

In this example, a policy named send-direct is applied at the global level, another policy named send-192.168.0.1 is applied at the group level, and a third policy named send-192.168.20.1 is applied at the neighbor level.

A key point, and one that is often misunderstood and that can lead to problems, is that in such a configuration, only the most explicit policy is applied. A neighbor-level policy is more explicit than a group-level policy, which in turn is more explicit than a global policy.

The neighbor 172.16.2.2 is subjected only to the send-192.168.20.1 policy. The neighbor 172.16.3.3, lacking anything more specific, is subjected only to the send-192.168.0.1 policy. Meanwhile, neighbor 172.16.4.4 in group other-group has no group or neighbor-level policy, so it uses the send-direct policy.

If you need to have neighbor 172.16.2.2 perform the function of all three policies, you can write and apply a new neighbor-level policy that encompasses the functions of the other three, or you can apply all three existing policies, as a chain, to neighbor 172.16.2.2.

Topology

Figure 1 shows the sample network.

Figure 1: Applying Routing Policies to BGPApplying Routing Policies to BGP

CLI Quick Configuration shows the configuration for all of the devices in Figure 1.

The section #d99e203__d99e457 describes the steps on Device R1.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Device R4

Procedure

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure an IS-IS default route policy:

  1. Configure the device interfaces.

  2. Enable OSPF, or another interior gateway protocols (IGP), on the interfaces.

  3. Configure static routes.

  4. Enable the routing policies.

  5. Configure BGP and apply the export policies.

  6. Configure the router ID and autonomous system (AS) number.

  7. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by issuing the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

Confirm that the configuration is working properly.

Verifying BGP Route Learning

Purpose

Make sure that the BGP export policies are working as expected by checking the routing tables.

Action
Meaning

On Device R1, the show route protocol direct command displays two direct routes: 172.16.1.1/32 and 10.10.10.0/30. The show route protocol static command displays two static routes: 192.168.0.1/32 and 192.168.20.1/32.

On Device R2, the show route protocol bgp command shows that the only route that Device R2 has learned through BGP is the 192.168.20.1/32 route.

On Device R3, the show route protocol bgp command shows that the only route that Device R3 has learned through BGP is the 192.168.0.1/32 route.

On Device R4, the show route protocol bgp command shows that the only routes that Device R4 has learned through BGP are the 172.16.1.1/32 and 10.10.10.0/30 routes.

Verifying BGP Route Receiving

Purpose

Make sure that the BGP export policies are working as expected by checking the BGP routes received from Device R1.

Action
Meaning

On Device R2, the route receive-protocol bgp 172.16.1.1 command shows that Device R2 received only one BGP route, 192.168.20.1/32, from Device R1.

On Device R3, the route receive-protocol bgp 172.16.1.1 command shows that Device R3 received only one BGP route, 192.168.0.1/32, from Device R1.

On Device R4, the route receive-protocol bgp 172.16.1.1 command shows that Device R4 received two BGP routes, 172.16.1.1/32 and 10.10.10.0/30, from Device R1.

In summary, when multiple policies are applied at different CLI hierarchies in BGP, only the most specific application is evaluated, to the exclusion of other, less specific policy applications. Although this point might seem to make sense, it is easily forgotten during router configuration, when you mistakenly believe that a neighbor-level policy is combined with a global or group-level policy, only to find that your policy behavior is not as anticipated.

See Also

Example: Injecting OSPF Routes into the BGP Routing Table

This example shows how to create a policy that injects OSPF routes into the BGP routing table.

Requirements

Before you begin:

Overview

In this example, you create a routing policy called injectpolicy1 and a routing term called injectterm1. The policy injects OSPF routes into the BGP routing table.

Topology

Configuration

Configuring the Routing Policy

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To inject OSPF routes into a BGP routing table:

  1. Create the policy term.

  2. Specify OSPF as a match condition.

  3. Specify the routes from an OSPF area as a match condition.

  4. Specify that the route is to be accepted if the previous conditions are matched.

  5. Apply the routing policy to BGP.

Results

Confirm your configuration by entering the show policy-options and show protocols bgp commands from configuration mode. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Configuring Tracing for the Routing Policy

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

  1. Include a trace action in the policy.

  2. Configure the tracing file for the output.

Results

Confirm your configuration by entering the show policy-options and show routing-options commands from configuration mode. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying That the Expected BGP Routes Are Present

Purpose

Verify the effect of the export policy.

Action

From operational mode, enter the show route command.

Troubleshooting

Using the show log Command to Examine the Actions of the Routing Policy

Problem

The routing table contains unexpected routes, or routes are missing from the routing table.

Solution

If you configure policy tracing as shown in this example, you can run the show log ospf-bgp-policy-log command to diagnose problems with the routing policy. The show log ospf-bgp-policy-log command displays information about the routes that the injectpolicy1 policy term analyzes and acts upon.

Configuring Routing Policies to Control BGP Route Advertisements

All routing protocols use the Junos OS routing table to store the routes that they learn and to determine which routes they should advertise in their protocol packets. Routing policy allows you to control which routes the routing protocols store in and retrieve from the routing table. For information about routing policy, see the Routing Policies, Firewall Filters, and Traffic Policers User Guide.

When configuring BGP routing policy, you can perform the following tasks:

Applying Routing Policy

You define routing policy at the [edit policy-options] hierarchy level. To apply policies you have defined for BGP, include the import and export statements within the BGP configuration.

You can apply policies as follows:

  • BGP global import and export statements—Include these statements at the [edit protocols bgp] hierarchy level (for routing instances, include these statements at the [edit routing-instances routing-instance-name protocols bgp] hierarchy level).

  • Group import and export statements—Include these statements at the [edit protocols bgp group group-name] hierarchy level (for routing instances, include these statements at the [edit routing-instances routing-instance-name protocols bgp group group-name] hierarchy level).

  • Peer import and export statements—Include these statements at the [edit protocols bgp group group-name neighbor address] hierarchy level (for routing instances, include these statements at the [edit routing-instances routing-instance-name protocols bgp group group-name neighbor address] hierarchy level).

A peer-level import or export statement overrides a group import or export statement. A group-level import or export statement overrides a global BGP import or export statement.

To apply policies, see the following sections:

Applying Policies to Routes Being Imported into the Routing Table from BGP

To apply policy to routes being imported into the routing table from BGP, include the import statement, listing the names of one or more policies to be evaluated:

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

If you specify more than one policy, they are evaluated in the order specified, from first to last, and the first matching filter is applied to the route. If no match is found, BGP places into the routing table only those routes that were learned from BGP routing devices.

Applying Policies to Routes Being Exported from the Routing Table into BGP

To apply policy to routes being exported from the routing table into BGP, include the export statement, listing the names of one or more policies to be evaluated:

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

If you specify more than one policy, they are evaluated in the order specified, from first to last, and the first matching filter is applied to the route. If no routes match the filters, the routing table exports into BGP only the routes that it learned from BGP.

Setting BGP to Advertise Inactive Routes

By default, BGP stores the route information it receives from update messages in the Junos OS routing table, and the routing table exports only active routes into BGP, which BGP then advertises to its peers. To have the routing table export to BGP the best route learned by BGP even if Junos OS did not select it to be an active route, include the advertise-inactive statement:

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

Configuring BGP to Advertise the Best External Route to Internal Peers

In general, deployed BGP implementations do not advertise the external route with the highest local preference value to internal peers unless it is the best route. Although this behavior was required by an earlier version of the BGP version 4 specification, RFC 1771, it was typically not followed in order to minimize the amount of advertised information and to prevent routing loops. However, there are scenarios in which advertising the best external route is beneficial, in particular, situations that can result in IBGP route oscillation.

In Junos OS Release 9.3 and later, you can configure BGP to advertise the best external route into an internal BGP (IBGP) mesh group, a route reflector cluster, or an autonomous system (AS) confederation, even when the best route is an internal route.

Note:

In order to configure the advertise-external statement on a route reflector, you must disable intracluster reflection with the no-client-reflect statement.

When a routing device is configured as a route reflector for a cluster, a route advertised by the route reflector is considered internal if it is received from an internal peer with the same cluster identifier or if both peers have no cluster identifier configured. A route received from an internal peer that belongs to another cluster, that is, with a different cluster identifier, is considered external.

In a confederation, when advertising a route to a confederation border router, any route from a different confederation sub-AS is considered external.

You can also configure BGP to advertise the external route only if the route selection process reaches the point where the multiple exit discriminator (MED) metric is evaluated. As a result, an external route with an AS path worse (that is, longer) than that of the active path is not advertised.

Junos OS also provides support for configuring a BGP export policy that matches on the state of an advertised route. You can match on either active or inactive routes. For more information, see the Routing Policies, Firewall Filters, and Traffic Policers User Guide.

To configure BGP to advertise the best external path to internal peers, include the advertise-external statement:

Note:

The advertise-external statement is supported at both the group and neighbor level. If you configure the statement at the neighbor level, you must configure it for all neighbors in a group. Otherwise, the group is automatically split into different groups.

For a complete list of hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

To configure BGP to advertise the best external path only if the route selection process reaches the point where the MED value is evaluated, include the conditional statement:

Configuring How Often BGP Exchanges Routes with the Routing Table

BGP stores the route information it receives from update messages in the routing table, and the routing table exports active routes from the routing table into BGP. BGP then advertises the exported routes to its peers. By default, the exchange of route information between BGP and the routing table occurs immediately after the routes are received. This immediate exchange of route information might cause instabilities in the network reachability information. To guard against this, you can delay the time between when BGP and the routing table exchange route information.

To configure how often BGP and the routing table exchange route information, include the out-delay statement:

By default, the routing table retains some of the route information learned from BGP. To have the routing table retain all or none of this information, include the keep statement:

For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.

The routing table can retain the route information learned from BGP in one of the following ways:

  • Default (omit the keep statement)—Keep all route information that was learned from BGP, except for routes whose AS path is looped and whose loop includes the local AS.

  • keep all—Keep all route information that was learned from BGP.

  • keep none—Discard routes that were received from a peer and that were rejected by import policy or other sanity checking, such as AS path or next hop. When you configure keep none for the BGP session and the inbound policy changes, Junos OS forces readvertisement of the full set of routes advertised by the peer.

In an AS path healing situation, routes with looped paths theoretically could become usable during a soft reconfiguration when the AS path loop limit is changed. However, there is a significant memory usage difference between the default and keep all.

Consider the following scenarios:

  • A peer readvertises routes back to the peer from which it learned them.

    This can happen in the following cases:

    • Another vendor's routing device advertises the routes back to the sending peer.

    • The Junos OS peer’s default behavior of not readvertising routes back to the sending peer is overridden by configuring advertise-peer-as.

  • A provider edge (PE) routing device discards any VPN route that does not have any of the expected route targets.

When keep all is configured, the behavior of discarding routes received in the above scenarios is overridden.

Disabling Suppression of Route Advertisements

Junos OS does not advertise the routes learned from one EBGP peer back to the same external BGP (EBGP) peer. In addition, the software does not advertise those routes back to any EBGP peers that are in the same AS as the originating peer, regardless of the routing instance. You can modify this behavior by including the advertise-peer-as statement in the configuration. To disable the default advertisement suppression, include the advertise-peer-as statement:

Note:

The route suppression default behavior is disabled if the as-override statement is included in the configuration.

If you include the advertise-peer-as statement in the configuration, BGP advertises the route regardless of this check.

To restore the default behavior, include the no-advertise-peer-as statement in the configuration:

If you include both the as-override and no-advertise-peer-as statements in the configuration, the no-advertise-peer-as statement is ignored. You can include these statements at multiple hierarchy levels.

For a list of hierarchy levels at which you can include these statements, see the statement summary section for these statements.

See Also

Example: Configuring a Routing Policy to Advertise the Best External Route to Internal Peers

The BGP protocol specification, as defined in RFC 1771, specifies that a BGP peer shall advertise to its internal peers the higher preference external path, even if this path is not the overall best (in other words, even if the best path is an internal path). In practice, deployed BGP implementations do not follow this rule. The reasons for deviating from the specification are as follows:

  • Minimizing the amount of advertised information. BGP scales according to the number of available paths.

  • Avoiding routing and forwarding loops.

There are, however, several scenarios in which the behavior, specified in RFC 1771, of advertising the best external route might be beneficial. Limiting path information is not always desirable as path diversity might help reduce restoration times. Advertising the best external path can also address internal BGP (IBGP) route oscillation issues as described in RFC 3345, Border Gateway Protocol (BGP) Persistent Route Oscillation Condition.

The advertise-external statement modifies the behavior of a BGP speaker to advertise the best external path to IBGP peers, even when the best overall path is an internal path.

Note:

The advertise-external statement is supported at both the group and neighbor level. If you configure the statement at the neighbor level, you must configure it for all neighbors in a group. Otherwise, the group is automatically split into different groups.

The conditional option limits the behavior of the advertise-external setting, such that the external route is advertised only if the route selection process reaches the point where the multiple exit discriminator (MED) metric is evaluated. Thus, an external route is not advertised if it has, for instance, an AS path that is worse (longer) than that of the active path. The conditional option restricts external path advertisement to when the best external path and the active path are equal until the MED step of the route selection process. Note that the criteria used for selecting the best external path is the same whether or not the conditional option is configured.

Junos OS also provides support for configuring a BGP export policy that matches the state of an advertised route. You can match either active or inactive routes, as follows:

This qualifier only matches when used in the context of an export policy. When a route is being advertised by a protocol that can advertise inactive routes (such as BGP), state inactive matches routes advertised as a result of the advertise-inactive and advertise-external statements.

For example, the following configuration can be used as a BGP export policy toward internal peers to mark routes advertised due to the advertise-external setting with a user-defined community. That community can be later used by the receiving routers to filter out such routes from the forwarding table. Such a mechanism can be used to address concerns that advertising paths not used for forwarding by the sender might lead to forwarding loops.

Requirements

Junos OS 9.3 or later is required.

Overview

This example shows three routing devices. Device R2 has an external BGP (EBGP) connection to Device R1. Device R2 has an IBGP connection to Device R3.

Device R1 advertises 172.16.6.0/24. Device R2 does not set the local preference in an import policy for Device R1’s routes, and thus 172.16.6.0/24 has the default local preference of 100.

Device R3 advertises 172.16.6.0/24 with a local preference of 200.

When the advertise-external statement is not configured on Device R2, 172.16.6.0/24 is not advertised by Device R2 toward Device R3.

When the advertise-external statement is configured on Device R2 on the session toward Device R3, 172.16.6.0/24 is advertised by Device R2 toward Device R3.

When advertise-external conditional is configured on Device R2 on the session toward Device R3, 172.16.6.0/24 is not advertised by Device R2 toward Device R3. If you remove the then local-preference 200 setting on Device R3 and add the path-selection as-path-ignore setting on Device R2 (thus making the path selection criteria equal until the MED step of the route selection process), 172.16.6.0/24 is advertised by Device R2 toward Device R3.

Note:

To configure the advertise-external statement on a route reflector, you must disable intracluster reflection with the no-client-reflect statement, and the client cluster must be fully meshed to prevent the sending of redundant route advertisements.

When a routing device is configured as a route reflector for a cluster, a route advertised by the route reflector is considered internal if it is received from an internal peer with the same cluster identifier or if both peers have no cluster identifier configured. A route received from an internal peer that belongs to another cluster, that is, with a different cluster identifier, is considered external.

Topology

Figure 2 shows the sample network.

Figure 2: BGP Topology for advertise-externalBGP Topology for advertise-external

CLI Quick Configuration shows the configuration for all of the devices in Figure 2.

The section #d102e166__d102e344 describes the steps on Device R2.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Procedure

Step-by-Step Procedure

The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the Junos OS CLI User Guide.

To configure Device R2:

  1. Configure the device interfaces.

  2. Configure OSPF or another interior gateway protocol (IGP).

  3. Configure the EBGP connection to Device R1.

  4. Configure the IBGP connection to Device R3.

  5. Add the advertise-external statement to the IBGP group peering session.

  6. Configure the autonomous system (AS) number and the router ID.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying the BGP Active Path

Purpose

On Device R2, make sure that the 172.16.6.0/24 prefix is in the routing table and has the expected active path.

Action
Meaning

Device R2 receives the 172.16.6.0/24 route from both Device R1 and Device R3. The route from Device R3 is the active path, as designated by the asterisk (*). The active path has the highest local preference. Even if the local preferences of the two routes were equal, the route from Device R3 would remain active because it has the shortest AS path.

Verifying the External Route Advertisement

Purpose

On Device R2, make sure that the 172.16.6.0/24 route is advertised toward Device R3.

Action
Meaning

Device R2 is advertising the 172.16.6.0/24 route toward Device R3.

Verifying the Route on Device R3

Purpose

Make sure that the 172.16.6.0/24 prefix is in Device R3’s routing table.

Action
Meaning

Device R3 has the static route and the BGP route for 172.16.6.0/24.

Note that the BGP route is hidden on Device R3 if the route is not reachable or if the next hop cannot be resolved. To fulfill this requirement, this example includes a static default route on Device R3 (static route 0.0.0.0/0 next-hop 10.0.0.5).

Experimenting with the conditional Option

Purpose

See how the conditional option works in the context of the BGP path selection algorithm.

Action

  1. On Device R2, add the conditional option.

  2. On Device R2, check to see if the 172.16.6.0/24 route is advertised toward Device R3.

    As expected, the route is no longer advertised. You might need to wait a few seconds to see this result.

  3. On Device R3, deactivate the then local-preference policy action.

  4. On Device R2, ensure that the local preferences of the two paths are equal.

  5. On Device R2, add the as-path-ignore statement.

  6. On Device R2, check to see if the 172.16.6.0/24 route is advertised toward Device R3.

    As expected, the route is now advertised because the AS path length is ignored and because the local preferences are equal.

See Also

Example: Configuring BGP Prefix-Based Outbound Route Filtering

This example shows how to configure a Juniper Networks router to accept route filters from remote peers and perform outbound route filtering using the received filters.

Requirements

Before you begin:

  • Configure the router interfaces.

  • Configure an interior gateway protocol (IGP).

Overview

You can configure a BGP peer to accept route filters from remote peers and perform outbound route filtering using the received filters. By filtering out unwanted updates, the sending peer saves resources needed to generate and transmit updates, and the receiving peer saves resources needed to process updates. This feature can be useful, for example, in a virtual private network (VPN) in which subsets of customer edge (CE) devices are not capable of processing all the routes in the VPN. The CE devices can use prefix-based outbound route filtering to communicate to the provider edge (PE) routing device to transmit only a subset of routes, such as routes to the main data centers only.

The maximum number of prefix-based outbound route filters that a BGP peer can accept is 5000. If a remote peer sends more than 5000 outbound route filters to a peer address, the additional filters are discarded, and a system log message is generated.

You can configure interoperability for the routing device as a whole or for specific BGP groups or peers only.

Topology

In the sample network, Device CE1 is a router from another vendor. The configuration shown in this example is on Juniper Networks Router PE1.

Figure 3 shows the sample network.

Figure 3: BGP Prefix-Based Outbound Route FilteringBGP Prefix-Based Outbound Route Filtering

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

PE1

Procedure

Step-by-Step Procedure

The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the Junos OS CLI User Guide.

To configure Router PE1 to accept route filters from Device CE1 and perform outbound route filtering using the received filters:

  1. Configure the local autonomous system.

  2. Configure external peering with Device CE1.

  3. Configure Router PE1 to accept IPv4 route filters from Device CE1 and perform outbound route filtering using the received filters.

  4. (Optional) Enable interoperability with routing devices that use the vendor-specific compatibility code of 130 for outbound route filters and the code type of 128.

    The IANA standard code is 3, and the standard code type is 64.

Results

From configuration mode, confirm your configuration by entering the show protocols and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying the Outbound Route Filter

Purpose

Display information about the prefix-based outbound route filter received from Device CE1.

Action

From operational mode, enter the show bgp neighbor orf detail command.

Verifying the BGP Neighbor Mode

Purpose

Verify that the bgp-orf-cisco-mode setting is enabled for the peer by making sure that the ORFCiscoMode option is displayed in the show bgp neighbor command output.

Action

From operational mode, enter the show bgp neighbor command.

See Also

Understanding the Default BGP Routing Policy on Packet Transport Routers (PTX Series)

On PTX Series Packet Transport Routers, the default BGP routing policy differs from that of other Junos OS routing devices.

The PTX Series routers are MPLS transit platforms that do IP forwarding, typically using interior gateway protocol (IGP) routes. The PTX Series Packet Forwarding Engine can accommodate a relatively small number of variable-length prefixes.

Note:

A PTX Series router can support full BGP routes in the control plane so that it can be used as a route reflector (RR). It can do exact-length lookup multicast forwarding and can build the multicast forwarding plane for use by the unicast control plane (for example. to perform a reverse-path forwarding lookup for multicast).

Given the PFE limitation, the default routing policy for PTX3000 and PTX5000 routers is for BGP routes not to be installed in the forwarding table. You can override the default routing policy and select certain BGP routes to install in the forwarding table.

The default behavior for load balancing and BGP routes on PTX Series routers is as follows. It has the following desirable characteristics:

  • Allows you to override the default behavior without needing to alter the default policy directly

  • Reduces the chance of accidental changes that nullify the defaults

  • Sets no flow-control actions, such as accept and reject

The default routing policy on the PTX Series routers is as follows:

As shown here, the junos-ptx-series-default policy is defined in [edit policy-options]. The policy is applied in [edit routing-options forwarding-table], using the default-export statement. You can view these default configurations by using the | display inheritance flag.

Also, you can use the show policy command to view the default policy.

CAUTION:

We strongly recommend that you do not alter the junos-ptx-series-default routing policy directly.

Junos OS chains the junos-ptx-series-default policy and any user-configured export policy. Because the junos-ptx-series-default policy does not use flow-control actions, any export policy that you configure is executed (by way of the implicit next-policy action) for every route. Thus you can override any actions set by the junos-ptx-series-default policy. If you do not configure an export policy, the actions set by junos-ptx-series-default policy are the only actions.

You can use the policy action install-to-fib to override the no-install-to-fib action.

Similarly, you can set the load-balance per-prefix action to override the load-balance per-packet action.

See Also

Example: Overriding the Default BGP Routing Policy on PTX Series Packet Transport Routers

This example shows how to override the default routing policy on packet transport routers, such as the PTX Series Packet Transport Routers.

Requirements

This example requires Junos OS Release 12.1 or later.

Overview

By default, the PTX Series routers do not install BGP routes in the forwarding table.

For PTX Series routers, the configuration of the from protocols bgp condition with the then accept action does not have the usual result that it has on other Junos OS routing devices. With the following routing policy on PTX Series routers, BGP routes do not get installed in the forwarding table.

No BGP routes are installed in the forwarding table. This is the expected behavior.

This example shows how to use the then install-to-fib action to effectively override the default BGP routing policy.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Installing Selected BGP Routes in the Forwarding Table

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the Junos OS CLI User Guide.

To install selected BGP routes in the forwarding table:

  1. Configure a list of prefixes to install in the forwarding table.

  2. Configure the routing policy, applying the prefix list as a condition.

  3. Apply the routing policy to the forwarding table.

Results

From configuration mode, confirm your configuration by entering the show policy-options and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying That the Selected Route Is Installed in the Forwarding Table

Purpose

Make sure that the configured policy overrides the default policy.

Action

From operational mode, enter the show route forwarding-table command.

Meaning

This output shows that the route to 66.0.0.1/32 is installed in the forwarding table.

See Also

Conditional Advertisement Enabling Conditional Installation of Prefixes Use Cases

Networks are usually subdivided into smaller, more-manageable units called autonomous systems (ASs). When BGP is used by routers to form peer relationships in the same AS, it is referred to as internal BGP (IBGP). When BGP is used by routers to form peer relationships in different ASs, it is referred to as external BGP (EBGP).

After performing route sanity checks, a BGP router accepts the routes received from its peers and installs them into the routing table. By default, all routers in IBGP and EBGP sessions follow the standard BGP advertisement rules. While a router in an IBGP session advertises only the routes learned from its direct peers, a router in an EBGP session advertises all routes learned from its direct and indirect peers (peers of peers). Hence, in a typical network configured with EBGP, a router adds all routes received from an EBGP peer into its routing table and advertises nearly all routes to all EBGP peers.

A service provider exchanging BGP routes with both customers and peers on the Internet is at risk of malicious and unintended threats that can compromise the proper routing of traffic, as well as the operation of the routers.

This has several disadvantages:

  • Non-aggregated route advertisements—A customer could erroneously advertise all its prefixes to the ISP rather than an aggregate of its address space. Given the size of the Internet routing table, this must be carefully controlled. An edge router might also need only a default route out toward the Internet and instead be receiving the entire BGP routing table from its upstream peer.

  • BGP route manipulation—If a malicious administrator alters the contents of the BGP routing table, it could prevent traffic from reaching its intended destination.

  • BGP route hijacking—A rogue administrator of a BGP peer could maliciously announce a network’s prefixes in an attempt to reroute the traffic intended for the victim network to the administrator’s network to either gain access to the contents of traffic or to block the victim’s online services.

  • BGP denial of service (DoS)—If a malicious administrator sends unexpected or undesirable BGP traffic to a router in an attempt to use all of the router’s available BGP resources, it might result in impairing the router’s ability to process valid BGP route information.

Conditional installation of prefixes can be used to address all the problems previously mentioned. If a customer requires access to remote networks, it is possible to install a specific route in the routing table of the router that is connected with the remote network. This does not happen in a typical EBGP network and hence, conditional installation of prefixes becomes essential.

ASs are not only bound by physical relationships but by business or other organizational relationships. An AS can provide services to another organization, or act as a transit AS between two other ASs. These transit ASs are bound by contractual agreements between the parties that include parameters on how to connect to each other and most importantly, the type and quantity of traffic they carry for each other. Therefore, for both legal and financial reasons, service providers must implement policies that control how BGP routes are exchanged with neighbors, which routes are accepted from those neighbors, and how those routes affect the traffic between the ASs.

There are many different options available to filter routes received from a BGP peer to both enforce inter-AS policies and mitigate the risks of receiving potentially harmful routes. Conventional route filtering examines the attributes of a route and accepts or rejects the route based on such attributes. A policy or filter can examine the contents of the AS-Path, the next-hop value, a community value, a list of prefixes, the address family of the route, and so on.

In some cases, the standard “acceptance condition” of matching a particular attribute value is not enough. The service provider might need to use another condition outside of the route itself, for example, another route in the routing table. As an example, it might be desirable to install a default route received from an upstream peer, only if it can be verified that this peer has reachability to other networks further upstream. This conditional route installation avoids installing a default route that is used to send traffic toward this peer, when the peer might have lost its routes upstream, leading to black-holed traffic. To achieve this, the router can be configured to search for the presence of a particular route in the routing table, and based on this knowledge accept or reject another prefix.

Example: Configuring a Routing Policy for Conditional Advertisement Enabling Conditional Installation of Prefixes in a Routing Table explains how the conditional installation of prefixes can be configured and verified.

See Also

Conditional Advertisement and Import Policy (Routing Table) with certain match conditions

BGP accepts all non-looped routes learned from neighbors and imports them into the RIB-In table. If these routes are accepted by the BGP import policy, they are then imported into the inet.0 routing table. In cases where only certain routes are required to be imported, provisions can be made such that the peer routing device exports routes based on a condition or a set of conditions.

The condition for exporting a route can be based on:

  • The peer the route was learned from

  • The interface the route was learned on

  • Some other required attribute

For example:

This is known as conditional installation of prefixes and is described in Example: Configuring a Routing Policy for Conditional Advertisement Enabling Conditional Installation of Prefixes in a Routing Table.

Conditions in routing policies can be configured irrespective of whether they are a part of the export or import policies or both. The export policy supports these conditions inherited from the routing policy based on the existence of another route in the routing policy. However, the import policy doesn't support these conditions, and the conditions are not executed even if they are present.

Figure 4 illustrates where BGP import and export policies are applied. An import policy is applied to inbound routes that are visible in the output of the show route receive-protocol bgp neighbor-address command. An export policy is applied to outbound routes that are visible in the output of the show route advertising-protocol bgp neighbor-address command.

Figure 4: BGP Import and Export PoliciesBGP Import and Export Policies

To enable conditional installation of prefixes, an export policy must be configured on the device where the prefix export has to take place. The export policy evaluates each route to verify that it satisfies all the match conditions under the from statement. It also searches for the existence of the route defined under the condition statement (also configured under the from statement).

If the route does not match the entire set of required conditions defined in the policy, or if the route defined under the condition statement does not exist in the routing table, the route is not exported to its BGP peers. Thus, a conditional export policy matches the routes for the desired route or prefix you want installed in the peers’ routing table.

To configure the conditional installation of prefixes with the help of an export policy:

  1. Create a condition statement to check prefixes.

  2. Create an export policy with the newly created condition using the condition statement.

  3. Apply the export policy to the device that requires only selected prefixes to be exported from the routing table.

See Also

Example: Configuring a Routing Policy for Conditional Advertisement Enabling Conditional Installation of Prefixes in a Routing Table

This example shows how to configure conditional installation of prefixes in a routing table using BGP export policy.

Requirements

This example uses the following hardware and software components:

  • M Series Multiservice Edge Routers, MX Series 5G Universal Routing Platforms, or T Series Core Routers

  • Junos OS Release 9.0 or later

Overview

In this example, three routers in three different autonomous systems (ASs) are connected and configured with the BGP protocol. The router labeled Internet, which is the upstream router, has five addresses configured on its lo0.0 loopback interface (172.16.11.1/32, 172.16.12.1/32, 172.16.13.1/32, 172.16.14.1/32, and 172.16.15.1/32), and an extra loopback address (192.168.9.1/32) is configured as the router ID. These six addresses are exported into BGP to emulate the contents of a BGP routing table of a router connected to the Internet, and advertised to North.

The North and South routers use the 10.0.89.12/30 and 10.0.78.12/30 networks, respectively, and use 192.168.7.1 and 192.168.8.1 for their respective loopback addresses.

Figure 5 shows the topology used in this example.

Figure 5: Conditional Installation of PrefixesConditional Installation of Prefixes

Router North exports the 172.16.0.0/16 BGP routes it learns from Router Internet to Router South. These routes might represent the routes owned by the Internet router's domain. In addition, when the specific 172.16.11.1/32 route is present, Router North also advertises a default route. The 172.16.11.1 route might represent the Internet router's link to a tier 1 transit peering provider that provides full internet connectivity.

Router South receives all six routes, but should only install the default route and one other specific route (172.16.11.1/32) in its routing table.

To summarize, the example meets the following requirements:

  • On North, send all 172.16/16 prefixes. In addition, also send 0/0 to South only if a particular route is present in the inet.0 routing table (in this example 172.16.11.1/32).

  • On South, accept and install the default route and the 172.16.11.1/32 route in the routing and forwarding tables. Drop all other routes. Consider that South might be a lower-end device that cannot accept a full Internet routing table. As a result the operator only wants South to have the default route and one other specific prefix.

The first requirement is met with an export policy on North:

The logic of the conditional export policy can be summarized as follows: If 0/0 is present, and if 172.16.11.1/32 is present, then send the 0/0 prefix. This implies that if 172.16.11.1/32 is not present, then do not send 0/0.

The second requirement is met with an import policy on South:

In this example, four routes are dropped as a result of the import policy on South. This is because the export policy on North leaks all of the routes received from Internet, and the import policy on South excludes some of these routes.

It is important to understand that in Junos OS, although an import policy (inbound route filter) might reject a route, not use it for traffic forwarding, and not include it in an advertisement to other peers, the router retains these routes as hidden routes. These hidden routes are not available for policy or routing purposes. However, they do occupy memory space on the router. A service provider filtering routes to control the amount of information being kept in memory and processed by a router might want the router to entirely drop the routes being rejected by the import policy.

Hidden routes can be viewed by using the show route receive-protocol bgp neighbor-address hidden command. The hidden routes can then be retained or dropped from the routing table by configuring the keep all | none statement at the [edit protocols bgp] or [edit protocols bgp group group-name] hierarchy level.

The rules of BGP route retention are as follows:

  • By default, all routes learned from BGP are retained, except those where the AS path is looped. (The AS path includes the local AS.)

  • By configuring the keep all statement, all routes learned from BGP are retained, even those with the local AS in the AS path.

  • By configuring the keep none statement, BGP discards routes that were received from a peer and that were rejected by import policy or other sanity checking. When this statement is configured and the inbound policy changes, Junos OS re-advertises all the routes advertised by the peer.

When you configure keep all or keep none and the peers support route refresh, the local speaker sends a refresh message and performs an import evaluation. For these peers, the sessions do not restart. To determine if a peer supports refresh, check for Peer supports Refresh capability in the output of the show bgp neighbor command.

CAUTION:

If you configure keep all or keep none and the peer does not support session restart, the associated BGP sessions are restarted (flapped).

Topology

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Router Internet

Router North

Router South

Configuring Conditional Installation of Prefixes

Step-by-Step Procedure

The following example requires that you navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the Junos OS CLI User Guide.

To configure conditional installation of prefixes:

  1. Configure the router interfaces forming the links between the three routers.

  2. Configure five loopback interface addresses on Router Internet to emulate BGP routes learned from the Internet that are to be imported into the routing table of Router South, and configure an additional address (192.168.9.1/32) that will be configured as the router ID.

    Also, configure the loopback interface addresses on Routers North and South.

  3. Configure the static default route on Router North to be advertised to Router South.

  4. Define the condition for exporting prefixes from the routing table on Router North.

  5. Define export policies (into-bgp and conditional-export-bgp ) on Routers Internet and North respectively, to advertise routes to BGP.

    Note:

    Ensure that you reference the condition, prefix_11 (configured in Step 4), in the export policy.

  6. Define an import policy (import-selected-routes) on Router South to import some of the routes advertised by Router North into its routing table.

  7. Configure BGP on all three routers to enable the flow of prefixes between the autonomous systems.

    Note:

    Ensure that you apply the defined import and export policies to the respective BGP groups for prefix advertisement to take place.

  8. Configure the router ID and autonomous system number for all three routers.

    Note:

    In this example, the router ID is configured based on the IP address configured on the lo0.0 interface of the router.

Results

From configuration mode, confirm your configuration by issuing the show interfaces, show protocols bgp, show policy-options, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Device Internet

Device North

Device South

If you are done configuring the routers, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Verifying BGP

Purpose

Verify that BGP sessions have been established between the three routers.

Action

From operational mode, run the show bgp neighbor neighbor-address command.

  1. Check the BGP session on Router Internet to verify that Router North is a neighbor.

  2. Check the BGP session on Router North to verify that Router Internet is a neighbor.

Check the following fields in these outputs to verify that BGP sessions have been established:

  • Peer—Check if the peer AS number is listed.

  • Local—Check if the local AS number is listed.

  • State—Ensure that the value is Established. If not, check the configuration again and see show bgp neighbor for more details on the output fields.

Similarly, verify that Routers North and South form peer relationships with each other.

Meaning

BGP sessions are established between the three routers.

Verifying Prefix Advertisement from Router Internet to Router North

Purpose

Verify that the routes sent from Router Internet are received by Router North.

Action

  1. From operational mode on Router Internet, run the show route advertising-protocol bgp neighbor-address command.

    The output verifies that Router Internet advertises the routes 172.16.11.1/32, 172.16.12.1/32, 172.16.13.1/32, 172.16.14.1/32, 172.16.15.1/32, and 192.168.9.1/32 (the loopback address used as router ID) to Router North.

  2. From operational mode on Router North, run the show route receive-protocol bgp neighbor-address command.

    The output verifies that Router North has received all the routes advertised by Router Internet.

Meaning

Prefixes sent by Router Internet have been successfully installed into the routing table on Router North.

Verifying Prefix Advertisement from Router North to Router South

Purpose

Verify that the routes received from Router Internet and the static default route are advertised by Router North to Router South.

Action

  1. From operational mode on Router North, run the show route 0/0 exact command.

    The output verifies the presence of the static default route (0.0.0.0/0) in the routing table on Router North.

  2. From operational mode on Router North, run the show route advertising-protocol bgp neighbor-address command.

    The output verifies that Router North is advertising the static route and the 172.16.11.1/32 route received from Router Internet, as well as many other routes, to Router South.

Verifying BGP Import Policy for Installation of Prefixes

Purpose

Verify that the BGP import policy successfully installs the required prefixes.

Action

See if the import policy on Router South is operational by checking if only the static default route from Router North and the 172.16.11.1/32 route from Router South are installed in the routing table.

From operational mode, run the show route receive-protocol bgp neighbor-address command.

The output verifies that the BGP import policy is operational on Router South, and only the static default route of 0.0.0.0/0 from Router North and the 172.16.11.1/32 route from Router Internet have leaked into the routing table on Router South.

Meaning

The installation of prefixes is successful because of the configured BGP import policy.

Verifying Conditional Export from Router North to Router South

Purpose

Verify that when Device Internet stops sending the 172.16.11.1/32 route, Device North stops sending the default 0/0 route.

Action

  1. Cause Device Internet to stop sending the 172.16.11.1/32 route by deactivating the 172.16.11.1/32 address on the loopback interface.

  2. From operational mode on Router North, run the show route advertising-protocol bgp neighbor-address command.

    The output verifies that Router North is not advertising the default route to Router South. This is the expected behavior when the 172.16.11.1/32 route is not present.

  3. Reactivate the 172.16.11.1/32 address on Device Internet’s loopback interface.

Verifying the Presence of Routes Hidden by Policy (Optional)

Purpose

Verify the presence of routes hidden by the import policy configured on Router South.

Note:

This section demonstrates the effects of various changes you can make to the configuration depending on your needs.

Action

View routes hidden from the routing table of Router South by:

  • Using the hidden option for the show route receive-protocol bgp neighbor-address command.

  • Deactivating the import policy.

  1. From operational mode, run the show route receive-protocol bgp neighbor-address hidden command to view hidden routes.

    The output verifies the presence of routes hidden by the import policy (172.16.12.1/32, 172.16.13.1/32, 172.16.14.1/32, and 172.16.15.1/32) on Router South.

  2. Deactivate the BGP import policy by configuring the deactivate import statement at the [edit protocols bgp group group-name] hierarchy level.

  3. Run the show route receive-protocol bgp neighbor-address operational mode command to check the routes after deactivating the import policy.

    The output verifies the presence of previously hidden routes (172.16.12.1/32, 172.16.13.1/32, 172.16.14.1/32, and 172.16.15.1/32).

  4. Activate the BGP import policy and remove the hidden routes from the routing table by configuring the activate import and keep none statements respectively at the [edit protocols bgp group group-name] hierarchy level.

  5. From operational mode, run the show route receive-protocol bgp neighbor-address hidden command to check the routes after activating the import policy and configuring the keep none statement.

    The output verifies that the hidden routes are not maintained in the routing table because of the configured keep none statement.

See Also

Implicit filter for Default EBGP Route Propagation Behavior without Policies

SUMMARY This section talks about using an implicit filter to regulate the EBGP route propagation behavior when there is no explicit policy configured.

Benefits

This feature provides the following benefits:

  • Regulates BGP implementation—Prevents EBGP speakers from becoming a silent pass-through where it accepted and advertised all routes by default. This feature effectively brings down the increase in transit traffic on leaf autonomous systems, especially when they are multi-homed to any upstream Internet Service Providers. Thus, it also prevents silent dropping of traffic, Denial of Service, and global internet outages.

  • Implicit filter—The configuration facilitates the use of an implicit filter, where the default behavior is still set to receive and advertise all routes by default. The configuration statement only adds an option to specify enable or disable for accept, reject, reject-always clauses, when required. The implicit filter ensures that the users with existing deployments that rely on the default BGP policy do not experience operational disruptions.

Overview

BGP is the current inter-domain Autonomous protocol used for global Internet routing. It also supports various services such as VPNs, and link state, which are not intended for global usage.

BGP implementation, including the default EBGP behavior is guided by RFC4271, A Border Gateway Protocol 4 (BGP-4). However, it does not provide any explicit guidance on specifying what routes should be distributed. This leads to the original BGP implementation being a silent pass-through for routes without any filtering and therefore, causing an increase in traffic, resulting in global Internet outages.

Starting in Junos OS Release 20.3R1, we have introduced an implicit filter defaults ebgp no-policy at the existing [edit protocols bgp] hierarchy level. The configuration separates the default policy for receive and advertise, into separate clauses (accept, reject, or reject-always) to permit the behavior to vary independently.

If there is no explicit policy configured, the implicit filter allows you to enable the default eBGP receive and advertise behavior in one of three states as follows:

Values

Default Policy

What it does

accept

receive

Accepts to receive all routes (also the default behavior).

advertise

Accepts to advertise all routes (also the default behavior).

reject

receive

Rejects to receive routes of type inet unicast and inet6 unicast in instance types primary, vrf, virtual-router, and non-forwarding.

advertise

Rejects to advertise routes of type inet unicast and inet6 unicast in instance types primary, vrf, virtual-router, and non-forwarding.

reject-always

receive

Rejects to receive all routes.

advertise

Rejects to advertise all routes.

See Also