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Local Preference for BGP Routes

 

Understanding Route Preference Values (Administrative Distance)

The Junos OS routing protocol process assigns a default preference value (also known as an administrative distance) to each route that the routing table receives. The default value depends on the source of the route. The preference value is a value from 0 through 4,294,967,295 (232 – 1), with a lower value indicating a more preferred route. Table 1 lists the default preference values.

Table 1: Default Route Preference Values

How Route Is Learned

Default Preference

Statement to Modify Default Preference

Directly connected network

0

System routes

4

Static and Static LSPs

5

static

Static LSPs

6

MPLS preference

Note: In Junos OS Releases prior to 10.4, if you configure a static MPLS LSP using the static-path statement, the default preference value is 5. Starting in Junos OS Release 10.4, if you configure a static-label-switched-path the default preference value is 6. The previous configuration statement static-path is hidden in Junos OS Release 10.4 and later releases.

RSVP-signaled LSPs

7

RSVP preference as described in the MPLS Applications Feature Guide

LDP-signaled LSPs

9

LDP preference, as described in the MPLS Applications Feature Guide

OSPF internal route

10

OSPF preference

access-internal route

12

access route

13

IS-IS Level 1 internal route

15

IS-IS preference

IS-IS Level 2 internal route

18

IS-IS preference

Redirects

30

Kernel

40

SNMP

50

Router discovery

55

RIP

100

RIP preference

RIPng

100

RIPng preference

PIM

105

Multicast Protocols Feature Guide

DVMRP

110

Multicast Protocols Feature Guide

Aggregate

130

aggregate

OSPF AS external routes

150

OSPF external-preference

IS-IS Level 1 external route

160

IS-IS external-preference

IS-IS Level 2 external route

165

IS-IS external-preference

BGP

170

BGP preference, export, import

MSDP

175

Multicast Protocols Feature Guide

In general, the narrower the scope of the statement, the higher precedence its preference value is given, but the smaller the set of routes it affects. To modify the default preference value for routes learned by routing protocols, you generally apply routing policy when configuring the individual routing protocols. You also can modify some preferences with other configuration statements, which are indicated in the table.

Example: Configuring the Preference Value for BGP Routes

This example shows how to specify the preference for routes learned from BGP. Routing information can be learned from multiple sources. To break ties among equally specific routes learned from multiple sources, each source has a preference value. Routes that are learned through explicit administrative action, such as static routes, are preferred over routes learned from a routing protocol, such as BGP or OSPF. This concept is called administrative distance by some vendors.

Requirements

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

Overview

Routing information can be learned from multiple sources, such as through static configuration, BGP, or an interior gateway protocol (IGP). When Junos OS determines a route’s preference to become the active route, it selects the route with the lowest preference as the active route and installs this route into the forwarding table. By default, the routing software assigns a preference of 170 to routes that originated from BGP. Of all the routing protocols, BGP has the highest default preference value, which means that routes learned by BGP are the least likely to become the active route.

Some vendors have a preference (distance) of 20 for external BGP (EBGP) and a distance of 200 for internal BGP (IGBP). Junos OS uses the same value (170) for both EBGP and IBGP. However, this difference between vendors has no operational impact because Junos OS always prefers EBGP routes over IBGP routes.

Another area in which vendors differ is in regard to IGP distance compared to BGP distance. For example, some vendors assign a distance of 110 to OSPF routes. This is higher than the EBGP distance of 20 , and results in the selection of an EBGP route over an equivalent OSPF route. In the same scenario, Junos OS chooses the OSPF route, because of the default preference 10 for an internal OSPF route and 150 for an external OSPF route, which are both lower than the 170 preference assigned to all BGP routes.

In a multivendor environment, you might want to change the preference value for BGP routes so that Junos OS chooses an EBGP route instead of an OSPF route. To accomplish this goal, one option is to include the preference statement in the EBGP configuration. To modify the default BGP preference value, include the preferece statement, specifying a value from 0 through 4,294,967,295 (232 – 1).

Tip

Another way to achieve multivendor compatibility is to include the advertise-inactive statement in the EBGP configuration. This causes the routing table to export to BGP the best route learned by BGP even if Junos OS did not select it to be an active route. 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. The advertise-inactive statement causes Junos OS to advertise the best BGP route that is inactive because of IGP preference. When you use the advertise-inactive statement, the Junos OS device uses the OSPF route for forwarding, and the other vendor’s device uses the EBGP route for forwarding. However, from the perspective of an EBGP peer in a neighboring AS, both vendors’ devices appear to behave the same way.

Topology

In the sample network, Device R1 and Device R2 have EBGP routes to each other and also OSPF routes to each other.

This example shows the routing tables in the following cases:

  • Accept the default preference values of 170 for BGP and 10 for OSPF.

  • Change the BGP preference to 8.

Figure 1 shows the sample network.

Figure 1: BGP Preference Value Topology
BGP Preference Value 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.

Device R1

Device R2

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 CLI User Guide.

To configure Device R1:

  1. Configure the interfaces.
  2. Configure the local autonomous system.
  3. Configure the external peering with Device R2.
  4. Configure OSPF.
  5. Configure the routing policy.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, 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.

Repeat these steps on Device R2.

Verification

Confirm that the configuration is working properly.

Verifying the Preference

Purpose

Make sure that the routing tables on Device R1 and Device R2 reflect the fact that Device R1 is using the configured EBGP preference of 8, and Device R2 is using the default EBGP preference of 170.

Action

From operational mode, enter the show route command.

user@R1> show route
user@R2> show route

Meaning

The output shows that on Device R1, the active path to Device R2’s loopback interface (10.255.14.177/32) is a BGP route. The output also shows that on Device R2, the active path to Device R1’s loopback interface (10.255.71.24/32) is an OSPF route.

Example: Using Routing Policy to Set a Preference Value for BGP Routes

This example shows how to use routing policy to set the preference for routes learned from BGP. Routing information can be learned from multiple sources. To break ties among equally specific routes learned from multiple sources, each source has a preference value. Routes that are learned through explicit administrative action, such as static routes, are preferred over routes learned from a routing protocol, such as BGP or OSPF. This concept is called administrative distance by some vendors.

Requirements

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

Overview

Routing information can be learned from multiple sources, such as through static configuration, BGP, or an interior gateway protocol (IGP). When Junos OS determines a route’s preference to become the active route, it selects the route with the lowest preference as the active route and installs this route into the forwarding table. By default, the routing software assigns a preference of 170 to routes that originated from BGP. Of all the routing protocols, BGP has the highest default preference value, which means that routes learned by BGP are the least likely to become the active route.

Some vendors have a preference (distance) of 20 for external BGP (EBGP) and a distance of 200 for internal BGP (IGBP). Junos OS uses the same value (170) for both EBGP and IBGP. However, this difference between vendors has no operational impact because Junos OS always prefers EBGP routes over IBGP routes.

Another area in which vendors differ is in regard to IGP distance compared to BGP distance. For example, some vendors assign a distance of 110 to OSPF routes. This is higher than the EBGP distance of 20 , and results in the selection of an EBGP route over an equivalent OSPF route. In the same scenario, Junos OS chooses the OSPF route, because of the default preference 10 for an internal OSPF route and 150 for an external OSPF route, which are both lower than the 170 preference assigned to all BGP routes.

This example shows a routing policy that matches routes from specific next hops and sets a preference. If a route does not match the first term, it is evaluated by the second term.

Topology

In the sample network, Device R1 and Device R3 have EBGP sessions with Device R2.

On Device R2, an import policy takes the following actions:

  • For routes received through BGP from next-hop 10.0.0.1 (Device R1), set the route preference to 10.

  • For routes received through BGP from next-hop 10.1.0.2 (Device R3), set the route preference to 15.

Figure 2 shows the sample network.

Figure 2: BGP Preference Value Topology
BGP Preference Value Topology

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

The section Step-by-Step Procedure 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

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 CLI User Guide.

To configure Device R2:

  1. Configure the device interfaces.
  2. Configure the local autonomous system.
  3. Configure the routing policy that sends direct routes.
  4. Configure the routing policy that changes the preference of received routes.
  5. Configure the external peering with Device R2.
  6. Apply the set-preference policy as an import policy.

    This affects Device R2’s routing table and has no impact on Device R1 and Device R3.

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 Preference

Purpose

Make sure that the routing tables on Device R1 and Device R2 reflect the fact that Device R1 is using the configured EBGP preference of 8, and Device R2 is using the default EBGP preference of 170.

Action

From operational mode, enter the show route protocols bgp command.

user@R2> show route protocols bgp

Meaning

The output shows that on Device R2, the preference values have been changed to 15 for routes learned from Device R3, and the preference values have been changed to 10 for routes learned from Device R1.

Understanding the Local Preference Metric for Internal BGP Routes

Internal BGP (IBGP) sessions use a metric called the local preference, which is carried in IBGP update packets in the path attribute LOCAL_PREF. When an autonomous system (AS) has multiple routes to another AS, the local preference indicates the degree of preference for one BGP route over the other BGP routes. The BGP route with the highest local preference value is preferred.

The LOCAL_PREF path attribute is always advertised to IBGP peers and to neighboring confederations. It is never advertised to external BGP (EBGP) peers. The default behavior is to not modify the LOCAL_PREF path attribute if it is present.

The default LOCAL_PREF path attribute value of 100 applies at export time only, when the routes are exported from the routing table into BGP.

If a BGP route is received without a LOCAL_PREF attribute, the route is stored in the routing table and advertised by BGP as if it were received with a LOCAL_PREF value of 100. A non-BGP route that is advertised by BGP is advertised with a LOCAL_PREF value of 100 by default.

Example: Configuring the Local Preference Value for BGP Routes

This example shows how to configure local preference in internal BGP (IBGP) peer sessions.

Requirements

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

Overview

To change the local preference metric advertised in the path attribute, you must include the local-preference statement, specifying a value from 0 through 4,294,967,295 (232 – 1).

There are several reasons you might want to prefer one path over another. For example, compared to other paths, one path might be less expensive to use, might have higher bandwidth, or might be more stable.

Figure 3 shows a typical network with internal peer sessions and multiple exit points to a neighboring AS.

Figure 3: Typical Network with IBGP Sessions and Multiple Exit Points
Typical Network with IBGP Sessions
and Multiple Exit Points

To reach Device R4, Device R1 can take a path through either Device R2 or Device R3. By default, the local preference is 100 for either route. When the local preferences are equal, Junos OS has rules for breaking the tie and choosing a path. (See Understanding BGP Path Selection.) In this example, the active route is through Device R2 because the router ID of Device R2 is lower than the router ID of Device R3. The following example shows how to override the default behavior with an explicit setting for the local preference. The example configures a local preference of 300 on Device R3, thereby making Device R3 the preferred path to reach Device R4.

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

Configuring Device R1

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 Device R1:

  1. Configure the interfaces.
  2. Configure BGP.
  3. Configure OSPF.
  4. Configure a policy that accepts direct routes.Note

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

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

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, 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.

Configuring Device R2

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 Device R2:

  1. Configure the interfaces.
  2. Configure BGP.
  3. Configure OSPF.
  4. Configure a policy that accepts direct routes.Note

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

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

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, 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.

Configuring Device R3

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 Device R3:

  1. Configure the interfaces.
  2. Configure BGP.
  3. Configure OSPF.
  4. Configure a policy that accepts direct routes.Note

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

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

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, 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.

Configuring Device R4

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 Device R4:

  1. Configure the interfaces.
  2. Configure BGP.
  3. Configure a policy that accepts direct routes.Note

    Other useful options for this scenario might be to accept routes learned through OSPF or local routes.

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

Results

From configuration mode, confirm your configuration by entering the show interfaces, show policy-options, 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.

Checking the Active Path From Device R1 to Device R4

Purpose

Verify that the active path from Device R1 to Device R4 goes through Device R2.

Action

From operational mode, enter the show route protocol bgp command.

user@R1> show route protocol bgp

Meaning

The asterisk (*) shows that the preferred path is through Device R2. In the default configuration, Device R2 has a lower router ID than Device R3. The router ID is controlling the path selection.

Altering the Local Preference to Change the Path Selection

Purpose

Change the path so that it goes through Device R3.

Action

From configuration mode, enter the set local-preference 300 command.

[edit protocols bgp group internal]
user@R3# set local-preference 300
user@R3# commit

Rechecking the Active Path From Device R1 to Device R4

Purpose

Verify that the active path from Device R1 to Device R4 goes through Device R3.

Action

From operational mode, enter the show route protocol bgp command.

user@R1> show route protocol bgp

Meaning

The asterisk (*) shows that the preferred path is through Device R3. In the altered configuration, Device R3 has a higher local preference than Device R2. The local preference is controlling the path selection.

Example: Configuring BGP to Advertise Inactive Routes

By default, BGP readvertises only active routes. 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:

In Junos OS, BGP advertises BGP routes that are installed or active, which are routes selected as the best based on the BGP path selection rules. The advertise-inactive statement allows nonactive BGP routes to be advertised to other peers.

Note

If the routing table has two BGP routes where one is active and the other is inactive, the advertise-inactive statement does not advertise the inactive BGP prefix. This statement does not advertise an inactive BGP route in the presence of another active BGP route. However, if the active route is a static route, the advertise-inactive statement advertises the inactive BGP route.

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 (or advertise-external) statement.

For example, the following configuration can be used as a BGP export policy to mark routes advertised due to the advertise-inactive 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

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

Overview

In this example, Device R2 has two external BGP (EBGP) peers, Device R1 and Device R3.

Device R1 has a static route to 172.16.5/24. Likewise, Device R2 also has a static route to 172.16.5/24. Through BGP, Device R1 sends information about its static route to Device R2. Device R2 now has information about 172.16.5/24 from two sources—its own static route and the BGP-learned route received from Device R1. Static routes are preferred over BGP-learned routes, so the BGP route is inactive on Device R2. Normally Device R2 would send the BGP-learned information to Device R3, but Device R2 does not do this because the BGP route is inactive. Device R3, therefore, has no information about 172.16.5/24 unless you enable the advertise-inactive command on Device R2, which causes Device R2 to send the BGP-learned to Device R3.

Topology

Figure 4 shows the sample network.

Figure 4: BGP Topology for advertise-inactive
BGP Topology for advertise-inactive

CLI Quick Configuration shows the configuration for all of the devices in Figure 4.

The section Step-by-Step Procedure 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

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 CLI User Guide.

To configure Device R2:

  1. Configure the device interfaces.
  2. Configure the EBGP connection to Device R1.
  3. Configure the EBGP connection to Device R3.
  4. Add the advertise-inactive statement to the EBGP group peering session with Device R3.
  5. Configure the static route to the 172.16.5.0/24 network.
  6. Configure the autonomous system (AS) number.

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 172.16.5.0/24 prefix is in the routing table and has the expected active path.

Action

user@R2> show route 172.16.5

Meaning

Device R2 receives the 172.16.5.0/24 route from both Device R1 and from its own statically configured route. The static route is the active path, as designated by the asterisk (*). The static route path has the lowest route preference (5) as compared to the BGP preference (170). Therefore, the static route becomes active.

Verifying the External Route Advertisement

Purpose

On Device R2, make sure that the 172.16.5.0/24 route is advertised toward Device R3.

Action

user@R2> show route advertising-protocol bgp 10.0.0.6

Meaning

Device R2 is advertising the 172.16.5.0/24 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

user@R3> show route 172.16.5.0/24

Meaning

Device R3 has the BGP-learned route for 172.16.5.0/24.

Experimenting with the advertise-inactive Statement

Purpose

See what happens when the advertise-inactive statement is removed from the BGP configuration on Device R2.

Action

  1. On Device R2, deactivate the advertise-inactive statement.

  2. On Device R2, check to see if the 172.16.5.0/24 route is advertised toward Device R3.



    user@R2> show route advertising-protocol bgp 10.0.0.6

    As expected, the route is no longer advertised.

  3. On Device R3, ensure that the 172.16.5/24 route is absent from the routing table.



    user@R3> show route 172.16.5/24

Meaning

Device R1 advertises route 172.16.5/24 to Device R2, but Device R2 has a manually configured static route for this prefix. Static routes are preferred over BGP routes, so Device R2 installs the BGP route as an inactive route. Because the BGP route is not active, Device R2 does not readvertise the BGP route to Device R3. This is the default behavior in Junos OS. If you add the advertise-inactive statement to the BGP configuration on Device R2, Device R2 readvertises nonactive routes.

Release History Table
Release
Description
Starting in Junos OS Release 10.4, if you configure a static-label-switched-path the default preference value is 6.