BGP Overview

Connections between peering networks are typically made through an exterior gateway protocol (EGP), most commonly BGP. BGP is an EGP used primarily to establish point-to-point connections and transmit data between peer autonomous systems (ASs). Unlike RIP and OSPF links, BGP peering sessions must be explicitly configured at both ends. BGP must explicitly advertise the routes between its peers. The route advertisements determine prefix reachability and the way packets are routed between BGP neighbors. Because BGP uses the packet path to determine route selection, it is considered a path-vector protocol.

This overview contains the following topics:

BGP Messages for Session Establishment
BGP Messages for Session Maintenance
IBGP and EBGP
Route Selection
Local Preference
AS Path
Origin
Multiple Exit Discriminator
Scaling BGP for Large Networks

BGP Messages for Session Establishment

When the routers on either end of a BGP session first boot, the session between them is in the Idle state. The BGP session remains idle until a start event is detected. Typically, the start event is the configuration of a new BGP session or the resetting of an existing BGP session. At boot time, the start event is generated by the router as the BGP session is initiated.

After it detects a start event, the BGP host sends TCP request packets to its configured BGP neighbors. These packets are directed only to neighboring interfaces that have been explicitly configured as BGP neighbors. Upon receipt of the TCP request packet, the neighboring host generates a TCP response to complete the three-way handshake and establish a TCP connection between the peers. While this handshake is taking place, the BGP state for the connection is Connect. If a TCP timeout occurs while the originating host is waiting for a TCP response packet, the BGP state for the connection is Active. The Active state indicates that the router is actively listening for a TCP response and the TCP retry timer has been initiated.

Once a TCP connection has been established between both ends of a BGP session, the BGP session state is OpenSent, indicating that the originating router has generated an open message. The open message is an initial BGP handshake that must occur before any route advertisement can take place. Upon receipt of the open message, the neighboring router generates a keepalive message. Receipt of the keepalive message establishes a point-to-point connection, and the BGP session state transitions to Established. While the originating host waits for the keepalive response packet, the BGP session state is OpenConfirm.

BGP Messages for Session Maintenance

Once a BGP session has been established, the BGP peers exchange route advertisements by means of update messages. Update messages contain one or more route advertisements, and they can contain one or more prefixes that are to be removed from the BGP routing table. If the peers need to advertise multiple routes, they generate and send multiple update messages as they detect changes to the network. In the absence of changes to the routing table, no update messages are generated.

While a BGP session is active, each router on the BGP session generates keepalive messages periodically. The timing of these messages is determined by the hold time on the session. The hold time is a negotiated value specifying the number of seconds that can elapse without keepalive messages before BGP designates the link inactive. Three messages are sent during every hold time interval.

When a peer connection is closed (either by error or if the BGP session is closed), a notification message is generated and sent to the peer router that did not experience the error or did not terminate the BGP session.

IBGP and EBGP

Peer ASs establish links through an external peer BGP session. As a result, all route advertisement between the external peers takes place by means of the EBGP mode of information exchange. To propagate the routes through the AS and advertise them to internal peers, BGP uses IBGP. To advertise the routes to a different peer AS, BGP again uses EBGP.

BGP uses two primary modes of information exchange, internal BGP (IBGP) and external BGP (EBGP), to communicate with internal and external peers, respectively.

To avoid routing loops, IBGP does not advertise routes learned from an internal BGP peer to other internal BGP peers. For this reason, BGP cannot propagate routes throughout an AS by passing them from one router to another. To achieve an IBGP full mesh, you configure a direct peering session every host to every other host within the network. These sessions are configured on every router within the network, as type internal.

As a network grows, the full mesh requirement becomes increasingly difficult to manage. In a network with 1000 routers, the addition of a single router requires that all the routers in the network be modified to account for the new addition. To combat these scaling problems, BGP uses route reflection and BGP confederations.

Route Selection

The BGP route selection process compares BGP attributes to select a single best path or active route for each prefix in the routing table. The attributes are compared in a particular order. A local BGP router uses the following criteria, in the order presented, to select a route from the routing table for the forwarding table.

Next-hop accessibility—If the next hop is inaccessible, the local router does not consider the route. The router must verify that it has a route to the BGP next-hop address. If a local route to the next hop does not exist, the local route does not include the router in its forwarding table. If such a route exists, route selection continues.
Highest local preference—The local router selects the route with the highest local preference value. If multiple routes have the same preference, route selection continues. (See Local Preference.)
Shortest AS path—The local router selects the route with the fewest entries in the AS path. If multiple routes have the same AS path length, route selection continues.
Lowest origin—The local router selects the route with the lowest origin value. If multiple routes have the same origin value, route selection continues. (See Origin.)
Lowest MED value—The local router selects the route with the lowest multiple exit discriminator (MED) value, comparing the routes from the same AS only. If multiple routes have the same MED value, route selection continues. See Multiple Exit Discriminator.
Strictly external paths—The local router prefers strictly external (EBGP) paths over external paths learned through interior sessions (IBGP). If multiple routes have the same strictly external paths, route selection continues.
Lowest IGP route metric— The local router selects the path for which the next hop is resolved through the IGP route with the lowest metric. If multiple routes have the same IGP route metric, route selection continues.
Maximum IGP next hops—The local router selects the path for which the BGP next hop is resolved through the IGP route with the largest number of next hops. If multiple routes have the same number of next hops, route selection continues.
Shortest route reflection cluster list—The local router selects the path with the shortest route reflection cluster list. Routes without a cluster list are considered to have a cluster list of length 0. If multiple routes have the same route reflection cluster list, route selection continues.
Lowest router ID—The local router selects the route with the lowest IP address value for the BGP router ID. By default, the router IDs of routes received from different ASs are not compared. You can change this default behavior.
Lowest peer IP address—The local router selects the path that was learned from the neighbor with the lowest peer IP address.

For configuration instructions, see Configuring Routing Table Path Selection for BGP in the Junos Routing Protocols Configuration Guide.

Local Preference

The local preference is typically used to direct all outbound AS traffic to a certain peer. When you configure a local preference, all routes that are advertised through that peer are assigned the preference value. The preference is a numeric value, and higher values are preferred during BGP route selection. Figure 23 illustrates how to use local preference to determine BGP route selection.

Figure 23: Local Preference

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The network in Figure 23 shows two possible routes to the prefixes accessible through Host E. The first route, through Router A, uses an OC3 link to Router C and is then forwarded to Host E. The second route, through Router B, uses an OC48 link to Router D and is then forwarded to Host E. Although the number of hops to Host E is identical regardless of the route selected, the route through Router B is more desirable because of the increased bandwidth. To force traffic through Router B, you can set the local preference on Router A to 100 and the local preference on Router B to 300. During BGP route selection, the route with the higher local preference is selected.

Note: In contrast to almost every other metric associated with dynamic routing protocols, the local preference gives higher precedence to the larger value.

For configuration instructions, see Configuring the Local Preference Value for BGP Routes in the Junos Routing Protocols Configuration Guide.

AS Path

Routes advertised by BGP maintain a list of the ASs through which the route travels. This information is stored in the route advertisement as the AS path, and it is one of the primary criteria that a local router uses to evaluate BGP routes for inclusion in its forwarding table. Figure 24 shows how BGP creates an AS path.

Figure 24: BGP AS Path

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In the network shown in Figure 24, the route from Host A to Host B travels through two intermediate ASs. As the route advertisement is propagated through the BGP network, it accumulates an AS path number each time it exits one AS and enters another. Each AS number is prepended to the AS path, which is stored as part of the route advertisement. When the route advertisement first leaves Host B's AS, the AS path is 17. When the route is advertised between intermediate ASs, the AS number 7 is prepended to the AS path, which becomes 7 17. When the route advertisement exits the third AS, the AS path becomes 4 7 17. The route with the shortest AS path is preferred for inclusion into the BGP forwarding table.

Origin

The BGP router that first advertises a route assigns it one of the following values to identify its origin. During route selection, the lowest origin value is preferred.

0—The router originally learned the route through an IGP (OSPF, IS-IS, or a static route).
1—The router originally learned the route through an EGP (most likely BGP).
2—The route's origin is unknown.

Multiple Exit Discriminator

A multiple exit discriminator (MED) is an arbitrary metric assigned to a route to determine the exit point to a destination when all other factors are equal. By default, MED metrics are compared only for routes to the same peer AS, but you can also configure routing table path selection options for different ways of comparing MEDs.

Default MED Usage

Because the AS path rather than the number of hops between hosts is the primary criterion for BGP route selection, an AS with multiple connections to a peer AS can have multiple equivalent AS paths. When the routing table contains two routes to the same host in a neighboring AS, an MED metric assigned to each route can determine which to include in the forwarding table. The MED metric you assign can force traffic through a particular exit point in an AS.

Figure 25 illustrates how MED metrics are used to determine route selection.

Figure 25: Default MED Example

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Figure 25 shows AS 1 and AS 2 connected by two separate BGP links to Routers C and D. Host E in AS 1 is located nearer to Router C. Host F, also in AS 1, is located nearer to Router D. Because the AS paths are equivalent, two routes exist for each host, one through Router C and one through Router D. To force all traffic destined for Host E through Router C, the network administrator for AS 2 assigns an MED metric for each router to Host E at its exit point. An MED metric of 10 is assigned to the route to Host E through Router C, and an MED metric of 20 is assigned to the route to Host E through Router D. BGP routers in AS 2 then select the route with the lower MED metric for the forwarding table.

Additional MED Options for Path Selection

By default, only the MEDs of routes that have the same peer ASs are compared. However, you can configure the routing table path selection options listed in Table 5 to compare MEDs in different ways. The MED options are not mutually exclusive and can be configured in combination or independently. For the MED options to take effect, you must configure them uniformly all through your network. The MED option or options you configure determine the route selected. Thus we recommend that you carefully evaluate your network for preferred routes before configuring the MED options. See Configuring Routing Table Path Selection for BGP in the Junos Routing Protocols Configuration Guide.

Table 5: MED Options for Routing Table Path Selection

Option (Name)	Function	Use
Always comparing MEDs (always-compare-med)	Ensures that the MEDs for paths from peers in different ASs are always compared in the route selection process.	Useful when all enterprises participating in a network agree on a uniform policy for setting MEDs. For example, in a network shared by two ISPs, both must agree that a certain path is the better path to configure the MED values correctly.
Adding IGP cost to MED (med-plus-igp)	Before comparing MED values for path selection, adds to the MED the cost of the IGP route to the BGP next-hop destination. This option replaces the MED value for the router, but does not affect the IGP metric comparison. As a result, when multiple routes have the same value after the MED-plus-IPG comparison, and route selection continues, the IGP route metric is also compared, even though it was added to the MED value and compared earlier in the selection process.	Useful when the downstream AS requires the complete cost of a certain route that is received across multiple ASs.
Applying Cisco IOS nondeterministic behavior (cisco-non-deterministic)	Specifies the nondeterministic behavior of the Cisco IOS software: The active path is always first. All nonactive but eligible paths follow the active path and are maintained in the order in which they were received. Ineligible paths remain at the end of the list. When a new path is added to the routing table, path comparisons are made among all routes, including those paths that must never be selected because they lose the MED tie-breaking rule.	We recommend that you do not configure this option, because the nondeterministic behavior sometimes prevents the system from properly comparing the MEDs between paths.

Option (Name)

Function

Use

Always comparing MEDs (always-compare-med)

Ensures that the MEDs for paths from peers in different ASs are always compared in the route selection process.

Useful when all enterprises participating in a network agree on a uniform policy for setting MEDs. For example, in a network shared by two ISPs, both must agree that a certain path is the better path to configure the MED values correctly.

Adding IGP cost to MED (med-plus-igp)

Before comparing MED values for path selection, adds to the MED the cost of the IGP route to the BGP next-hop destination.

This option replaces the MED value for the router, but does not affect the IGP metric comparison. As a result, when multiple routes have the same value after the MED-plus-IPG comparison, and route selection continues, the IGP route metric is also compared, even though it was added to the MED value and compared earlier in the selection process.

Useful when the downstream AS requires the complete cost of a certain route that is received across multiple ASs.

Applying Cisco IOS nondeterministic behavior (cisco-non-deterministic)

Specifies the nondeterministic behavior of the Cisco IOS software:

The active path is always first. All nonactive but eligible paths follow the active path and are maintained in the order in which they were received. Ineligible paths remain at the end of the list.
When a new path is added to the routing table, path comparisons are made among all routes, including those paths that must never be selected because they lose the MED tie-breaking rule.

We recommend that you do not configure this option, because the nondeterministic behavior sometimes prevents the system from properly comparing the MEDs between paths.

Scaling BGP for Large Networks

BGP is not a flooding protocol like RIP or OSPF. Instead, it is a peering protocol that exchanges routes with fully meshed peers only. However, in large networks, the full mesh requirement causes scaling problems. BGP combats scaling problems using route reflectors and confederations.