© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-1 BGP Overview Processing BGP Routes

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-1

BGP Overview

Processing BGP Routes


Outline

• Overview

• Receiving Routing Updates

• Building the BGP Table

• BGP Route Selection Criteria

• BGP Route Propagation

• Building the IP Routing Table

• Advertising Local Networks

• Automatic Summarization

• Summary


Receiving Routing Updates

Small BGP Network


Receiving Routing Updates (Cont.)

Information from the BGP tables is exchanged after adjacency establishment.


Building the BGP Table

All inbound updates are placed into the BGP table.


BGP Route Selection Criteria

• Exclude routes with inaccessible next hop

• Prefer highest weight (local to router)

• Prefer highest local preference (global within AS)

• Prefer routes that the router originated

• Prefer shortest AS path (only length is compared)

• Prefer lowest origin code (IGP < EGP < Incomplete)

• Prefer lowest MED

• Prefer external (EBGP) paths over internal (IBGP)

• For IBGP paths, prefer path through closest IGP neighbor

• For EBGP paths, prefer oldest (most stable) path

• Prefer paths from router with the lowest BGP router-ID


BGP Route Selection Criteria (Cont.)

The best routes to the destination networks are selected from the BGP table.


BGP Route Propagation

The best BGP routes are propagated to BGP neighbors.


The best BGP routes are copied into the IP routing table based on administrative distance.

Building the IP Routing Table


Advertising Local Networks

• The BGP router process keeps a list of local networks (defined with the network command or through redistribution).

• The BGP process periodically scans the IP routing table and inserts or revokes routes from the BGP routing table based on their presence in the IP routing table.


Advertising Local Networks (Cont.)

The BGP route is revoked after the network is removed from the routing table.


Advertising Local Networks (Cont.)

The BGP route is advertised after the network appears in the routing table.


Automatic Summarization

• Automatic summarization is enabled by default.

• Enable automatic summarization when:

– Summarization of IGP-to-BGP redistributed routes to major network boundary required

– Using classful network command to summarize subnets to a major network boundary

• Disable automatic summarization when:

– Summarization on IGP-to-BGP redistribution not desired

– Using classless variant of the network command


Automatic Summarization (Cont.)

Classful network summary is inserted into BGP table.

One subnet and one host route for 197.1.1.0 exist in the routing table.

Automatic summarization is enabled for BGP.

BGP has been configured to locally announce 197.1.1.0.


Automatic Summarization (Cont.)

Classful network summary is inserted into BGP table.

One subnet and two host routes for 172.16.0.0 exist in the routing table.

Automatic summarization is enabled for BGP.

BGP has been configured to redistribute Open Shortest Path First (OSPF) into BGP.


Summary

• After BGP sessions are established between BGP routers, they can start exchanging routing updates.

• All updates that are received from BGP neighbors are stored in the BGP table, regardless of whether they are used.

• The route selection process takes into account various BGP attributes that are attached to the route, as well as local decisions (indicated with weights).

• Only the best BGP routes are propagated to other BGP routers.


Summary (Cont.)

• Only the best BGP routes are installed in the local IP routing table.

• Every BGP router can also originate the routes in BGP. The routes to be originated are entered manually in the BGP routing process or redistributed into BGP from an IGP.

• Automatic summarization is enabled by default in BGP.


Documents

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-1 BGP Overview Processing BGP Routes