© 2005 Cisco Systems, Inc. All rights reserved. BGP v BGP Transit Autonomous Systems Configuring a Transit AS

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Outline Overview Configuring IBGP Neighbors Configuring IBGP Sessions Between Loopback Interfaces Configuring BGP Synchronization Changing the Administrative Distance of BGP Routes Scalability Limitations of IBGP-Based Transit Backbones Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring IBGP Neighbors neighbor ip-address remote-as as-number router(config-router)# This command configures a BGP neighbor. The AS number configured determines whether the session is an EBGP session (neighbor AS is different from local AS) or IBGP session (same AS number). neighbor ip-address description text router(config-router)# Attaches optional description to a neighbor

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring IBGP Sessions Between Loopback Interfaces neighbor ip-address update-source interface router(config-router)# This command configures the source interface for the TCP session that carries BGP traffic. For IBGP sessions, the source interface is a loopback address. The source address configured on one peering router must match the destination address configured on the othera BGP session will not start otherwise. Make sure that your loopback interfaces are announced in the backbone IGP.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Configuring BGP Synchronization no synchronization router(config-router)# This command disables synchronization between BGP and an IGP. Modern transit autonomous systems do not need synchronization because they do not rely on redistribution of BGP routes into an IGP. BGP synchronization has to be disabled in modern transit AS designs on all BGP routers.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Changing the Administrative Distance of BGP Routes distance bgp external internal local router(config-router)# This command sets the AD for EBGP, IBGP, and local routes. This change applies only to routes received after the command has been entered (similar to filters). Defaults: EBGP routes have a distance of 20; IBGP and local routes have a distance of 200. The defaults are usually correct; do not change them.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Scalability Limitations of IBGP-Based Transit Backbones Transit backbone requires IBGP full mesh between all core routers. Large number of TCP sessions Unnecessary, duplicate routing traffic There are two scalability solutions: Route reflectors BGP confederations

© 2005 Cisco Systems, Inc. All rights reserved. BGP v Summary To configure an IBGP neighbor, use the neighbor command, specifying a remote AS number matching the AS number of the local router. When you configure IBGP sessions between loopback interfaces, the interfaces must be announced in the backbone IGP. Configuring BGP neighbors using the neighbor update-source command ensures that the source address of the outgoing TCP connection is correct by referring to the interface that has the correct IP address. You should disable BGP synchronization in all modern transit AS designs on all BGP routers by using the no form of the synchronization command. Although you can change the administrative distances of BGP routes by using the distance bgp router configuration command, you typically should not change the default settings for EBGP (20) and IBGP (200). The full-mesh IBGP requirement in the transit AS creates scalability issues in the number of TCP sessions and unnecessary, duplicate routing traffic. IBGP scalability solutions to these issues exist.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.22-9