Cisco Systems 78-11741-02 Indicating Backdoor Routes, Setting Administrative Distance, IPC-325

Configuring BGP

Configuring Advanced BGP Features

Indicating Backdoor Routes

You can indicate which networks are reachable by using a backdoor route that the border router should use. A backdoor network is treated as a local network, except that it is not advertised. To configure backdoor routes, use the network backdoor command, beginning in router configuration mode:

Command	Purpose
Router(config-router)# network ip-addressbackdoor	Indicates reachable networks through backdoor routes.

Modifying Parameters While Updating the IP Routing Table

By default, when a BGP route is put into the IP routing table, the MED is converted to an IP route metric, the BGP next hop is used as the next hop for the IP route, and the tag is not set. However, you can use a route map to perform mapping. To modify metric and tag information when the IP routing table is updated with BGP learned routes, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# table-mapmap-name	Applies a route map to routes when updating the IP routing
	table.

Setting Administrative Distance

Administrative distance is a measure of the preference of different routing protocols. BGP uses three different administrative distances: external, internal, and local. Routes learned through external BGP are given the external distance, routes learned with iBGP are given the internal distance, and routes that are part of this autonomous system are given the local distance. To assign a BGP administrative distance, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# distance bgp	Assigns a BGP administrative distance.
external-distance internal-distance local-distance

Changing the administrative distance of BGP routes is considered dangerous and generally is not recommended. The external distance should be lower than any other dynamic routing protocol, and the internal and local distances should be higher than any other dynamic routing protocol.

Adjusting BGP Timers

BGP uses certain timers to control periodic activities such as the sending of keepalive messages and the interval after not receiving a keepalive message after which the Cisco IOS software declares a peer dead. By default, the keepalive timer is 60 seconds, and the hold-time timer is 180 seconds.You can adjust these timers. When a connection is started, BGP will negotiate the hold time with the neighbor. The smaller of the two hold times will be chosen. The keepalive timer is then set based on the negotiated hold time and the configured keepalive time.

Cisco IOS IP Configuration Guide

IPC-325