Cisco ASR 9000 Series Configuration Guide

•
172.16.0.1
◦
POI=IGP
◦
cost community ID=2
◦
cost number=100
•
192.168.0.1
◦
POI=IGP
◦
cost community ID=1
◦
cost number=200
The single advertised path includes the aggregate cost communities as follows:
{POI=IGP, ID=1, Cost=2147483647} {POI-IGP, ID=2, Cost=2147483647}
Influencing Route Preference in a Multiexit IGP Network
This figure shows an IGP network with two autonomous system boundary routers (ASBRs) on the edge. Each
ASBR has an equal cost path to network 10.8/16.
Figure 1: Multiexit Point IGP Network
Both paths are considered to be equal by BGP. If multipath loadsharing is configured, both paths to the routing
table are installed and are used to balance the load of traffic. If multipath load balancing is not configured,
the BGP selects the path that was learned first as the best path and installs this path to the routing table. This
behavior may not be desirable under some conditions. For example, the path is learned from ISP1 PE2 first,
but the link between ISP1 PE2 and ASBR1 is a low-speed link.
The configuration of the cost community attribute can be used to influence the BGP best-path selection process
by applying a lower-cost community value to the path learned by ASBR2. For example, the following
configuration is applied to ASBR2:
RP/0/RSP0/CPU0:router(config)# route-policy ISP2_PE1
RP/0/RSP0/CPU0:router(config-rpl)# set extcommunity cost (1:1)
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
36 OL-30423-03
Implementing BGP
BGP Cost Community

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English