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Implementing BGP on Cisco IOS XR Software
Information About Implementing BGP on Cisco IOS XR Software
RC-25
Cisco IOS XR Routing Configuration Guide
OL-14356-01
• 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
Figure 1 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/RP0/CPU0:router(config)# route-policy ISP2_PE1
RP/0/RP0/CPU0:router(config-rpl)# set extcommunity cost (1:1)
The preceding route policy applies a cost community number of 1 to the 10.8.0.0 route. By default, the
path learned from ASBR1 is assigned a cost community number of 2147483647. Because the path
learned from ASBR2 has a lower-cost community number, the path is preferred.
BGP Cost Community Support for EIGRP MPLS VPN PE-CE with Back-door Links
Back-door links in an EIGRP MPLS VPN topology is preferred by BGP if the back-door link is learned
first. (A back-door link, or route, is a connection that is configured outside of the VPN between a remote
and main site; for example, a WAN leased line that connects a remote site to the corporate network.)
89045
ASBR1ASBR2
ISP2 PE2
ISP2 PE1
10.8/16
ISP1 PE1
172.19.20/24
172.19.20/24
10.8/16
ISP1 PE2