3
On R3:
bgp: prefix p/n has:
path 1(bestpath) with LB 180 {learned from R1}
path 2(ibgp multipath) with LB 310 {learned from R2}
Configuring Policy-based Link Bandwidth: Example
This is a sample configuration for policy-based DMZ link bandwidth. The link-bandwidth ext-community
can be set on a per-path basis either at the neighbor-in or neighbor-out policy attach-points. The
dmz-link-bandwidth knob is configured under eBGP neighbor configuration mode. All paths received from
that particular neighbor will be marked with the link-bandwidth extended community when sent to iBGP
peers.
1
Configure inbound or outbound route-policy.
extcommunity-set bandwidth dmz_ext
1:1290400000
end-set
!
route-policy dmz_rp
set extcommunity bandwidth dmz_ext
pass
end-policy
!
neighbor 10.0.101.1
remote-as 1001
address-family ipv4 unicast
route-policy dmz_rp in <<< Inbound route-policy.
route-policy pass out
!
2
Configure dmz-link-bandwidth under BGP neighbor.
neighbor 10.0.101.2
remote-as 1001
dmz-link-bandwidth <<< Under neighbor.
address-family ipv4 unicast
route-policy pass in
route-policy pass out
!
For more information on policy-based extended community set, see the Implementing Routing Policy chapter
in Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide .
BGP Best Path Algorithm
BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines
the best path to install in the IP routing table and to use for forwarding traffic. This section describes the Cisco
IOS XR software implementation of BGP best-path algorithm, as specified in Section 9.1 of the Internet
Engineering Task Force (IETF) Network Working Group draft-ietf-idr-bgp4-24.txt document.
The BGP best-path algorithm implementation is in three parts:
• Part 1—Compares two paths to determine which is better.
• Part 2—Iterates over all paths and determines which order to compare the paths to select the overall best
path.
• Part 3—Determines whether the old and new best paths differ enough so that the new best path should
be used.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
39
Implementing BGP
BGP Best Path Algorithm
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