Configuring Policy-Based Link Bandwidth: Example; Bgp Best Path Algorithm - Cisco ASR 9000 Series Routing Configuration Manual

Implementing BGP
3 On R3:
bgp: prefix p/n has:
path 1(bestpath)
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
end-set
!
route-policy dmz_rp
end-policy
!
2 Configure dmz-link-bandwidth under BGP neighbor.
neighbor 10.0.101.2
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
• Part 3—Determines whether the old and new best paths differ enough so that the new best path should
with LB 180 {learned from R1}
1:1290400000
set extcommunity bandwidth dmz_ext
pass
neighbor 10.0.101.1
remote-as 1001
address-family ipv4 unicast
route-policy dmz_rp in
route-policy pass out
!
remote-as 1001
dmz-link-bandwidth
address-family ipv4 unicast
route-policy pass in
route-policy pass out
!
path.
be used.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
<<< Inbound route-policy.
<<< Under neighbor.
BGP Best Path Algorithm
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