Cisco NCS 5500 Series

2
Eliminate the current best path.
3
Eliminate all the internal paths for the prefix.
4
From the remaining paths, eliminate all the paths that have the same next hop as that of the current best
path.
5
Rerun the best path algorithm on the remaining set of paths to determine the best–external path.
BGP considers the external and confederations BGP paths for a prefix to calculate the best–external path.
BGP advertises the best path and the best–external path as follows:
• On the primary PE—advertises the best path for a prefix to both its internal and external peers
• On the backup PE—advertises the best path selected for a prefix to the external peers and advertises the
best–external path selected for that prefix to the internal peers
Configure Best-External Path Advertisement
Perform the following tasks to advertise the best–external path to the iBGP and route-reflector peers:
SUMMARY STEPS
1.
configure
2.
router bgp as-number
3.
Do one of the following
•
address-family { vpnv4 unicast | vpnv6 unicast }
•
vrfvrf-name{ipv4 unicast|ipv6 unicast}
4.
advertise best-external
5.
commit
DETAILED STEPS
Step 1
configure
Step 2
router bgp as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 100
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP
routing process.
Step 3
Do one of the following
•
address-family { vpnv4 unicast | vpnv6 unicast }
•
vrfvrf-name{ipv4 unicast|ipv6 unicast}
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
28
Implementing BGP
BGP Best-External Path