Connectivity Guide

BGP ensures that the rst AS of the rst AS segment is always the AS of the peer, otherwise the update drops
and the counter increments. The no version of this command turns o the default.
NOTE: To congure these settings for a non default VRF instance, you must rst enter the ROUTER-
CONFIG-VRF sub mode using the following commands:
1 Enter the ROUTER BGP mode using the router bgp as-number command.
2 From the ROUTER BGP mode, enter the ROUTER BGP VRF mode using the vrf vrf-name
command.
Example
OS10(conf-router-bgp-1)# enforce-first-as
Supported Releases 10.3.0E or later
fall-over
Enables or disables BGP session fast fall-over for BGP neighbors.
Syntax
fall-over
Parameters None
Default Disabled
Command Mode ROUTER-NEIGHBOR
Usage Information Congure the BGP fast fall-over on a per-neighbor or peer-group basis. When you enable this command on a
template, it simultaneously enables on all peers that inherit the peer group template. When you enable fall-over,
BGP tracks IP reachability to the peer remote address and the peer local address. Whenever either address
becomes unreachable — no active route exists in the routing table for peer IPv6 destinations or local address —
BGP brings down the session with the peer. The no version of this command disables fall-over.
Example
OS10(conf-router-neighbor)# fall-over
Supported Releases 10.3.0E or later
fast-external-fallover
Resets BGP sessions immediately when a link to a directly connected external peer fails.
Syntax
fast-external-fallover
Parameters None
Default Not congured
Command Mode ROUTER-BGP
Usage Information Fast external fall-over terminates the EBGP session immediately after the IP unreachability or link failure is
detected. This only applies after you manually reset all existing BGP sessions. For the conguration to take eect,
use the clear ip bgp command. The no version of this command disables fast external fallover.
NOTE: To congure these settings for a non default VRF instance, you must rst enter the ROUTER-
CONFIG-VRF sub mode using the following commands:
1 Enter the ROUTER BGP mode using the router bgp as-number command.
2 From the ROUTER BGP mode, enter the ROUTER BGP VRF mode using the vrf vrf-name
command.
366 Layer 3