Deployment Guide

Usage Information
This is enabled by default; that is, for all updates received from EBGP peers, BGP ensures that the rst AS of the
rst AS segment is always the AS of the peer. If not, the update is dropped and a counter is incremented. To view
the failed enforce-rst-as check counter, use the show ip bgp ipv6 unicast neighbors command.
If you disable enforce-first-as, view it using the show ip protocols command.
Related Commands
show ip bgp ipv6 unicast neighbors — displays IPv6 routing information exchanged by BGP neighbors.
show ip protocols — views information on routing protocols.
bgp fast-external-fallover
Enable the fast external fallover feature, which immediately resets the BGP session if a link to a directly connected external peer fails.
Syntax
bgp fast-external-fallover
To disable fast external fallover, use the no bgp fast-external-fallover command.
Defaults Enabled.
Command Modes ROUTER BGP
Supported Modes Full–Switch
Command History
Version Description
9.9(0.0) Introduced on the FN IOM.
9.2(0.0) Introduced on the MXL 10/40GbE Switch IO Module.
Usage Information The bgp fast-external-fallover command appears in the show config command output.
bgp four-octet-as-support
Enable 4-byte support for the BGP process.
Syntax
bgp four-octet-as-support
To disable fast external fallover, use the no bgp four-octet-as-support command.
Defaults Disabled (supports 2-Byte format)
Command Modes ROUTER BGP
Supported Modes Full–Switch
Command History
Version Description
9.9(0.0) Introduced on the FN IOM.
9.2(0.0) Introduced on the MXL 10/40GbE Switch IO Module.
Usage Information
Routers supporting 4-Byte ASNs advertise that function in the OPEN message. The behavior of a 4-Byte router is
slightly dierent depending on whether it is speaking to a 2-Byte router or a 4-Byte router.
672 IPv6 Border Gateway Protocol (IPv6 BGP)