Cisco Nexus 7000 Series - Configuring as Confederations; Configuring Ebgp Multihop; Disabling a Fast External Fallover; Limiting the AS-Path Attribute

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11-25
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 11 Configuring Advanced BGP
Configuring Advanced BGP
Configuring eBGP Multihop
You can configure the eBGP time-to-live (TTL) value to support eBGP multihop. In some situations, an
eBGP peer is not directly connected to another eBGP peer and requires multiple hops to reach the remote
eBGP peer. You can configure the eBGP TTL value for a neighbor session to allow these multihop
sessions.
To configure eBGP multihop, use the following command in neighbor configuration mode:
Disabling a Fast External Fallover
By default, the Cisco Nexus 7000 Series device supports fast external fallover for for neighbors in all
VRFs and address-families (IPv4 or IPv6). Typically, when a BGP router loses connectivity to a directly
connected eBGP peer, BGP triggers a fast external fallover by resetting the eBGP session to the peer.
You can disable this fast external fallover to limit the instability caused by link flaps.
To disable fast external fallover, use the following command in router configuration mode:
Limiting the AS-path Attribute
You can configure eBGP to discard routes that have a high number of AS numbers in the AS-path
attribute.
To discard routes that have a high number of AS numbers in the AS-path attribute, use the following
command in router configuration mode:
Configuring AS Confederations
To configure an AS confederation, you must specify a confederation identifier. To the outside world, the
group of autonomous systems within the AS confederation look like a single autonomous system with
the confederation identifier as the autonomous system number.
Command Purpose
ebgp-multihop ttl-value
Example:
switch(config-router-neighbor)#
ebgp-multihop 5
Configures the eBGP TTL value for eBGP
multihop. The range is from 2 to 255. You must
manually reset the BGP sessions after using this
command.
Command Purpose
no fast-external-fallover
Example:
switch(config-router)# no
fast-external-fallover
Disables a fast external fallover for eBGP peers.
This command is enabled by default.
Command Purpose
maxas-limit number
Example:
switch(config-router)# maxas-limit 50
Discards eBGP routes that have a number of
AS-path segments that exceed the specified limit.
The range is from 1 to 2000.