Specifications

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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 6 Configuring Advanced BGP
Configuring Advanced BGP
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 Local AS Support
The local AS feature allows a router to appear to be a member of a second autonomous system (AS), in
addition to its real AS. Local AS allows two ISPs to merge without modifying peering arrangements.
Routers in the merged ISP become members of the new autonomous system but continue to use their old
AS numbers for their customers.
Local AScan only be used for true eBGP peers. You cannot use this feature for two peers that are
members of different confederation sub-autonomous systems.
To configure eBGP local AS support, use the following command in neighbor configuration mode:
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.
Command Purpose
local-as number [no-prepend [replace-as
[dual-as]]]
Example:
switch(config-router-neighbor)# local-as
1.1
Configures eBGP to prepend the local AS number
to the AS_PATH attribute.
The local-as number can be a 16-bit integer or a
32-bit integer in the form of a higher 16-bit decimal
number and a lower 16-bit decimal number in xx.xx
format.
The no-prepend keyword ensures that the local-as
number is not prepended to any downstream BGP
neighbors except for the partner who is peering
with the local-as number.
The replace-as keyword ensures that only the
local-as number of the peering session is
prepended to the AS_PATH attribute. The
autonomous-system number from the local BGP
routing process is not prepended.
The dual-as keyword configures the eBGP
neighbor to establish a peering session using the
real autonomous-system number (from the local
BGP routing process) or by using the
autonomous-system number configured as the
Local AS).