Remove And Replace Private As Numbers From As Path In Bgp; Bgp Update Message Error Handling - Cisco NCS 5500 Series Configuration Manual

Information about Implementing BGP

Remove and Replace Private AS Numbers from AS Path in BGP

Private autonomous system numbers (ASNs) are used by Internet Service Providers (ISPs) and customer
networks to conserve globally unique AS numbers. Private AS numbers cannot be used to access the global
Internet because they are not unique. AS numbers appear in eBGP AS paths in routing updates. Removing
private ASNs from the AS path is necessary if you have been using private ASNs and you want to access the
global Internet.
Public AS numbers are assigned by InterNIC and are globally unique. They range from 1 to 64511. Private
AS numbers are used to conserve globally unique AS numbers, and they range from 64512 to 65535. Private
AS numbers cannot be leaked to a global BGP routing table because they are not unique, and BGP best path
calculations require unique AS numbers. Therefore, it might be necessary to remove private AS numbers from
an AS path before the routes are propagated to a BGP peer.
External BGP (eBGP) requires that globally unique AS numbers be used when routing to the global Internet.
Using private AS numbers (which are not unique) would prevent access to the global Internet. The remove
and replace private AS Numbers from AS Path in BGP feature allows routers that belong to a private AS to
access the global Internet. A network administrator configures the routers to remove private AS numbers from
the AS path contained in outgoing update messages and optionally, to replace those numbers with the ASN
of the local router, so that the AS Path length remains unchanged.
The ability to remove and replace private AS numbers from the AS Path is implemented in the following
ways:
• The remove-private-as command removes private AS numbers from the AS path even if the path
contains both public and private ASNs.
• The remove-private-as command removes private AS numbers even if the AS path contains only private
AS numbers. There is no likelihood of a 0-length AS path because this command can be applied to eBGP
peers only, in which case the AS number of the local router is appended to the AS path.
• The remove-private-as command removes private AS numbers even if the private ASNs appear before
the confederation segments in the AS path.
• The replace-as command replaces the private AS numbers being removed from the path with the local
AS number, thereby retaining the same AS path length.
The feature can be applied to neighbors per address family (address family configuration mode). Therefore,
you can apply the feature for a neighbor in one address family and not on another, affecting update messages
on the outbound side for only the address family for which the feature is configured.
Use show bgp neighbors and show bgp update-group commands to verify that the that private AS numbers
were removed or replaced.

BGP Update Message Error Handling

The BGP UPDATE message error handling changes BGP behavior in handling error UPDATE messages to
avoid session reset. Based on the approach described in IETF IDR I-D:draft-ietf-idr-error-handling, the Cisco
IOS XR BGP UPDATE Message Error handling implementation classifies BGP update errors into various
categories based on factors such as, severity, likelihood of occurrence of UPDATE errors, or type of attributes.
Errors encountered in each category are handled according to the draft. Session reset will be avoided as much
as possible during the error handling process. Error handling for some of the categories are controlled by
configuration commands to enable or disable the default behavior.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
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Implementing BGP