Chapter 50
| IP Routing Commands
Policy-based Routing for BGP
– 1162 –
Example
Console(config)#route-map RD permit 8
Console(config-route-map)#match origin igp
Console(config-route-map)#set weight 30
Console(config-route-map)#
match pathlimit as This command sets the maximum AS path length allowed for propagation of more
specific prefixes to match in routing messages. Use the no form to remove this
entry from a route map.
Syntax
match pathlimit as as-limit
no match pathlimit as
as-limit – Maximum AS path length. (Range: 1-4294967295)
Command Mode
Route Map
Command Usage
â—† To perform inter-domain traffic engineering, a multi-homed site can advertise
its prefix to all of its neighbors via an aggregate address, and also advertise
more specific prefixes to a subset of its neighbors. The longest match lookup
algorithm then causes traffic for the more specific prefixes to be forwarded to
the subset of neighbors with the more specific prefix.
These longer prefixes may be advertised in addition to an aggregate, even
when the aggregate advertisement is sufficient for basic reachability. This type
of inter-domain traffic engineering is a widely used phenomenon that is
contributing to growth in the size of the global routing table.
Traffic engineering via longer prefixes is only effective when the longer prefixes
have a different next hop from the less specific prefix. Thus, past the point
where the next hops become identical, the longer prefixes provide no value
whatsoever. This command can be used to limit the radius of propagation of
more specific prefixes by adding a count of the ASes that may be traversed by
the more specific prefix.
â—† Private AS numbers [RFC1930] and confederation AS members [RFC3065]
found in the AS_PATH are not counted. AS numbers found within an AS_SET are
not counted and an entire AS_SET is counted as a single AS. Each instance of an
AS number that appears multiple times in an AS_PATH is counted.
If the AS_PATHLIMIT attribute is attached to a prefix by a private AS, then when
the prefix is advertised outside of the parent AS, the AS number contained in
the AS_PATHLIMIT attribute should be replaced by the AS number of the parent
AS.
Similarly, if the AS_PATHLIMIT attribute is attached to a prefix by a member of a
confederation, then when the prefix is advertised outside of the confederation
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