ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Policy-based Routing for BGP
– 1907 –
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.
◆ 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 boundary, then the AS number of the
confederation member inside of the AS_PATHLIMIT attribute should be
replaced by the confederation's AS number.
EXAMPLE
Console(config)#route-map RD permit 8
Console(config-route-map)#match pathlimit as 5
Console(config-route-map)#on match goto 20
Console(config-route-map)#
match peer This command sets the peer address to match in routing messages. Use
the no form to remove this entry from a route map.
SYNTAX
match peer {peer-address | local}
no match peer [peer-address | local]
peer-address – IP address of neighboring router sending routing
messages.
local – Static or redistributed routes.
COMMAND MODE
Route Map