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16-6 Riverstone Networks RS Switch Router User Guide Release 8.0
Basic BGP Tasks BGP Configuration Guide
To export all active routes from 284 or 813 or 814 or 815 or 816 or 3369 or 3561 to autonomous system 64800.
16.2.7 Using the AS Path Prepend Feature
When BGP compares two advertisements of the same prefix that have differing AS paths, the default action is to prefer
the path with the lowest number of transit AS hops; in other words, the preference is for the shorter AS path length.
The AS path prepend feature is a way to manipulate AS path attributes to influence downstream route selection. AS
path prepend involves inserting the originating AS into the beginning of the AS prior to announcing the route to the
exterior neighbor.
Lengthening the AS path makes the path less desirable than would otherwise be the case. However, this method of
influencing downstream path selection is feasible only when comparing prefixes of the same length because an
instance of a more specific prefix always is preferable.
On the RS, the number of instances of an AS that are put in the route advertisement is controlled by the
as-count
option of the
bgp set peer-host
command.
The following is an example:
Notes on Using the AS Path Prepend Feature
Use the
as-count
option for external peer-hosts only.
If the
as-count
option is entered for an active BGP session, routes will not be resent to reflect the new setting. To
have routes reflect the new setting, you must restart the peer session. To do this:
1.
Enter Configure mode.
2.
Negate the command that adds the peer-host to the peer-group. (If this causes the number of
peer-hosts in the peer-group to drop to zero, then you must also negate the command that creates the
peer group.)
ip-router policy create aspath-regular-expression someAspath
".*(284|813|814|815|816|3369|3561) .*"
ip-router policy create bgp-export-destination to-64800 autonomous-system 64800
ip-router policy create aspath-export-source allRoutes aspath-regular-expression
someAspath origin any protocol all
ip-router policy export destination to-64800 source allRoutes network all
#
# insert two instances of the AS when advertising the route to this peer
#
bgp set peer-host 194.178.244.33 group nlnet as-count 2
#
# insert three instances of the AS when advertising the route to this
# peer
#
bgp set peer-host 194.109.86.5 group webnet as-count 3