Chapter 10: BGP Configuration Guide
110
DIGITAL GIGAswitch/Router User Reference Manual
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 GSR, 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:
a.
Enter Configure mode.
b. 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.)
c.
Exit Configure mode.
d. Re-enter Configure mode.
#
# 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
