Information Technology Reference
In-Depth Information
Configurations
Conditional advertisement has two forms: advertisement of some prefixes when some other
prefixes do not exist and advertisement of some prefixes when they do exist. The prefixes
to be advertised are defined by a special route map called
advertise-map
. The condition is
defined by a route map called
non-exist-map
for conditions that do not exist or by a route
map called
exist-map
for conditions that do exist.
The first form of conditional advertisement is configured as follows:
neighbor advertise-map map1 non-exist-map map2
The route map associated with the non-exist-map specifies the prefix (or prefixes) that the
BGP speaker tracks. Only permit is accepted; any deny is ignored. When a match is made,
the status of the advertise-map is Withdraw; when no match is made, the status becomes
Advertise.
Within the non-exist-map, a match statement for the prefix is required. You can configure it
with a prefix list or a standard access list. Only an exact match is supported. Additionally,
AS_PATH and community can be matched.
The route map associated with the advertise-map defines the prefix (or prefixes) that are
advertised to the specific neighbor when the prefixes in the non-exist-map no longer exist—
that is, when the status is Advertise. When the status is Withdraw, the prefix or prefixes
defined in the advertise-map are not advertised or withdrawn. Note that the advertise-map
applies only on the outbound direction, which is in addition to the other outbound filters.
The second form of conditional advertisement is configured as follows:
neighbor advertise-map map1 exist-map map2
In this case, the route map associated with the exist-map specifies the prefix (or prefixes)
that the BGP speaker tracks. The status is Advertise when the match is positive—that is,
when the tracked prefix exists. The status is Withdraw if the tracked prefix does not exist.
The route map associated with the advertise-map defines the prefix (or prefixes) that are
advertised to the specific neighbor when the prefix in the exist-map exists. Prefixes in both
route maps must exist in the local BGP RIB.
Examples
Figure 4-2 shows a topology of a conditional advertisement that tracks the nonexistence
of a prefix. AS 100 is multihomed to AS 200 and AS 300, with the link to AS 300 as the
primary connection. The address block of AS 100 is assigned from AS 300, within the range
of 172.16.0.0/16. The address block 172.16.1.0/24 is not to be advertised to AS 200 unless
the link to AS 300 fails. AS 300 sends 172.16.2.0/24 to AS 100, and it is tracked by the non-
exist-map on R1. Example 4-14 shows R1's BGP configuration. Note that the community
100:300 is set and matched for the prefix to be tracked to ensure that the prefix is indeed
from AS 300.
