Friday, February 26, 2010

Advertise maps in BGP

A recent discussion on groupstudy pushed me into labbing BGP again.
In BGP, Advertise maps are used for two functions;
1. Conditional Advertisement
2. Route aggregation.

In conditional advertisement, Advertise maps are with an EXIST-MAP (or NON-EXIST map) to perform conditonal advertisement. Here the advertise-map specifies a route-map that matches the prefixes that would be advertised ONLY if the prefixes in the EXIST-MAP exist in the routing table.

The syntax is "neighbor ip-address advertise-map map-name {exist-map|non-exist} map-name"

The other use of advertise-maps is in specifying what attribute would be carried along in the as-set attributes of an aggregate during summarization.

Assume we have R1, R2,R3 and R4 in AS 1, 2,3 and 4 respectively
R1 -- R4 --- R3

R1, R2, and R3 advertise 150.1.x.0/24 into bgp where x is the router number.

R4 aggregates the routes to 150.1..0.0/16 with as-set attribute.
By default, none of the routers get the update anymore since their
individual routes are a part of the summary.

Using advertise map, we want to make R1 and R3 get the summary; so we only advertise the attributes of the prefix form R2 with the summary.

Using as path access-lists and route-maps on R4 we have,

ip as-path access-list 1 permit ^2$
route-map adv permit 10
match as-path 1

router bgp 4
aggregate-address as-set summary-only advertise-map adv

Now, R1 and R3 get the summary, R2 doesn't because its AS number is carried along with the summary

R1(config-router)#do sh ip bg | i
*> 0 0 4 2 i

R2(config-router)#do sh ip bg | i

Ok. That's it for now. Back to security :-) I was trying to look into NAC with the CTA and CSA. Fun stuff :D



  1. Why I really don't like BGP is what I really don't get. How did you go about it though?

  2. Why I really don't like BGP is what I really don't get. How did you go about it though?

  3. A good place to start is routing TCP/IP volume I and 2.