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BGP Advertise MAP

Hello,

I'm trying to wrap my head around the "neighbor x.x.x.x advertise-map" command.

So the gist of what I understand is it has two route-maps RM1 and RM2.

RM1 consists of networks you want to advertise.

RM2 consists of networks your watching for in your BGP table.

If the routes in route-map RM2 dissappear out of the BGP table you then start advertising the routes in route-map RM1. If the routes in RM2 reappear you stop advertising the routes in route-map RM1.

So if anyone wants to verify that this is how advertise map works I would appreciate it. My heads starting to explode with BGP and I'm second guessing myself everywhere that I'm right.

Patrick

I'm sure I could put this in the certifications training section but seems like those forums aren't checked as often.

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Re: BGP Advertise MAP

A few more things:

A good way to verify this is by using the 'show ip bgp neighbors x.x.x.x' command. It displays a line starting with: "Condition-map....". If the prefixes are not being advertised, it will display a status of 'Withdraw', if they are, it will display 'Advertise'.

I've found a couple of other things (the hard way) about conditional advertisement:

- when you use a prefix-list to match on the tracked prefixes, it has to use an exact match i.e. you cannot use the 'le' or 'ge' keywords

- you have to use a prefix-list to match on the tracked prefixes i.,e you cannot just match on something like a community

Hope that helps, dude...

Paresh

4 REPLIES
Purple

Re: BGP Advertise MAP

Hi Patrick,

Your understanding is pretty much correct. The advertise-map matches on the routes you want to advertise and the 'other' map watches prefixes in your BGP table. The thing to be aware of is that there are two types of the 'other' map. You can have either an exist-map or a non-exist-map.

Depending on the form you use (exist or non-exist), the prefixes matched by the advertise-map are advertised if:

- the tracked prefix is not present (in case of a non-exist-map)

- the tracked prefix is present (in case of a exist-map)

That probably does not help in making it less complicated for you but that's what it is :-)

Paresh.

Purple

Re: BGP Advertise MAP

A few more things:

A good way to verify this is by using the 'show ip bgp neighbors x.x.x.x' command. It displays a line starting with: "Condition-map....". If the prefixes are not being advertised, it will display a status of 'Withdraw', if they are, it will display 'Advertise'.

I've found a couple of other things (the hard way) about conditional advertisement:

- when you use a prefix-list to match on the tracked prefixes, it has to use an exact match i.e. you cannot use the 'le' or 'ge' keywords

- you have to use a prefix-list to match on the tracked prefixes i.,e you cannot just match on something like a community

Hope that helps, dude...

Paresh

Re: BGP Advertise MAP

Paresh,

Appreciate it I was haveing some problems with both the two types of maps. It ended up being a problem with the first map that I wanted to advertise. The routes weren't actually in my BGP tables and I needed to put them there.

Patrick

Cisco Employee

Re: BGP Advertise MAP

There is acttually two forms of this command.

R1(config-router)#nei 2.2.2.2 advertise-map test ?

exist-map advertise prefix only if prefix is in the condition exists

non-exist-map advertise prefix only if prefix in the condition does not

exist

The first one works with an exist-map and will only advertise prefixes specified in RM1 based on the existence of prefixes in RM2 in the BGP table.

The second form works with a non-exist-map and behaves as you explained, which is that prefixes configured in RM1 are only advertised based on the non-existence of RM2 prefixes in the bgp table.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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