[c-nsp] BGP Multihomed Selective/Conditional Advertisement

[Tony]

--- On Sun, 26/10/08, tkacprzynski at SpencerStuart.com <tkacprzynski at SpencerStuart.com> wrote:

> From: tkacprzynski at SpencerStuart.com <tkacprzynski at SpencerStuart.com>
> Subject: RE: [c-nsp] BGP Multihomed Selective/Conditional Advertisement
> To: td_miles at yahoo.com, cisco-nsp at puck.nether.net
> Date: Sunday, 26 October, 2008, 3:54 AM
> I'm not sure if I can have multiple advertise-maps also,
> but can't find any documentation on it either. Does
> anyone else know?
>  
> I tried something similar to what you posted:
>  

Yep, I saw an email with a different subject that had pretty much what I said, sorry about that, I started reading this thread and responded to it before I read the other one.

> neighbor COGENT_NEIGHBOUR_IP advertise-map
> ADVERTISE_WITH_COMMUNITIES exist-map DEFAULT-ATT-PREFIX
> neighbor COGENT_NEIGHBOUR_IP advertise-map ADVERTISE_ALL
> non-exist-map DEFAULT-ATT-PREFIX
> 
> but I was tracking for the same route-map
> DEFAULT-ATT-PREFIX on both, not NON_EXIST_ATT_PREFIX and
> EXIST_ATT_PREFIX. Do you think that could matter?
> 

You should be able to track the same prefix for both if it's valid to have an exist and nonexist map for the same neighbour (which I still don't know if that is valid).


> With the above config it only matched on my first
> advertise-map and keeps it in "withdraw state".
> 
> >The other tip I can give if you try setting something
> like this up is to NOT track the default route from ATT.
> 
>  >Find a prefix that they use on their core network (ie.
> an ATT subnet that if you weren't seeing it would mean
> bad things had happened) and track that one.
> 
> Why do you think a different route be different? The reason
> I ask is that i'm planning on only getting a default
> rotue from ATT. 
> 

When I first tried doing this (but only using nonexist) I couldn't get it to work properly. Link to ISP_A would go down and the router would start advertising subnet to ISP_B then link A would come up a again and sometimes it would fail back, sometimes not.

I opened a TAC case and one of the first things the Cisco guys said "thou shouldst not track thy default route for exist or nonexist advertisements".

You need to track another route that if it ceases to exist means that you link to ATT is cactus. In the scenario when I was doing this, we ended up tracking a /16 that the ISP used on their core national backbone. If that route ceased to exist, then our link to the ISP might still be up, but their network would be screwed big time so we should start advertising to ISP_B.

I have no idea what ATT offer as options for routes to send you, but hopefully they'll offer something like "default + local". Local routes being ones that are originated from within their ASN only.

Once you are getting more than the default route, you just filter out everything except the default & your tracking route anyway, like this:

===
ip prefix-list att-all seq 10 permit 0.0.0.0/0
ip prefix-list att-all seq 20 permit 2.2.0.0/16
!
! you need to substitute 2.2.0.0/16 for the ATT route you're tracking
!
route-map from-att permit 10
 match ip address prefix-list att-all
!
neighbor ATT_NEIGHBOR_IP route-map from-att in
===

If you're concerned about bogging your router down, I wouldn't be. I've done this on both a 1751 & 1861 router where the number of routes received from the ISP when I asked for "default + local" was over 14,000. It just chucks out all the others and only puts the two (default + tracking route) in the route table.


>  
> 
> Another thing I tried was 
> 
> neighbor COGENT_NEIGHBOUR_IP advertise-map
> ADVERTISE_WITH_COMMUNITIES exist-map DEFAULT-ATT-PREFIX
> neighbor COGENT_NEIGHBOUR_IP route-map ADVERTISE_ALL out
> 
> So the idea was to advertise the route-map once
> advertise-map withdraws the ADVERTISE_WITH_COMMUNITIES.
> Coudn't get that to work either.
> 
> Does anyone know where advertise-map fits with the order of
> exporting routes?
> 

I have no idea and I can't find any good doco on using advertise maps with tracking maps. All I can find is command reference stuff, which doesn't give any real idea on how you can/can't use the stuff.