[c-nsp] BGP Multihomed Selective/Conditional Advertisement

[Tony]

I'm not sure if you can have multiple "advertise-map" for one BGP neighbour ?

If you can, then you could have this kind of setup:


neighbor COGENT_NEIGHBOUR_IP advertise-map ADVERTISE_WITH_COMMUNITIES exist-map EXIST_ATT_PREFIX
neighbor COGENT_NEIGHBOUR_IP advertise-map ADVERTISE_ALL non-exist-map NON_EXIST_ATT_PREFIX


I've only set this up for conditional advertisement when a prefix DID NOT exist, so I don't know if you can do both at the same kind and I can't find doco to say either way.

There might be some permutation or variation of this command (or in combination with something else) that may help you achieve what you need to.

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.


regards,
Tony.

--- On Sat, 25/10/08, tkacprzynski at SpencerStuart.com <tkacprzynski at SpencerStuart.com> wrote:

> From: tkacprzynski at SpencerStuart.com <tkacprzynski at SpencerStuart.com>
> Subject: [c-nsp] BGP Multihomed Selective/Conditional Advertisement
> To: cisco-nsp at puck.nether.net
> Date: Saturday, 25 October, 2008, 8:07 AM
> I have been trying to figure out how to do this and maybe
> someone will be able to help me out.
>  
> I have two ISP connections ISP ATT and ISP Cogent. 
>  
> (ISP Cogent)    (ISP ATT)
>      |                       |
>   RO ------------------- R1
>  
>  
> ATT would be used for primarily internet and access to our
> webservers.
>  
> Cogent would be primarily used to access Cognet's
> network that use VPN for incoming connections only. I do not
> want to have other networks besides Cogent's network
> using this path to access our webserver.
>  
> I would like to have each other act as a backup for one
> another. For instance if ATT fails I want everyone on the
> internet use Cogent to access me. If Cogent fails I want
> everyone on the internet and the VPN connections on
> Cogent's network to use ATT.
>  
> So basically what I was thinking to setup is to accept a
> default router from ATT and Cogent. Lower the local
> preference of Cogent and that way I would accomplish using
> ATT as primary internet access.
>  
> The tricky part is with Cogent and using then to only
> access their local networks. Looking through communities I
> found out Cogent's communities that would not export my
> route to their peers and keep it internal within their AS.
> This works fine but the problem now is how do I failover if
> ATT fails? How do I automatically change my not-export
> community I'm sending to Cogent to start adverting the
> route to its peers?
>  
> I looked at conditional advertisement, I was able to
> basically send the route map with not-export communities to
> Cogent if the default route from ATT is present. The problem
> with this is that once the default router disappears it
> doesn't advertise anything to Cogent, none of my routes
> are advertised to Cogent.
>  
> I'm not sure if I could do this sort of a double
> condition such as 
>  
> if ATT's default route is present send out to Cogent a
> route map with prefixes to not-export my routes
> if ATT's default route is not present sent to Cogent a
> route map without any communities on my routes
>  
> Basically I'm trying to figure out how I can have
> multihoming, but with the constrains that I want 1 ISP to be
> used for internet and the other to only access their AS, but
> still have the capability to automatically failover in case
> one of the circuits dies.
>  
> Thank you for any input or help.
>  
>  
> Tom Kacprzyński
> Network Engineer
>  
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