BGP multi-homed (default routes + full routes)

[Josiah Inubio]

Hi Everyone,

Is it possible and advisable to advertised/received default routes and full routes on a multi-homed bgp setup? 

Thanks

[Masoud Pourshabanian]

Hello,

It is completely depending on what you are going to do.

Advertising Full BGP routes is not suggested if you are an end customer. it may cause your AS become a transit AS. Costumers usually only advertises their own IP addresses to avoid that.

If you are going to implement full load balancing across both links, you need to receive full BGP routes. However, receiving full BGP routes is very intensive in terms of memory and processing. It will be also very complex  so troubleshooting is difficult.

Some ISPs gives you an option to receive a default route and partial routes. You can consider one of your link as a primary link and other as backup. Then you can send traffic of partial routes through your backup link and other traffic through your primary link by implementing policy.

Load balancing is not going to be even but your primary link probably would have higher bandwidth than your backup.

Hope it helps,

Masoud

[Saurabh Gera]

Yep, I agree with Masoud it will be Load Sharing.

** Do Not Advertise Full Routing Table **, Just advertise your Public Routes and make sure to put an ACL on your public Interface.

If you are going to implement full load balancing across both links, you need to receive full BGP routes. However, receiving full BGP routes is very intensive in terms of memory and processing. It will be also very complex  so troubleshooting is difficult. --> Check your IPOP Hardware/Router's Capacity.

You might end up doing asymmetric routing too, I will suggest use primary and back up, unless you are highly skilled.

Regards,

Saurabh Gera

[Joseph W. Doherty]

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Posting

Yes it's possible, advisable is debatable.

When you take full routes, you're routers than have the option of using the "best" path to any destination, but as BGP (by default) only considers the better path as having the fewest ASs to cross, the "best" path might not actually be the "best" path.

As Masoud also notes, you can take (or just only accept) partial routes.  For example, you might use a default and take prefixes that only come from your ISP AS or just one AS hop from it.

If your routers support PfR, it can select the actual best performing path, and it can work with just using defaults to using full Internet tables.  I found PfR works very well with defaults.