BGP - full internet routes vs default route

Answered Question
Mar 16th, 2009

Hello,

Given 2 x BGP routers dual-homed to 2 x different ISPs running in active/standby. Should I be getting the full internet routes from the ISPs or not?

I guess in my configuration the 'default routes' should be more then enough compared with 'full internet routes'.

I thought I heard that I should be running 'full internet routes' only if you're transit AS.

What do you think?

Thanks

Alex

I have this problem too.
0 votes
Correct Answer by JamesLuther about 7 years 8 months ago

Hi,

If you are running active/passive then default routes are sufficient.

However it's not true to say that full internet routes are only used if you're a transit AS. If you wished to use both of your links then you could get full internet routes from two different ISPs and BGP would prefer one ISP for some traffic and the other for ISP for other traffic.

Of course you don't want to be a transit AS betwen the ISPs so you would also use a filter list outbound.

Regards

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Correct Answer
JamesLuther Mon, 03/16/2009 - 01:52

Hi,

If you are running active/passive then default routes are sufficient.

However it's not true to say that full internet routes are only used if you're a transit AS. If you wished to use both of your links then you could get full internet routes from two different ISPs and BGP would prefer one ISP for some traffic and the other for ISP for other traffic.

Of course you don't want to be a transit AS betwen the ISPs so you would also use a filter list outbound.

Regards

aberdan Mon, 03/16/2009 - 02:46

Jamees,

Thanks for you answer. I think I could still do load balance the traffic even if I get the 0/0 but not in a very optimal way (some dst could be closer from a specific ISP).

Regards,

Alex

Joseph W. Doherty Mon, 03/16/2009 - 13:02

If you want optimal flow performance (easiest outbound), with or without full BGP routes, OER/PfR is an interesting Cisco technology.

aberdan Mon, 03/16/2009 - 13:13

Thanks Joseph. Your option could be considered as well but additional resources are required and I still think you need to see the prefixes in that case to decided what path is 'better' (which ISP).

Thanks all for getting me in the right direction.

rgds,

Alex

Joseph W. Doherty Mon, 03/16/2009 - 14:22

Yes, additional resources are required to run OER or PfR, but depending on configuration, perhaps not as much as processing full multiple Internet BGP tables.

As to seeing prefixes to see 'better' path, remember all BGP sees as best is shortest AS path (excluding network engineering). What OER/PfR can see, is actual, at the moment, flow performance. A longer AS path might perform much, much better than a shorter path (more available bandwidth, less latency, less congestion, etc). Cisco's new name, Performance Routing, is well selected.

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