Re: BGP over satellite link

[Brian Candler <B.Candler at pobox.com>]

On Sat, Apr 05, 2003 at 04:04:28AM -0800, Joe Joe wrote:
> 
>    Hello,
> 
>    I'm a newbie on this list. 

Welcome!

>    I need to run BGP with my upstream provider
>    because I will be multihomed to another provider, since we are not
>    directly connected, how do I go about the config.

How exactly do you mean "we are not directly connected?" Normally you would
have a border router which connects to the leased line or satellite link,
and at the other end of the link is your provider's border router. If this
is not what you are intending to have, can you draw an ASCII diagram?

>    Secondly, am I to
>    use my router interface IP address or the loopback address to run the
>    BGP session with my upstream.

Well, this is by agreement with your provider. Normally it makes sense to
use your router interface IP (typically assigned by your provider as a /30
subnet). This is so that if the link fails, the BGP keepalives will fail to
get through. BGP will thus notice the failed link, withdraw the routes, and
your traffic will get rerouted via your other provider. You don't want the
BGP session between yourself and provider A to be re-routed via provider B's
link!

However, if I remember correctly, BGP sets an IP TTL of 1 on its packets
anyway by default, so it should work even if you use a loopback address. In
that case it will try to reroute the BGP packets via provider B, but because
provider A's router is more than one hop away when reached via provider B,
the BGP session between you and provider A will still fail when the link
goes down.

It can be preferential to use a loopback address in some cases, for example
if you are running two 2M links in parallel to the same far-end router to
simulate a 4M link:

              serial0   2M
                ,----------------.
             you                  provider
                `----------------'
              serial1   2M

In this case, you do want the BGP session to stay up even if one of the two
2M lines goes down, so you run it between loopbacks, with static routes to
the loopback address of the far-end router. If both 2M lines are up, the
routers will round-robin the traffic between them.

Regards,

Brian.

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