From: Rick Tyrell (rtyrell@gmail.com)
Date: Tue Feb 17 2009 - 12:25:36 ARST
That is a great email, I never thought of it that way. Thank you.
-Rick
On Tue, Feb 17, 2009 at 7:45 AM, Marko Milivojevic <markom@markom.info>wrote:
> > Because Ebgp TTL is 1 by default but for IBGP TTL is 255 by default
> ...
> > That's the way the RFC was written. It assumes EBGP peering will be done
> on
> > a directly connected physical link to a neighbor.
>
> While these are surely the technical reasons, which explain the
> behaviour, they do nothing to help understand the deeper reason behind
> it.
>
> The reason lies in the very fact that BGP is not really a routing
> protocol. It's a technical tool to implement non-technical decisions.
>
> You must have noticed in the process of learning about BGP that most
> of the tasks are pretty arbitrary, like "send traffic for X via Y" and
> how easy it is to implement them. For example, in real life, sometimes
> "the best path" is "the most expensive one" and we are supposed to use
> it only when we must, not all the time... which would be the case with
> most IGP's.
>
> The reason for different behaviour in eBGP and iBGP is simple - the
> primary function of BGP is to exchange information _between_
> autonomous systems. The "routing" within AS is left for IGP.
>
> When BGP speaks to another AS, it assumes that it's the only source of
> routing information and it requires the other neighbor to be directly
> connected for this particular reason.
>
> On the other hand, when peering with internal neighbor, BGP assumes
> that it's not the sole source of routing information and it accepts
> the existence of IGP that will take care of intra-AS transport. In the
> wonderful world of SP networks, this combined with MPLS transport
> allows for great scalability and interesting solutions (BGP-free core,
> etc.).
>
> --
> Marko
> CCIE #18427 (SP)
>
>
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>
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