From: James (james@towardex.com)
Date: Wed Sep 15 2004 - 13:30:18 GMT-3
On Wed, Sep 15, 2004 at 11:43:52AM -0400, Jongsoo.Kim@Intelsat.com wrote:
> James
>
> BGP protocol will choose which next hop to use for a BGP route.
> But still it is IGP's decision which interface to use to reach the next hop.
>
> Nexthop 10.90.90.1 for 2.2.2.0/29 is selected on R6 by iBGP process learned
> from R1.
> However, which interface R6 to use to reach 10.90.90.1 is based on IGP
> process ( OSPF in this case).
> So R6 will forward any packet of 2.2.2.0/29 to R4 via F/R interface.
>
> In the same way, R4 learned 2.2.2.0/29 from two eBGP connections from R1 and
> R6.
> But it chooses R6's peer address as nexthop as you said higher IP address
> (unless bgp loadbalance is enable.)
> To determine which interface to use to reach 10.6.6.6, R4 will check IGP and
> forward to F/R interface going to R6.
>
> If you make bgp nexthop unchanged on R1 so that R4 choose 10.90.90.1 as
> nexthop for 2.2.2.0/29, then still R4 will rely on IGP to select the proper
> interface to forward.
Yes, being a programmer myself writing own BGPd implementation, I am familiar
with how BGP works to select best path ;-)
I think it was more of confusion between us in previous email exchanges.
I may have misunderstood what you were saying in the first email..
Anyhow, 10.90.x.x is clearly closer to reach via R1 in R4's perspective at
least if your setup your IGP as the book requested. IGP may reroute, but as
you say, things like that in lab is probably rare.
Thanks,
-J
-- James Jun TowardEX Technologies, Inc. Technical Lead Network Design, Consulting, IT Outsourcing james@towardex.com Boston-based Colocation & Bandwidth Services cell: 1(978)-394-2867 web: http://www.towardex.com , noc: www.twdx.net
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