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North American Network Operators Group

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Re: iBGP next hop and multi-access media

  • From: John M. Brown
  • Date: Tue Oct 08 10:08:36 2002

Please do NOT confuse PHYSICAL plumbing with LOGICAL plumbing.

Based on your description, router A and B   ARE NOT  on the same
broadcast domain, with respect to 172.16.16/24.

THey are on the same broadcast domain as 10.10.10.0/30

But thats it.


In otherwords, No it is NOT technically possible for B to tell
A that what it wants is on the same media.

Welcome to one of the few down falls of loading up multiple nets
on the same physical interface.


On Sun, Oct 06, 2002 at 04:25:00PM -0400, Ralph Doncaster wrote:
> 
> A and B are connected via the same multi-access media.  It is technically
> possible for B to tell A "you can reach 172.16.16.0/24 on the same media
> that you receive this update on".  However what people seem to be saying
> is that there is no dynamic routing protocol that implements this.
> 
> Ralph Doncaster
> principal, IStop.com 
> 
> On Sun, 6 Oct 2002, Stephen J. Wilcox wrote:
> 
> > 
> > I dont understand this..
> > 
> > A wants to get to a network which it [thinks it] is not connected to, the only
> > route is via B. therefore you must advertise the route from B with next hop B
> > 
> > the only possible way (at least in ethernet IP) that A can send direct onto the
> > ethernet segment is if it is connected to the other (172.16) network and if
> > youre not willing to do that then your solution is not possible
> > 
> > Steve
> > 
> > On Sun, 6 Oct 2002, Ralph Doncaster wrote:
> > 
> > > 
> > > Background: 
> > > Router A and B are connected via a common ethernet segment 1. Router A
> > > uses 10.10.10.1/30, and Router B uses 10.10.10.2/30. Router B also has
> > > another subnet configured for ethernet segment 1; 172.16.16.0/24.
> > > 
> > > When I setup a situation like the above, with Router B advertising the
> > > 172.16.16.0/24 to router A, router A sees a next hop of 10.10.10.2.  This
> > > is not good since packets from A going to the 172.16.16 subnet get sent to
> > > Router B, which then ARPs the desitnation, instead of just being ARPed by
> > > router A.
> > > 
> > > I don't want to turn on ICMP redirects on B since they're insecure and
> > > ugly.  I've also made sure I'm not using next-hop self.  Is there a way to
> > > make this work?
> > > 
> > > Ralph Doncaster
> > > principal, IStop.com 
> > > 
> > > 
> > > 
> > 
> > 
> 




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