[c-nsp] eBGP via loopback

[Ibrahim Abo Zaid]

also i want to draw attention that when establishing eBGP over loopback it
will need ebgp multihop to be configured as well because the default TTL
over directly connected interfaces is 1 and in case of loopback it is
reachable over just single hop

some IOS version defaults multihop to 255 so the neighbor loopback can be
reachable by any route not just the directly connected link which causes
sub-optimal or routing loops


so you need to configure ebgp multihop x where x is the actual IP hops to
the neighbor


best regards
--Ibrahim

On Fri, Jun 13, 2008 at 6:07 PM, Steve Bertrand <steve at ibctech.ca> wrote:

> Aaron wrote:
>
>> Did you setup ebgp multihop since you are doing peering to the loopbacks?
>>
>
> Yes.
>
>  Curious on why you would want to use the loopback instead of the interface
>> for ebgp. Definitely not the recommended way unless you are trying to load
>> balance on multiple links.
>>
>
> Here is my (slightly edited) response to someone else who emailed me
> off-list:
>
> To be honest, the loopback over eBGP was not an intended design goal. It
> just so happened that I had this particular router in an iBGP mesh (lab
> environment), and realized I wanted to push the router to a remote location
> and gain practical experience on how to conceptually implement a no-export
> community for a private ASN.
>
> I overlooked the loopback addresses when I put the router in its own AS,
> due to the fact the PtP addresses did not need to change. I just threw in
> the ebgp-multihop blindingly instead of renumbering the neighbors and the
> update-source.
>
> Most likely, I would not have even payed any attention to the configuration
> until later review if I hadn't have had the packet loss problem.
>
> ---
>
> For the sake of completeness, the router that was swallowing the packets
>  is running Quagga on FreeBSD. I had forgotten to set the ip.forwarding
> sysctl variable to true.
>
>
> Steve
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