[c-nsp] OSPF design (danger will)

[Benny Amorsen]

William Cooper <wcooper02 at gmail.com> writes:

> On Mon, Oct 25, 2010 at 4:07 PM, Benny Amorsen <benny+usenet at amorsen.dk> wrote:

>> Actually it does, in some cases. BGP cannot maintain 2 links to the same
>> neighbour, and so it does not work if you have redundant links (except
>> for LACP links and similar). That is when you need OSPF so you can peer
>> on the loopback addresses.

> Doesn't multi-path fulfill this requirement?

No multipath is a way to install multiple routes into the FIB. That is
all well and good but it is an entirely separate problem.

BGP cannot maintain two sessions to the same neighbour. Imagine router A
having two ethernet links to router B, with router A having addresses
1.1.1.1/24 and 2.2.2.1/24, and router B having addresses 1.1.1.2/24 and
2.2.2.2/24. Then you could set up two BGP neighbours on router A,
1.1.1.2 and 2.2.2.2. However, the second session won't work, because it
has the same router ID as the first session.

Hence why you need to add 3.3.3.1/32 as loopback on router A, 3.3.3.2/32
on router B, run OSPF to get the correct redundant routing of the
loopbacks, and peer on 3.3.3.x.


/Benny