There seems to be one major advantage of using loopback addresses for
OSPF-the highest loopback will be chosen for the RID and it stays up as long
as the router is up and you can use any address you want. There are other
advantages but not as significant as this.
If you use a loopback address which is a legal IP address, and treat it like
any other legal interface, you can ping and route to that interface from
anywhere. If you use an unpublished address, you cannot ping or route to the
interface from elsewhere.
In the case of virtual links, it is advisable to use any physical interface
IP address on the router. This allows the routers on either end of the link
to find each other. It does not have to be the loopback interface. In fact
it should not be the loopback interface unless you are using a legal router
address for the loopback.
Bottom Line: If you have addresses to spare, use a legal address for the
loopback.
If you do not have addresses to spare use an "unpublished"
address. Do not advertise or distribute this address. Only the router on
which it is configured should have it as a route(and RID). All others will
see it only as an RID.
For virtual links, any interface address configured on the
router would do the trick. The OSPF routers configured with the statement
know what to do with the packets regardless of which interface it comes in
on. Of course it may be better to choose the interfaces which are closest to
each other.
This does not cover every possible scenario, but in the special case of
virtual links, it is not necessary to do static or advertise the loopback.
Winston.
-----Original Message-----
From: Keith Townsend [mailto:[EMAIL PROTECTED]]
Sent: Thursday, November 09, 2000 8:01 PM
To: [EMAIL PROTECTED]
Subject: OSPF Area virtual links
When creating a virtual link between an ABR non-directly connected are to an
ABR in the backbone should you always use the loopback address for the
virtual link. Doyle does this on page 943 of his Routing TCP/IP book.
Secondly is there a route to the loopback of these two routers. Maybe I'm
forgetting something but how do the two routers find routes to each other if
you're not advertising L0 and there is no static routes to the L0.
Thanks,
Keith
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