One or two comments inset.
Chuck's Text
>>I would venture a guess that the BDR
>>would be promoted because even though there is an alternative route to the
>>DR loopback, hellos go only to adjacent routers, and the DR is no longer
>>adjacent.
>
>Well, I proved my point. Under this scenario, when I unplug the DR ethernet
>port, the BDR becomes the DR.
>
>1) I am correct that in the case of the ethernet DR becoming disabled, the
>BDR still becomes the DR.
>
>
>Conclusion:
>
>With regards to OSPF, at least, the idea that the loopback interface
>provides any kind of reliability is in and of itself not true. Problems
>arise with the advertising of the loopback throughout the OSPF domain,
>particularly when the area 0 connection is lost, and the alternate routes do
>not propogate.
>
>This of course is not an issue with IGRP or EIGRP or even RIP, which do not
>have the same restriction with regards to routing behaviours. So while in
>OSPF the existence of a loopback interface might prove to be of no use, and
>probably more often than one might care to imagine, it still will prove
>quite useful within other routing protocols.
A couple quick notes. The resiliency that using a loopback interface on an ospf
router has little to do with individual segments. Naturally, if you lose an interface
into a multi access segment you will lose your adjacencies on that segment.
However, let consider a router with 4 interfaces. Without using a loopback interface,
if the interface with the highest IP address dies, the routers RID changes. What this
means is that not only does the router lose its adjacencies on the defective
interface, it also must changes its RID to the next highest address. This will force
it to lose and rebuild all other adjacencies and as far as I know, for every router in
the area to recalculate their SPF tree's. Using a loopback address, in the same
situation, this failure would be contained to the one segment.
Pete
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