From: Nigel Taylor (nigel_taylor@xxxxxxxxxxx)
Date: Sun Dec 24 2000 - 17:08:57 GMT-3
Chuck,
You raise a good point.. I was looking at a couple of scenarios where the
loopback doesn't
allow for any real advantages. I've come to believe that the roll the
loopback plays in much
of the network is from a design standpoint. In making this a basic design
practice it provides
the operational engineer(s) with a more deterministic approach to
trouble-shooting. Also, within various designs which is becoming more
common-placed is backup paths/Dual paths. Case in point,
is the use of "ip ospf priority X" command in assuring which router(s) in
your
AS(non-broadcast/broadcast) is eligible to become DRs/BDRs.
Another good example is the use of loopback in connecting I-BGP peers. In
making
this connection through the use of loopback interfaces provides a number of
different routes
through the use of the IGP however if a single physical connection existed
between these devices this
would of course make no difference.
In looking at our testing however I do have one question? I breaking the
connection between
R2 and R3, area 0 is now partitioned from both areas in the AS. So without
this physical
connection to the backbone area how would the virtual link(even with the use
of an IGP get to
the backbone area 0.
When looking at the use and benefits of using the loopback devices I think
most should identify if any single point of failure exist and work to
provide some type of backup path(HSRP, etc..) this way the loopback
interface could complete the process as part of network HA.
Nigel
----- Original Message -----
From: Chuck Larrieu <chuck@cl.cncdsl.com>
To: CCIE_Lab Groupstudy List <ccielab@groupstudy.com>
Sent: Sunday, December 24, 2000 1:12 PM
Subject: OSPF Virtual Links and RIDs
> Just got through with one of the multiprotocol redistribution practice
labs
> ( Mentor Labs 4141 )
>
> Got a question regarding virtual links and loopback RIDs.
>
> In the realm of OSPF, when direct ( meaning through the OSPF process )
> access to a particular router is lost, does that mean that any virtual
link
> associated with that router is lost? Well, yes, I know, and duh!
>
> But my question has to do with the placement of the RID into the routing
> process.
>
> The deal is that there is an alternative link to the OSPF area 0. However
it
> is through a different routing protocol. All routes are redistributed
> through that protocol, and when the direct i.e. OSPF link between the two
> endpoints of the virtual link are severed, even though the route to the
RID
> is seen via the redistribution process, the virtual link apparently does
not
> come back up.
>
> This leads back to the question of the value of loopback addresses as a
cure
> all for routing process interruptions. In the scenario I ran, there was a
> classic virtual link.
>
> R1---------R2-------R3 connected via serial links
> Area2....area1.....area0
>
> All routers have loopbacks, which under the rules of the game have become
> the RID's
>
> There is also an external routing domain connecting R1 and R3 via the
> ethernet ports. Redistribution is established, and works just fine.
>
> When I severed the serial link between R2 and R3, the virtual link goes
> down, and does not re-establish itself, even though the RID is being
> advertised as a route into the exterior domain, and remains in the routing
> tables of all routers as external routes.
>
> I kinda expected this behaviour, but it still raises the question of the
> supposed benefit of loopbacks as an interface that is "always up" and
> therefore advantageous to use.
>
> One of those "pitfalls" someone was asking about a couple of weeks back, I
> suppose.
>
> Chuck
> ----------------------
> I am Locutus, a CCIE Lab Proctor. Xx_Brain_dumps_xX are futile. Your life
as
> it has been is over ( if you hope to pass ) From this time forward, you
will
> study US!
> ( apologies to the folks at Star Trek TNG )
>
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