Re: Question on OSPF virtual-links computed via a router with in RFC-3137 "overload"

[Liem Nguyen <[e-mail]>]

Don,

On Fri, Jul 30, 2004 at 04:53:28PM -0400, Don Goodspeed wrote:
> All,
>
> I have a question on a topology where I mix virtual-links with
> a router set in a RFC-3137 compliant "stub router" or "overload"
> state.
>
>  BB<->Router A <--> Router B <--> Router C <--> Router D
> Area 0  ABR   Area 1       Area 1  ABR   Area 2
>
> If everything lines up, Router A is an ABR between area 0
> and area 1.  Router C is an ABR between area 1 and area 2.
>
> I configure a virtual-link between routers A and C so router
> D can communicate to the backbone routers in area 0.
>
> If Router B is then set to an overload state compliant with
> RFC-3137, it advertises it's transit links with a metric of
> 65535.  Router C then computes the cost of the virtual-link
> to A as 65535 + the metric of its link to B.
>
> My question is should the virtual-adjacency between A and C
> be declared down, or should A and C advertise the virtual-link
> metric in their router LSAs as 65535?  This issue does not

The latter.
Note that RFC3137 is a little different than the IS-IS overload
feature in that RFC3137 only discourages transit traffic through the
"stub" router; however, if there are no alternate paths,
the "stub" router can still be used.

Liem


> seem to be covered in either RFC-3137 or RFC-2328.
>
> This could also be done without an RFC-3137 situation by
> setting the path metrics such that the cost of the virtual-
> link also exceeds 65535.
>
> My feeling is that since the path metric exceeds 65535 the
> virtual adjacency should be torn down (since there is no better
> path between A and C).
>
> Any other opinions?
> Don
>
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--

Liem Nguyen