From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sat Jan 05 2008 - 22:43:32 ARST
I'm seeing slightly different behaviour myself. Virtual links seem to
have a maximum metric of 65535, which is also the maximum cost allowed for
an individual OSPF interface (so V.L.'s have the same maximum cost limit
as a direct connection).
OSPF routes consisting of multiple links can accumulate a higher cost, and
a virtual link which transits multiple links can be assigned a different
cost depending on the version of IOS:
Topology is as follows:
R2----R3----R4
R2 runs 12.4(5b)
R3 runs 12.2(15)T7
R4 runs 12.2(16)T7
The behaviour I'm seeing can be summaried as follows:
- If one of the routers terminating the virtual link has an outgoing
(directly connected) interface cost of 65535 the V.L. goes down.
- If the outgoing interface costs of the terminating routers are each
individually lower than 65535 the link stays up.
- Transit routers can have interface cost of 65535 without bringing the
virtual link down.
- The router running 12.2 uses a maximum metric of 65535 for the virtual
link, even if the total cost of all transit links is higher.
- The router running 12.4 uses a maximum metric of 65535 for the virtual
link, but the total metric rolls back to -1 when it increases past 65535.
As an example,
- If outgoing interfaces from R2 towards R4 have costs of 65534 and 65535,
R2 shows a cost of 65533 for the virtual link.
- If outgoing interfaces from R4 towards R2 have costs of 65534 and 64, R4
shows a cost of 65535 for the virtual link.
Paul.
> cost 65535 is the highest cost of OSPF if you set all your interfaces to
> cost of 65535 it is like you are doing stub router in EIGRP, there is a
> command for that if you want:
>
> max-metric router-lsa ?
> external-lsa Override external-lsa metric with max-metric value
> include-stub Set maximum metric for stub links in router-LSAs
> on-startup Set maximum metric temporarily after reboot
> summary-lsa Override summary-lsa metric with max-metric value
>
> the Virtual Link is not an actual logical interface like tunnel there is
> no
> traffic going over the virtual link it is used to bridge the backbone area
> (transit area) of OSPF so if you have a stub router you cant have also
> use
> it to transit traffic.
>
>
>
> On Jan 5, 2008 10:44 PM, Luan Nguyen <luan.m.nguyen@gmail.com> wrote:
>
>> Do you guys know that virtual link depends on cost? If the cost =>65535
>> then it will die?
>> If you set the auto-cost reference-bandwidth to say...100000, then most
>> likely, virtual link over serial T1 links will die.
>> If you have a requirement to change the auto-
>> 100000,
>> how do you keep the virtual link up?
>>
>> thanks.
>>
>> -lmn
>>
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>
>
>
> --
> Shiran Guez
> MCSE CCNP NCE1
> http://cciep3.blogspot.com
> http://www.linkedin.com/in/cciep3
>
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