From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Sun Jan 06 2008 - 08:23:58 ARST
Hi Luan,
The solution seems to be to use a recent version of IOS for the two
routers terminating the virtual link. Previously, if auto-cost reference
bandwidth was sufficiently high, the router would assign a cost of up to
and including 65535 to the local outoing interface. A local outgoing
interface cost of 65535 causes a virtual link to fail.
IOS 12.4 (and perhaps others) assign a local interface cost of up to
65534, which means virtual links stay up.
As I mentioned before, the virtual link itself may have a different cost
assigned if it passes over multiple links. Earlier versions use a simple
sum , with an absolute max of 65535. The later IOS version I checked uses
uses a rolling cycle where after 65535 it counts to 0,1,2,3 etc. Neither
version of ios gives a virtual link a cost higher than 65535.
Note that a cost of 65535 accumulated over multiple links will not cause
the virtual link to fail, it has to be on the local interface of a
terminating router to take it down.
Incidently I miscalculated when I mentioned -1 (I shouldn't do even simple
sums so late at night). Should have said that cost cycles back to 0 after
65535, which is consistent with the example I gave earlier.
My understanding is that the OSPF cost of 65535 on a transit router does
not have particular significance other than making that router unlikely to
be used for transit traffic. If no alternative paths are available it
will be used.
Paul.
> Any magic command to keep the virtual link up then? or you just have to
> use
> tunnel in stead of virtual link?
>
> Thanks.
>
> -lmn
>
> On Jan 5, 2008 7:43 PM, Paul Cosgrove <paul.cosgrove@heanet.ie> wrote:
>
>>
>> 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
>> >
>> > _______________________________________________________________________
>> > Subscription information may be found at:
>> > http://www.groupstudy.com/list/CCIELab.html
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