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Re: OSPF virtual link and cost posted 01/06/2008
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unless you use, no capability transit, this is true.

if you have no capability transit, the traffic will be routed via the
virtual link itself.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/rte_osph.htm#wp999437

Joseph.

On Jan 6, 2008 11:09 AM, Scott Vermillion <scott_ccie_list@xxxxxxxxx> wrote:

> Hey Shiran,
>
> I did some battle with VLs probably a few months back.  While I couldn't
> offer up the link offhand, IIRC, Cisco states that the reason you can't
> punch a VL through a stub area is because payload traffic is not actually
> sent "through" the VL; rather, it is routed natively through the
> non-backbone area supporting the VL.  Thus, all routers must have full
> routing knowledge for the OSPF domain. Is that what you were basically
> saying or do you have a different take on this?
>
> Regards,
>
> Scott
>
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
> shiran guez
> Sent: Saturday, January 05, 2008 11:45 PM
> To: paul.cosgrove@xxxxxxxxx
> Cc: Luan Nguyen; ccielab@xxxxxxxxxxxxxx
> Subject: Re: OSPF virtual link and cost
>
> if you see a cost of -1 then it is more then likely a bug, you can see a
> higher accumulative distance of the links in your OSPF domain but this
> router will not be used as a transit
>
> So if you have the following senarion
>                                          _
>                                         /   R3
> R5--------R1*-------*R2--------<
>                                         \_ R4
>
> the red link between R1 and R2 is set with the highest cost and you
> advertise all the links to the OSPF, *you will not see* the link between
> R1
> to R5 on router's R2 R3 and R4 only in there database as the routers will
> know that a router that advertise a link with 65535 cost do not want to be
> a
> transit and will never will be even if you had a redundant link between R1
> and R2 and you wanted to set one of them as backup then setting it to
> 65535
> will not work as it will never become transit, you can set it only to a
> high
> metric but not to the highest.
>
> so with that to say OSPF Area 0 by definition is a transit area cant be a
> stub area, so by setting links in (Virtual links) trough a stub you are
> violating the protocol definition.
>
> another note to that: this is the reason why you cant put a virtual link
> trough a stub area only tunnel interface that you advertise the tunnel to
> area 0.
>
>
>
> On Jan 6, 2008 2:43 AM, Paul Cosgrove <paul.cosgrove@xxxxxxxxx> 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@xxxxxxxxx> 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
> > >
> > >
> >
> >
> >
>
>
> --
> Shiran Guez
> MCSE CCNP NCE1
> http://cciep3.blogspot.com
> http://www.linkedin.com/in/cciep3
>
> _______________________________________________________________________
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>
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