From: Huan Pham (huan.pham@valuenet.com.au)
Date: Wed Apr 16 2008 - 20:58:57 ART
Hi Mike,
I got the same problem few weeks ago, and it took me quite sometime to fix.
I still do not have a good explanation for it. I believe that it is just a
bug.
Given the fact that the cost of virtual link is the total cost of the link
on the path, I have few interesting observation for the below simple
scenario including 3 routers, R1---R2---R3 and the Virtual link is between
R1 and R3.
R1 ------- R2 -------- R3
| |
+---------VL------------+
If you set the cost of 65535 on ether R1 or R3 interface, the VL does not
come up. Maybe the router just consider the VL cost infinite in that
scenario, and does not bother try to bring it up. Interestingly enough, if
you set up the same max cost 65535 on R2 interfaces, the VL is fine.
If you set up cost of 65534 on R1 interface, then cost of 1 on R2 interface
pointing to R3, then the total cost of virtual link on the direction from R1
to R3 is 65535, but the virtual link is showing up, without any problem.
So Virtual link does not come up only when the cost of the exiting interface
of either ends of the virtual link is set to maximum.
BTW, it seems to me that in OSPF, if the total cost of a path exceeds the
max of 65535, then 65535 is subtracted from it, resulting in an unexpected
small number. This would potentially cause OSPF to function in a very
unexpected way, specifically on the best path selection decision. A longer
path could be preferred over a shorter one.
I will be very careful with the OSPF cost or link bandwidth manipulation if
I am given a scenario where I have to set the reference bandwidth to a very
high number.
Cheers,
Huan
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Mike
Harrison
Sent: Thursday, 17 April 2008 5:25 AM
To: Cisco certification
Subject: Virtual Links and max ciost
Hi
I have found a small issue with virtual links. If the the cost of the link
is 65535 (max ospf can use) then the virtual link wont come up;
OSPF_VL0 is down, line protocol is down
Internet Address 0.0.0.0/0, Area 0
Process ID 1, Router ID 150.11.4.4, Network Type VIRTUAL_LINK, Cost: 65535
FastEthernet0/0 is up, line protocol is up
Internet Address 129.11.45.4/24, Area 1
Process ID 1, Router ID 150.11.4.4, Network Type BROADCAST, Cost: 65535
But if you reduce it by 1 then it comes up;
OSPF_VL0 is up, line protocol is up
Internet Address 129.11.45.4/24, Area 0
Process ID 1, Router ID 150.11.4.4, Network Type VIRTUAL_LINK, Cost: 65534
FastEthernet0/0 is up, line protocol is up
Internet Address 129.11.45.4/24, Area 1
Process ID 1, Router ID 150.11.4.4, Network Type BROADCAST, Cost: 65534
When it is maxed the ospf neighbor comes up and routes are valid, just
virtual links wont come up. I realise that this isnt going to happen in real
life unless you crank up the auto-cost ref bw and have a 64k interface -
just curious if anyone has an explaintion.
Mike
Pass the CCIE in six weeks, Guaranteed!
http://www.certscience.com/CCIE
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