From: Huan Pham (huan.pham@valuenet.com.au)
Date: Thu Apr 17 2008 - 23:00:17 ART
> Also does a virtual link care about the cost of the path through the
> transit area when it establishes an link ??
The virtual link cost path is the "path cost", i.e. the sum of cost of link
along the path. Whether or not Cisco cares about that when it establishes a
virtual link, I doubt it does! However, once the virtual link is
established, its cost is used in shortest path calculation, like any other
physical links.
Due to the "circular" characteristic of a virtual link cost (i.e. after
65535, it starts at 0, 1, 2 again) you will run into a scenario where a
longer path (physically) is selected over a shorted path.
Try setting up a scenario where you have two virtual links over different
areas. Change interface cost so that one of the virtual link ends up being a
very small number. That "small number" is picked by Cisco over the other one
(which in fact shorter, and should be preferred). Very interesting to watch,
but very frustrating if we are to troubleshout.
E.g. in the scenario below, normally traffic is load-balance between R3 and
R4.
------- R3-------
/ \
R1 R2
\ /
------- R4-------
R1#sh ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
VL1 1 0 14.0.0.1/8 128 P2P 1/1
VL0 1 0 13.0.0.1/8 128 P2P 1/1
Lo0 1 0 1.1.1.1/32 1 LOOP 0/0
Se1/1 1 3 13.0.0.1/8 64 P2P 1/1
Se1/0.1 1 4 14.0.0.1/8 64 P2P 1/1
R1#sh ip route 2.2.2.2
Routing entry for 2.2.2.2/32
Known via "ospf 1", distance 110, metric 129, type intra area
Last update from 13.0.0.3 on Serial1/1, 00:20:14 ago
Routing Descriptor Blocks:
* 14.0.0.4, from 2.2.2.2, 00:20:14 ago, via Serial1/0.1
Route metric is 129, traffic share count is 1
13.0.0.3, from 2.2.2.2, 00:20:14 ago, via Serial1/1
Route metric is 129, traffic share count is 1
Now if you change the cost of interface from R1 to R4 to 65354, this link
will be ended up being the preferred exit point to go to R2 (excluding any
traffic routed intra-area, which is not via virtual-link)
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int s1/0.1
R1(config-subif)#ip ospf cost 65534
R1(config-subif)#
R1#sh ip ospf int brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
VL1 1 0 14.0.0.1/8 62 P2P 1/1
VL0 1 0 13.0.0.1/8 128 P2P 1/1
Lo0 1 0 1.1.1.1/32 1 LOOP 0/0
Se1/1 1 3 13.0.0.1/8 64 P2P 1/1
Se1/0.1 1 4 14.0.0.1/8 65534 P2P 1/1
R1#sh ip ospf int s1/0.1 | in Cost
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 65534
R1#sh ip route 2.2.2.2
Routing entry for 2.2.2.2/32
Known via "ospf 1", distance 110, metric 63, type intra area
Last update from 14.0.0.4 on Serial1/0.1, 00:00:02 ago
Routing Descriptor Blocks:
* 14.0.0.4, from 2.2.2.2, 00:00:02 ago, via Serial1/0.1
Route metric is 63, traffic share count is 1
Pass the CCIE in six weeks, Guaranteed!
http://www.certscience.com/CCIE
This archive was generated by hypermail 2.1.4 : Thu May 01 2008 - 08:25:51 ART