From: Huan Pham (huan.pham@valuenet.com.au)
Date: Thu Apr 17 2008 - 00:08:54 ART
Hi Brian,
Thanks for highlighting the RFC section about Virtual link. However, I still
do not get what the reason really is when Virtual link does not work in one
scenario while it is OK on the others (even though as far as the total path
cost is concerned, they are the same).
At least, I do not think that CISCO follows the RFC recommendation strictly
in this regard. That makes our brain hurt.
No matter you change the path cost by tweaking intermediate router(s)
virtual link still comes up. But as soon as we set the cost of 65353 on the
router that Virtual link ends, it just stops working.
I relab in the following scenario to show you what I means.
Topology:
R1 --------- R3 ---------- R2
R1#
conf t
int s1/1
ip add 13.0.0.1 255.0.0.0
no shut
int loop0
ip add 1.1.1.1 255.255.255.255
router ospf 1
net 13.0.0.1 0.0.0.0 area 2
net 1.1.1.1 0.0.0.0 area 0
area 2 virtual-link 2.2.2.2
R3#
conf t
int s1/2
ip add 13.0.0.3 255.0.0.0
no shut
int s1/3
ip add 23.0.0.3 255.0.0.0
no shut
int loop0
ip add 3.3.3.3 255.255.255.255
router ospf 1
router-id 3.3.3.3
net 13.0.0.3 0.0.0.0 area 2
net 23.0.0.3 0.0.0.0 area 2
net 3.3.3.3 0.0.0.0 area 0
R2#
conf t
int s1/1
ip add 23.0.0.2 255.0.0.0
no shut
int loop0
ip add 2.2.2.2 255.255.255.255
router ospf 1
router-id 2.2.2.2
net 23.0.0.2 0.0.0.0 area 2
net 2.2.2.2 0.0.0.0 area 0
area 2 virtual-link 1.1.1.1
Virtual link comes up nicely.
R1#sh ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
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 2 13.0.0.1/8 64 P2P 1/1
Now if we change the cost of the interface on R1 to 65535, the virtual link
go down after we reset OSPF.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#int s1/1
R1(config-if)#ip ospf cost 65535
R1(config-if)#
R1#
*Mar 1 00:14:40.891: %SYS-5-CONFIG_I: Configured from console by console
R1#sh ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
VL0 1 0 13.0.0.1/8 63 P2P 1/1
Lo0 1 0 1.1.1.1/32 1 LOOP 0/0
Se1/1 1 2 13.0.0.1/8 65535 P2P 1/1
R1#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
R1#sh ip ospf int brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
VL0 1 0 0.0.0.0/0 65535 DOWN 0/0
Lo0 1 0 1.1.1.1/32 1 LOOP 0/0
Se1/1 1 2 13.0.0.1/8 65535 P2P 1/1
I can make the virtual-link cost to the maximum of 65535 by changing link
cost on R2, and the virtual link is still up.
R3(config)#int s1/3
R3(config-if)#ip ospf cost 65471
R1#sh ip ospf int brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
VL0 1 0 13.0.0.1/8 65535 P2P 1/1
Lo0 1 0 1.1.1.1/32 1 LOOP 0/0
Se1/1 1 2 13.0.0.1/8 64 P2P 1/1
R1#sh ip ospf virtual-links | in Virtual|Cost
Virtual Link OSPF_VL0 to router 2.2.2.2 is up
Transit area 2, via interface Serial1/1, Cost of using 65535
How can we make the virtual link cost of more than 65535? We simply cann't!
R3(config)#int s1/3
R3(config-if)#ip ospf cost 65535 ! that is the max we can set the cost for a
link
The virtual link cost from R1 to R2 via R3 will be 64 + 65535 = (guess
what?) 63 :-))
The virtual link still survive this maneuver.
R1#sh ip ospf virtual-links | in Virtual|Cost
Virtual Link OSPF_VL0 to router 2.2.2.2 is up
Transit area 2, via interface Serial1/1, Cost of using 63
If we set the link cost on R1 to 65354, the virtual link cost from R1 to R2
via R3 will be 65534 + 65535 = 65533 (after subtracted 65536):-))
R1(config)#int s1/1
R1(config-if)#ip ospf cost 65534
R1#sh ip ospf virtual-links | in Virtual|Cost
Virtual Link OSPF_VL0 to router 2.2.2.2 is up
Transit area 2, via interface Serial1/1, Cost of using 65533
Appreciate if you can shed more light on this.
Cheers,
Huan
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Brian Dennis
Sent: Thursday, 17 April 2008 12:02 PM
To: Mike Harrison; Cisco certification
Subject: Re: Virtual Links and max ciost
http://blog.internetworkexpert.com/2008/04/16/ospf-virtual-links-and-max-cos
t/
Brian Dennis, CCIE4 #2210 (R&S/ISP-Dial/Security/SP)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: +1-775-544-1653 (Outside the US and Canada)
>----- Original Message -----
Subject: Virtual Links and max ciost
Date: Wed, April 16, 2008 12:25
From: "Mike Harrison" <michael.h4@blueyonder.co.uk>
> 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
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
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