From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Thu Apr 17 2008 - 06:49:10 ART
I've tested this before myself and saw the same behaviour.
Whilst the reference in the RFC appears to indicate that if the sum of
the costs of all links along the path exceed 65535, then the virtual
link should be considered inoperational; cisco routers do not take the
virtual link down unless one of the endpoint routers has a cost of 65535
on the outgoing interface it uses for the virtual link.
I found that the way that the virtual link cost is calculated also
changes in different IOS versions. 12.2 versions I tested used 65535 as
the virtual link cost even if the sum of the intermediate link costs was
higher. Not sure when this change occured, but later versions (e.g.
12.4) use a circular count, where after 65535 it begins again at 0,1,2,3
etc.
Commands which, if applied to either endpoint router, can kill your
virtual-links by setting the ospf outgoing interface cost to 65535 include:
a) ip ospf cost 65535
b) bandwidth <low value>
c) auto-cost reference bandwidth <high value>
d) max-metric router-lsa
Paul.
Huan Pham wrote:
> 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
>>
>>
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>
>
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>
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>
>
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