From: Mike Harrison (michael.h4@blueyonder.co.uk)
Date: Thu Apr 17 2008 - 16:41:00 ART
Brian and Huan
Thanks for the investigations. Huan is right - it does seem like the
virtual link wont come up if the interface cost is 65353, but the path cost
can be higher and the virtual link works. I guess that it depends on the
interpretation of "underlying path" in the RFC - does this mean path or
interface?
Also does a virtual link care about the cost of the path through the transit
area when it establishes an link ??
Thanks guys
Mike
----- Original Message -----
From: "Huan Pham" <huan.pham@valuenet.com.au>
To: "'Brian Dennis'" <bdennis@internetworkexpert.com>
Cc: "'Cisco certification'" <ccielab@groupstudy.com>
Sent: Thursday, April 17, 2008 4:08 AM
Subject: RE: Virtual Links and max cost
> 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!
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>> _______________________________________________________________________
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>
>
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>
>
> Pass the CCIE in six weeks, Guaranteed!
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> _______________________________________________________________________
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