I came out with the same idea as you have when I first learned this. it
doesn't make sense.
OK, here's my understanding. Pls correct me if I am wrong.
When you setup the VL, you are using the routerID. The routerID used by OSPF
as
1 neighbor ID
2 Link state ID
3 Link ID
So as you could see, it's only ID. OSPF never use these IP addresses as the
destination of the advertisement packet.
Let's say there's a network that is running OSPF. Each router has been setup
a loopback interface. The routerID will be the ip address of the loopback.
But you don't have to put these loopback into OSPF(though it's strongly
recommended and always a good practice). OSPF will be working fine. you
could use all the ospf features without any problem. The only problem is the
loopbacks are not reachable.
The VL is treated as a unnumbered point to point interface. It will be
assigned an IP however. The IP address will be the interface it associated
to. When a router received a LSA, it will do some checks. One of those check
is the area ID. If a packet is received on a non-backbone interface with
areaID=0, then this router will know that the packet is received through a
VL, so the receiving router will check the source routerID. it has to match
the routerID you locally configure for the VL.(There are some other checks
of course).
Now go back to your question, these two interfaces do not have to be in the
same subnet(I will never put them in the same subnet. By doing that, you
will never be able to reach each other even you enable routing on those
interfaces) and you don't need to be able to ping each other to make the VL
work.
Hope this is help.
-ns
-----Original Message-----
From: Dhiman Barman [mailto:dhiman@cs.bu.edu]
Sent: 4 April 2002 3:18 PM
To: Shi, Ning
Subject: Re: [nsp] virtual link
doesn't make sense.
Shi, Ning claims the following:
> They don't have to be in the same subnet. Actually, the routerID don't
even
> has to be in the routing table to setup the VL. But like you said, you
> would't be able to ping each other.
>
> -ns
>
> -----Original Message-----
> From: Dhiman Barman [mailto:dhiman@cs.bu.edu]
> Sent: 4 April 2002 12:24 AM
> To: cisco-nsp@puck.nether.net
> Subject: [nsp] virtual link
>
>
> Hi,
> I have set up a virtual link between two routers.
>
> 192.168.4.1/24 192.168.3.2
> Router1--------virtual link--------------------Router2
> Area 2 Area 2
>
> (other interfaces not shown, Router 1 has an interface on area 0)
>
> Conf on router1:
> area 2 virtual-link 192.168.3.2 (also the router ID)
>
> conf on router2:
> area 2 virtual-link 192.168.4.1 (also the router ID)
>
>
> Is it necessary for the two addresses to be on the same network ?
> Because I cannot ping the other interface from one. Getting
> the message on Router 1:
> recv pkt from 192.168.3.2, ethernet 0/0 area 0.0.0.2: src not on
> the same network.
>
>
> -Dhiman
>
>
-- Technological progress has merely provided us with more efficient means for going backwards. -- Aldous Huxley
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