Subject: Re: Doubt regarding ARP cache updation

From: basila <[email-masked]>
Date: Thu, 22 Jan 98 17:23:00 PST

Peter,

Thanks for detailed explanation. It helped us a lot to understand the draft 
better. As you said the draft should be updated to explain better. Even some 
of the explanation you have given can also get into the draft.

Thanks a lot once again

Regards
Rao & Basil

V.R.N. Rao & Anton Basil
Future Software Pvt., Ltd.,
481, Nandanam,
Madras - 600 035.

Voice : 91-44-4340323, 4330550 (Extn : 331)
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 ----------
From: Peter Hunt
To: basila
Cc: 'SMTP:[email-masked]'
Subject: Re: Doubt regarding ARP cache updation
Date: Wednesday, January 21, 1998 8:38AM

Rao & Basil,

        thanks for giving more detail; sorry I didn't get back to
you yesterday. I think I can help clarify things.

> The following are the doubts:
>
> 1) Whether the topology depicted above is allowed? What i feel it
> is the extension of the sample configuration specified in
> section 4.2 of "draft-ietf-vrrp-spec-04.txt"

Yes, the configuration you've described is allowed.

> If the topology depicted above is allowed, i have the following doubts
>
> 2) When IP A becomes master router for VRID 1, it is responsible for
> forwarding IP packets sent for IP A, IP B and IP C. Am i correct?

No, that is not correct. A virtual router only contains the IP
addresses for one of the routers. Or, to put it another way,
backup routers of a virtual router don't have their addresses
backed up by that virtual router.

Let me illustrate this using a simpler configuration, and then
I'll come back to the configuration you've described:

                       VRID=1
                      +-----+      +-----+
                      |     |      |     |
                      | MR1 |      | BR1 |
                      |     |      |     |
                      +-----+      +-----+
         IP A ---------->*            *<---------- IP B
                         |            |
                         |            |
                         |            |
       ------------------+------------+--------------

In this configuration, IPA is backed up using virtual router VRID1.
The router on the left is the owner of IP A; whenever it is up, it
will forward packets for IP A. The router on the right is a backup
router for VRID1. Whenever the router on the left fails, it will
take over responsibility for forwarding packets for IP A.

The router on the right is configured with IP B, which it uses
to access the LAN. IP B is not backed up using VRID 1. If the router
on the right fails, nothing adopts IP B. In order to back up IP B,
you'd have to configure a second virtual router, with the router
on the right as owner, and the router on the left as backup.

When first configured, the router on the left will send a gratuitous
ARP request for IP A, using 00:00:5E:XX:XX:01. The router on the
right will send one for IP B using it's device's physical MAC address.
The two routers will respond to ARP requests the same way.

If the router on the left fails, the router on the right will adopt
the information for VRID 1. Until the router on the left comes back,
the router on the right will respond to ARP requests for IP A, using
00:00:5E:XX:XX:01, *and* respond to ARP requests for IP B using
it's device's physical MAC address.

In the configuration you've specified, IP A will be backed up by
VRID 1, and IP B will be backed up by VRID 2. IP C and IP D won't
be backed up; you would need another virtual router for each of
these addresses to implement failover for them.

So whoever is master of VRID 1 will respond to ARP requests for
IP A, using 00:00:5E:XX:XX:01, and whoever is master of VRID 2
will respond to ARP requests for IP B, using 00:00:5E:XX:XX:02.
ARP requests for IP C and IP D will only ever be responded to by
the routers which are shown to have them now. Those responses will
contain the physical LAN addresses of those routers.

I hope this helps clarify how virtual routers work in VRRP2. I
also hope I haven't put you to sleep with my long-winded
description.

Your question makes me think that we may need to clarify the
definitions and example text in the draft to explain this better.

Do you think we need to do this?

 --
| Peter Hunt                              |  Phone: +1 408 990 2093
| Ipsilon Networks Inc.                   |  Fax:   +1 408 743 5677
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