Subject: [VRRP] Potential Forwarding Loop

From: "Chiu Hsia Peng" <[email-masked]>
Date: Wed, 20 Aug 2008 20:32:37 +0800

Hi,
The following is stated in the RFC 3768.

8.4.  Potential Forwarding Loop

   A VRRP router SHOULD not forward packets addressed to the IP
Address(es) it becomes Master for if it is not the owner.  Forwarding
these packets would result in unnecessary traffic.  Also in the case
   of LANs that receive packets they transmit (e.g., token ring) this
 can result in a forwarding loop that is only terminated when the IP
TTL expires.

...
I am confused about the above description. Since 6.4.3 already
describe: A master, .. MUST NOT accept packets addressed to the IP
address(es) associated with the virtual router if it is not the IP
address owner. I think if a VRRP router doesn't accept a packet, it
can be implied that the router doesn't forward it either.

What's the purpose of this section?
Could someone give me a concrete example about the potential forwarding loop?

 Thanks.
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---

From: "Don Provan" <[email-masked]>
Date: Wed, 20 Aug 2008 16:48:41 -0700

By "accept", 6.4.3 is speaking of receiving the packets into
its IP layer for IP receive handling. The point in this section
is that the master should not pretend to actually *be* the node
with the VR's IP address other than to answer ARPs for it.
(Unless, of course, it *is* the VR IP address's owner.)

This has nothing to do with accepting the packets into
the system to begin with and considering them for forwarding,
although I suppose I can see why you didn't find that clear.
But you can read these two sections as saying, "do not accept
packets addressed to VR addresses you do not own either for
local reception or for forwarding." The thought processes that
led to the two observations don't really intersect, so I think
that's why they two issues are raised separately and without
any relation even though, as you're pointing out, they're
really two parts of the same concept: ignore packets addressed
to a VR IP address you don't own.

I hope that clears things up, because I'm going to ignore your
question about the forwarding loop: it's a somewhat obscure
point, anyway, and it is sufficient to understand that forwarding
the VR IP address owner's packets is a bad idea. Is there some
reason you want to do it, anyway, or are your just interested
in the clarification?
-don

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of
> Chiu Hsia Peng
> Sent: Wednesday, August 20, 2008 5:33 AM
> To: [email-masked]
> Subject: [VRRP] Potential Forwarding Loop
> 
> 
> Hi,