Subject: Re: why Virtual IP addresses and not Virtual IP subnets

From: Yaron Presente <[email-masked]>
Date: Tue, 03 Jul 2001 10:52:06 +0300

Hi,
Thanks for the answers. However, I still have some concerns.
Peter Hunt wrote:

> The router backing up the virtual address has its own "real" address
> configured in that subnet. When the master fails, the backup VRRP
> router installs the virtual address as an additional address in that
> subnet. So it doesn't need to know the mask length of the network as
> part of VRRP; it already knows it because it has its own address
> configured in that subnet.

>        Peter.

I couldn't find in the RFC where it says or implies
that a backup router should have a real address for
every IP address that it backs up.

Vivek Menezes wrote:
> There is no requirement for subnet because the Master only represents
the IP
> address
> to Mac address mapping. It is supposed to route all packets destined to
the
> mac address.

> Vivek.

TRUE. Here my problem is more related to the implementation side.
I know that VRRP only gives solution to a default gateway failure,
that is for traffic generated by hosts in an upstream direction.
However, I think that it is reasonable to assume that at least in
a symmetric topology, downstream traffic would also have to be routed
by this backup router. in order to do this, the router has to know
(and advertise) his local IP subnets, and the IP <=> Mac mapping is
not enough to make the necessary routing decisions.
Thanks,
Yaron