Subject: Virtual IPs & Virtual MACs...

From: Badri Srinivasan S <[email-masked]>
Date: Wed, 04 Apr 2001 11:13:23 +0530

Hi All..

When a host wants to send a packet to its default gateway (which in our
case is our VRRP Master), it send an ARP request for the virtual IP
addr..
My doubts in this reg are..
1. Since the IP addr in the ARP request sent by the host is virtual &
not real, how will the master respond to this ARP req..
2. If suppose the master responds with the virtual MAC in the ARP reply,
how does it pick up any frames with destn MAC addr = Virtual MAC addr
since the Ethernet card of the Master does'nt recognize the virtual MAC
as its own addr & hence will discard the packet..at the ethernet driver
level itself..

kindly clarify these doubts..

Thanks & Regards,
Badri

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From: "Danny J. Mitzel" <[email-masked]>
Date: Wed, 04 Apr 2001 11:24:02 -0700

On Wednesday, April 04, 2001 at 11:13 +0530,

>> not real, how will the master respond to this ARP req..

the ``virtual'' aspect of the IP address is that it can be dynamically
[re]mapped across a set of VRRP enabled routers.  resposibility for
replying to ARP, responsibility for IP forwarding service, etc. is
always explicitly assigned to one router [the current Master].  the
VRRP protocol specification mandates these operation and actions that
must be performed by the Master relating to ARP and IP forwarding.
thus this service will be provided or your router equipment is not
conformant to the protocol specification.

>> as its own addr & hence will discard the packet..at the ethernet driver

the VRRP protocol specification mandates the actions related to IP
forwarding, and the Master listening for the virtual MAC is a requirement.
how this is performed is an implementation decision.  many ethernet 
cards are programmable to listen for a set of ethernet addresses
[this is often used for instance in listening for specific IP multicast
group memberships].  in the worst case the implementation might put
the interface in promiscuous mode and filter in the driver.  whatever
the solution chosen, if the VRRP router is unable to listen and forward
for the VMAC then the equipment is not conformant and not implementing
VRRP.

danny