Subject: [VRRP] ARP REPLY IN VRRP

From: "ashish thakur" <[email-masked]>
Date: 4 Nov 2002 10:51:12 -0000

Hi ,

I have a doubt related to virtual router respone for ARP request . 
Consider two routers R1 and R2 as shown :

                      IF1  ----  IF2
                    ------| R1 |---------
                   |       ----          |
      ----         |                     |     ----
     | H1 |--------|                     |----| H2 |
      ----         |                     |     ----
                   |   IF1 ----  IF2     |
                    ------| R2 |---------
                           ----

R1 and R2 are the routers.

H1 is a host in private network
H2 is a host in public network

IF1 and IF2 are the interface numbers.

R1 and R2 are connected through both the interfaces.

Virtual router is configured such that R1 is made address owner 
and R2 as backup. If an ARP request is send to the virtual ip 
address (which is now same as the
R1 interface address) , Master (R1) will respond back with source 
harware address in ARP packet as the virtual router MAC address . 
If a Shutdown event is received, then R1 will :
        o Cancel the Master_Down_Timer
        o Transition to the {Initialize} state

This will make R2 to become Master . If we now send an ARP request 
to the virtual ip address, then R2 will respond back with source 
hardware address in ARP reply packet as the virtual router MAC 
address and also R1 will reply back with source hardware address 
in ARP reply as the physical MAC address of the IF1 interface . 
This leads to duplication of MAC address and most probably 
duplication of packets .

To avoid this phenomena i think that in the initialise stage the 
virtual router MUST NOT respond to ARP requests for the IP 
address(s) associated with the virtual router .

Any suggestions/comments if i am missing something out here ? or 
really some modification is required in the VRRP Initialize 
stage

Thanks,
Ashish Thakur

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From: Bill Fenner <[email-masked]>
Date: Mon, 4 Nov 2002 19:13:05 -0800

>...If we now send an ARP request 

Section 8.2 says:

   When a VRRP router restarts or boots, it SHOULD not send any ARP 
   messages with its physical MAC address for the IP address it owns, it
   should only send ARP messages that include Virtual MAC addresses.

This seems to preclude R1 replying with its physical MAC address.

  Bill