RE: Virtual MAC really necessary?

Using the physical MAC address may also contribute to opening of another
issue: a race conditions when two Standby virtual routers compete for Master
state. 

The specific situation occurs when two Masters become active approximately
at the same time. As a result of protocol defined arbitration mechanism, the
Master with lower primary IP address will be forced to switch to Backup
state.  It is fine as long as out of these two Masters, the legitimate one,
which will remain its status, will be also the last one who has sent the
gratuitous ARP (just considering the IPv4 case). 

However, what will happen if the gratuitous ARP from the legitimate Active
is overwritten in device by that one coming from virtual router that will
assume the Standby role?

After receiving advertisement from another virtual router, should not the
legitimate Active re-broadcast the gratuitous ARP - just in any case - even
after a short delay? 

Sylwester

 

 

  _____  

From: [email protected] [mailto:[email protected]] 
Sent: Monday, January 14, 2008 12:00 PM
To: Roland Abt
Cc: [email protected]
Subject: Re: [VRRP] Virtual MAC really necessary?

 


The advantage of the virtual mac is that it is a multicast address so both
devices should receive the traffic regardless of their failover state.
Also, the virtual mac allows failover with out any of the devices having to
change their arp entry.  The gratuitous arp just serves to update the
forwarding table in the switch(s).  If the gateway mac was suddenly a
physical address you may run into issues if some of the attached devices
don't receive the gratuitous arp or don't process it because of security. 

  




"Roland Abt" <[email protected]> 

01/14/08 11:31 AM 


To

[email protected] 


cc

 


Subject

[VRRP] Virtual MAC really necessary?

 


 

 




Hello everybody

I want to implement my own VRRP protocol. The protocol itself is not that  
tricky, but using a virtual MAC seems like a big hurdle to me. The  
standard states that the MAC address of the new Master has to be published  
in a gratuitous ARP (request/reply). I think this already should help to  
update the ARP table of every device. Does anybody know of a situation  
where publishing the physical MAC would cause any trouble? Are there any  
known devices that would not update their ARP table in case of such a  
gratuitous ARP?

Best regards and thanks in advance for your time,
Roland


PS:
According to the Ethereal Wiki ( <http://wiki.ethereal.com/Gratuitous_ARP>
http://wiki.ethereal.com/Gratuitous_ARP) a  
gratuitous ARP can be either a request or a reply. Any experience which  
one is better?

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