Subject: RE: Why Virtual MACs....?

From: Thomas Dalton <[email-masked]>
Date: Tue, 15 May 2001 12:22:59 +0100

Also another point against Scenario 1 is that some end stations' ARP tables
assume a relatively static mapping between MAC address and IP address, and
immediate changes in the hosts ARP table are not necessarily guaranteed.

Tom.

> -----Original Message-----
> From: Casey S. Ajalat [mailto:[email-masked]]
> Sent: Tuesday, May 15, 2001 11:32 AM
> To: Badri Srinivasan S
> Cc: VRRP
> Subject: RE: Why Virtual MACs....?
> 
> 
> Yet another way of looking at this is:
> 
> Scenario 1: Virtual IP but Unique MAC (Badri's suggestion)
> 
> - Master Router fails
> - Secondary now has IP (IP aliasing for example)
> - Secondary issues Grat ARP
> - Switch DB is not changed (at L2) but ALL hosts now update 
> their ARP-Cache 
> Tables to reflect new MAC for the same IP
> 
> Scenario 2: Virtual IP/MAC (RFC 2238)
> 
> - Master router fails
> - Secondary now owns BOTH Virtual IP/MAC
> - Secondary issues Grat ARP
> - Switch DB is updated (at L2), all hosts see Grat ARP but 
> their ARP-Cache 
> is unchanged
> 
> In the first example, the burden is on the host to make sure 
> they reference 
> the VRRP IP via the proper MAC address.  In the second (and 
> VRRP Standard), 
> the burden is on the switches to reflect the new location of 
> the virtual 
> MAC.  The latter is of course preferred.  In a Data Center 
> environment 
> where you potentially can have dozens of hosts using the same default 
> router (or hundreds in the case of RFC 3069) it quickly 
> becomes clear why 
> the current implementation of VRRP makes sense.  i.e. it is 
> significantly 
> quicker and more reliable for a couple of switches to update the new 
> location of a host/port (i.e. vMAC) in the switch MAC tables 
> than it is for 
> hundreds of hosts to update their ARP-Cache tables.
> 
> Casey
> 
> At 10:49 AM 4/5/2001, Fernandes, Flavio wrote:
> 
> >Another way of "looking" at what VRRP tries to address via 
> Virtual Macs:
> >
> >As VRRP was defined, a concern existed (and still exists) in 
> moving IP
> >address
> >ownership around.
> >
> >That is so because management stations normally use IPs as 
> indexes for
> >tables in
> >their monitoring scheme. It is a bad trouble to have 
> stations confused about
> >"who" the
> >IP owner really is for the purposes of walking SNMP tables, 
> ping, telnet,
> >etc...
> >
> >Thus, moving the ownership of Mac addresses was the solution 
> found (virtual
> >Macs to be
> >more exact. :)).
> >
> >-- flavio
> >
> >
> >-----Original Message-----
> >From: Vikram [mailto:[email-masked]]
> >Sent: Thursday, April 05, 2001 10:19 AM
> >To: Badri Srinivasan S; VRRP
> >Subject: Re: Why Virtual MACs....?
> >
> >
> >The moot purpose of the virtual mac is to allow the learning 
> bridges to
> >know which lan segment the virtual router is in other than 
> letting the
> >failover to be transparent(and fast) to endhosts.
> >
> >If I missed anything please fill me in.
> >
> >tks,
> >
> >vikram
> >
> >--- Badri Srinivasan S <[email-masked]> wrote:
> > > Hi:
> > >
> > > another doubt down the line...
> > >
> > > 1. Why is it necessary that the source MAC addr of the VRRP
> > > Advertisements
> > > must me set to the Virtual MAC addr...i really dont understand the
> > > real
> > > purpose of virtual MACs...
> > >
> > > Can we solve the prob without using vMACs but only 
> virtual IPs in the
> > > following way..
> > >
> > > -->Whenever a VRRP router becomes a master, add the virtual IP
> > > address
> > > to the master's ethernet by doing a IP aliasing..
> > > -->Then let the Master do a gratutious ARP for the virtual IP addr
> > > ----
> > > real MAC combination so that the hosts update their ARP cache with
> > > this.
> > >
> > > -->so when the host wants to send a pckt to the virtual 
> IP, its wiil
> > > look into its ARP cache & get the real MAC of the Master..
> > > or else , it can make  an ARP req  for the Virtual IP 
> addr , and the
> > > Master replies with its real MAC addr & not any vMAC..
> > >
> > > so the pckts ultimately reach the Master...
> > > When the Master goes to the backup state it removes the virtual IP
> > > alias..
> > > The new Master performs the same steps whenit starts just 
> as above..
> > > i.e --> Do an IP alias for the virtual IP
> > >     --> do a gratutious ARP with virtual IP --- the new 
> Masters real
> > > MAC...
> > >
> > >
> > > if it can be implemented like this , what is the purpose 
> of Virtual
> > > MAC..
> > >
> > > please reply..
> > >
> > > thanks & Regards,
> > > Badri
> > >
> >
> >
> >=====
> >----
> >NO sense being pessimistic. It wouldnt work anyway
> >
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