HSRP'ing two 3640's connected to a Catalyst 4003.

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by plethor » Wed, 19 Dec 2001 10:39:21



Hello.

Quick question about this setup...  I'm trying to figure out how the
switch isn't going to be confused when the standby router becomes
active.  Seems to me that the switch will continue to think that the
virtual MAC address is on whatever port the old active router is
connected to; hence, I'd be screwed until the ARP entry expires.

I'm assuming this isn't the case, so I'm wondering if anyone knows how
this is avoided.  When the standby router becomes active, does it, by
chance, send out a gratuitous ARP broadcast to announce itself?  

Thanks!

 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by c1sc0k1 » Wed, 19 Dec 2001 21:33:13


http://www.cisco.com/warp/public/cc/so/cuso/epso/entdes/hsrp_wp.htm

The K1d


Quote:> Hello.

> Quick question about this setup...  I'm trying to figure out how the
> switch isn't going to be confused when the standby router becomes
> active.  Seems to me that the switch will continue to think that the
> virtual MAC address is on whatever port the old active router is
> connected to; hence, I'd be screwed until the ARP entry expires.

> I'm assuming this isn't the case, so I'm wondering if anyone knows how
> this is avoided.  When the standby router becomes active, does it, by
> chance, send out a gratuitous ARP broadcast to announce itself?

> Thanks!


 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by Mortimer Mou » Wed, 19 Dec 2001 21:45:55


Quote:> When the standby router becomes active, does it, by
> chance, send out a gratuitous ARP broadcast to announce itself?  

Exactly so.
 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by plethor » Thu, 20 Dec 2001 02:39:56


Hello.

I read that, but I don't see how it answers my question.  Am I missing
something?



>http://www.cisco.com/warp/public/cc/so/cuso/epso/entdes/hsrp_wp.htm

>The K1d



>> Hello.

>> Quick question about this setup...  I'm trying to figure out how the
>> switch isn't going to be confused when the standby router becomes
>> active.  Seems to me that the switch will continue to think that the
>> virtual MAC address is on whatever port the old active router is
>> connected to; hence, I'd be screwed until the ARP entry expires.

>> I'm assuming this isn't the case, so I'm wondering if anyone knows how
>> this is avoided.  When the standby router becomes active, does it, by
>> chance, send out a gratuitous ARP broadcast to announce itself?

>> Thanks!

 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by c1sc0k1 » Thu, 20 Dec 2001 03:03:10


On re-reading your question that link might not address how the switch
handles the change and I apologize for that.  When the router interface goes
down the switch port also goes down and the arp entry is removed.  The
switch will arp for the virtual mac again and start talking on the other
port.

The k1d


> Hello.

> I read that, but I don't see how it answers my question.  Am I missing
> something?



> >http://www.cisco.com/warp/public/cc/so/cuso/epso/entdes/hsrp_wp.htm

> >The K1d



> >> Hello.

> >> Quick question about this setup...  I'm trying to figure out how the
> >> switch isn't going to be confused when the standby router becomes
> >> active.  Seems to me that the switch will continue to think that the
> >> virtual MAC address is on whatever port the old active router is
> >> connected to; hence, I'd be screwed until the ARP entry expires.

> >> I'm assuming this isn't the case, so I'm wondering if anyone knows how
> >> this is avoided.  When the standby router becomes active, does it, by
> >> chance, send out a gratuitous ARP broadcast to announce itself?

> >> Thanks!

 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by Barry Margoli » Thu, 20 Dec 2001 04:44:04




>On re-reading your question that link might not address how the switch
>handles the change and I apologize for that.  When the router interface goes
>down the switch port also goes down and the arp entry is removed.  The
>switch will arp for the virtual mac again and start talking on the other
>port.

Switches don't usually send ARP queries to build their CAM tables.  They
just look at source MAC addresses of traffic that passes through them.

And the router interface going down shouldn't cause the switch port to go
down.  There could be other devices on that switch port beside the router,
right?

--

Genuity, Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.

 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by Rainer Nage » Thu, 20 Dec 2001 06:25:02


On Tue, 18 Dec 2001 01:39:21 GMT,


> Quick question about this setup...  I'm trying to figure out how the
> switch isn't going to be confused when the standby router becomes
> active.  Seems to me that the switch will continue to think that the
> virtual MAC address is on whatever port the old active router is
> connected to; hence, I'd be screwed until the ARP entry expires.

> I'm assuming this isn't the case, so I'm wondering if anyone knows how
> this is avoided.  When the standby router becomes active, does it, by
> chance, send out a gratuitous ARP broadcast to announce itself?  

The switch learns the MAC on the new port by looking on the sorce MAC
address of received packets. As soon as the new active router (after a
takeover) send packets with the virtual MAC address, the switch will
notice the MAC address behind this port and will forward packets with
this MAC address as destination to this (new) port.

Ciao
--
Rainer Nagel

Duesseldorfer Linux User Group - http://www.dlug.de

 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by c1sc0k1 » Thu, 20 Dec 2001 06:57:32


Thanks for clarifying that!

The k1d




> >On re-reading your question that link might not address how the switch
> >handles the change and I apologize for that.  When the router interface
goes
> >down the switch port also goes down and the arp entry is removed.  The
> >switch will arp for the virtual mac again and start talking on the other
> >port.

> Switches don't usually send ARP queries to build their CAM tables.  They
> just look at source MAC addresses of traffic that passes through them.

> And the router interface going down shouldn't cause the switch port to go
> down.  There could be other devices on that switch port beside the router,
> right?

> --

> Genuity, Woburn, MA
> *** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to
newsgroups.
> Please DON'T copy followups to me -- I'll assume it wasn't posted to the

group.
 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by Harri Suomalaine » Thu, 20 Dec 2001 18:34:50



Quote:> On re-reading your question that link might not address how the switch
> handles the change and I apologize for that.  When the router interface
goes
> down the switch port also goes down and the arp entry is removed.

HSRP hello packects are sent from the active router as follows:
-Source MAC = HSRP virtual MAC
-Sourtce IP = real router IP
-Destination = Multicast 224.0.0.2

Therefore, when a router becomes active it will start sourcing packects
from HSRP MAC and the switch will update it's CAM table to include the
HSRP address in the new port. (Just like the mac had moved to another port).

Port will not always go down when router becomes passive and the other
router active. The idea is based on learning where the hello packets came.
--
Harri

 
 
 

HSRP'ing two 3640's connected to a Catalyst 4003.

Post by plethor » Fri, 21 Dec 2001 02:45:30


On Wed, 19 Dec 2001 09:34:50 GMT, "Harri Suomalainen"




>> On re-reading your question that link might not address how the switch
>> handles the change and I apologize for that.  When the router interface
>goes
>> down the switch port also goes down and the arp entry is removed.

>HSRP hello packects are sent from the active router as follows:
>-Source MAC = HSRP virtual MAC
>-Sourtce IP = real router IP
>-Destination = Multicast 224.0.0.2

>Therefore, when a router becomes active it will start sourcing packects
>from HSRP MAC and the switch will update it's CAM table to include the
>HSRP address in the new port. (Just like the mac had moved to another port).

This doesn't seem to be the case as per this document:
http://www.cisco.com/warp/public/473/62.shtml

"Routers running HSRP communicate HSRP information between each other,
via HSRP hello packets. These packets are sent to the destination IP
multicast address 224.0.0.2 (reserved multicast address used to
communicate to all routers) on User Datagram Protocol (UDP) port 1985.
These hello packets are sourced with the configured IP address on the
interface and the burned-in MAC address of the interface, as opposed
to the HSRP or virtual IP and MAC address. This use of source
addressing is necessary so that HSRP routers can correctly identify
each other.

The only exception to the above behavior is for Cisco 2500, 4000, and
4500 routers. These routers have Ethernet hardware that only
recognizes a single MAC address. Therefore, these routers will use the
HSRP MAC address when they are the active router, and their burned-in
address for HSRP hello packets."

Strangely enough, however, the RFC says this:

"If and only if the router is in the Active state, the router MUST use
the group's virtual MAC address as the source MAC address for its
Hello messages.  This is necessary in order to allow learning bridges
to be able to determine which LAN segment the virtual MAC address
currently belongs to."

My guess is that what Cisco's site says is accurate, primarily because
the RFC also states that a router must send out a gratuitous ARP
broadcast upon transitioning to the active state.  Seems to me this
would negate the need for the Hello messages to use the virtual MAC
address as the source.  

 
 
 

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