Stacking 3750G switches and SVI

Unanswered Question
May 14th, 2008

I have a general question about stacking 3750G switches.

Any SVIs configured on the stack would actually live on the master, true?

If the master fails, a new master is elected but the MAC for the SVI would change as a result.

Can using a single legged HSRP configuration speed convergence should the master fail?

It would be single legged since we are on a stack, not two or more individual 3750s.

Example:

interface Vlan9

ip address 10.100.9.10 255.255.255.0

standby ip 10.100.9.1

end

So my gateway for vlan 9 would be the standby address.

Is this the best way to do it?

I have this problem too.
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Edison Ortiz Wed, 05/14/2008 - 15:03

Any SVIs configured on the stack would actually live on the master, true?

The stack master contains the saved and running configuration files for the switch stack. The configuration files include the system-level settings for the switch stack and the interface-level settings for each stack member. Each stack member has a current copy of these files for back-up purposes.

Can using a single legged HSRP configuration speed convergence should the master fail?

You gain nothing with this design other than burning an IP address on that subnet.

If you aren't planning to have another device with HSRP, I recommend applying the 10.100.9.1 IP address to the physical interface.

HTH,

__

Edison.

Joseph W. Doherty Thu, 05/15/2008 - 03:52

Any SVIs configured on the stack would actually live on the master, true?

No, it should be as Edison notes.

If the master fails, a new master is elected but the MAC for the SVI would change as a result.

It does by default, although there's a later "stack-mac persistent timer" feature that can be set zero to keep the MAC from changing until complete stack reload.

Can using a single legged HSRP configuration speed convergence should the master fail?

Assuming the stack isn't configured to avoid MAC change, I recall when the MAC changes the stack does something (gratuitous ARP?) to try to inform clients that their gateway IP's MAC is different. (This is assuming you're also routing on the stack.) I would think using HSRP would preserve the gateway MAC and help avoid clients having a "stale" ARP entry for their gateway MAC address if they don't otherwise notice the gateway MAC change.

firemtngems Thu, 05/15/2008 - 07:44

Sorry I wasn't clear when I said the SVI lives on the master. What I meant was the MAC used for that SVI would come off the master.

I am not familiar with stack commands to help preserve the MAC address, that is why I brought up the idea of using HSRP. I'll look into the stack features to preserve the MAC (or just use HSRP).

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