Question about static mac addresses on a 3750

John Blakley · ‎03-03-2009

All,

I've got a situation that involves Microsoft's cluster service. It moves a Sql server to another physical server in case of a failover. The sql server's IP address doesn't change, but the mac address changes depending on the server that it moves to.

The problem that we're having is that the arp cache isn't timing out in the switch. Microsoft says that it's because the switch isn't forwarding gratuitous arp packets. How do I enable this?

I found that I can do an "ip arp gratuitous local", but I'm not sure that's going to do what I want.

Thanks,

John

HTH, John *** Please rate all useful posts ***

Mohamad Qayoom · ‎03-03-2009

You can use "arp timeout" command on the vlan interface on the router to change the arp timeout.

Thanks,

Mohamad

John Blakley · ‎03-03-2009

Mohamad,

We've literally let these servers sit for a day, mainly because we didn't know they "moved", and it still didn't time out. I'm not sure changing the timeout value will resolve the issue.

Thanks,

John

HTH, John *** Please rate all useful posts ***

Edison Ortiz · ‎03-03-2009

ARP provides IP communication within a Layer 2 broadcast domain by mapping an IP address to a MAC address.

You can't map multiple MACs to an IP so for this to work, the server needs to release the MAC address from the switch table. This is often triggered when the device is powered down. If the device didn't release the MAC address properly, then the switch will wait until the mac-address-table aging time removes the stale entry. By default is set to 5 minutes:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/command/reference/cli1.html#wp1862251

HTH,

__

Edison.

John Blakley · ‎03-03-2009

Edison,

It doesn't work. We've let the mac address table age out, and you still can't ping the device until you manually clear arp on the switch.

Here's MS explanation:

http://support.microsoft.com/kb/244331/EN-US/

Thanks,

John

HTH, John *** Please rate all useful posts ***

Edison Ortiz · ‎03-03-2009

Gratuitous ARP is enabled by default on switches and all devices within that Vlan will receive it.

What you are unable to do with Cisco switches is Gratuitous ARP over different subnets.

I don't think that's the case here.

So, you are saying that manually clearing the ARP table fixes the problem?

If you do a show ip arp before manually clearing the table, the MAC-to-IP mapping still points to the old MAC address?

__

Edison.

John Blakley · ‎03-03-2009

Yes, but I found this article that suggests a different IOS fixes my problem:

http://supportwiki.cisco.com/ViewWiki/index.php/Host_connectivity_or_ping_issues_due_to_ARP_or_CEF_table_changes_in_Catalyst_3750_switch_stack

This one says that .SE2 resolves it, and we're running SE1.

Thanks,

John

HTH, John *** Please rate all useful posts ***

John Blakley · ‎03-03-2009

Edison,

I didn't realize that I didn't tell you something, and this may affect this also.

The vlan SVI has multiple subnets on it:

10.125.99.1 secondary

10.128.100.1 secondary

10.129.100.1 secondary

10.125.100.1 primary

The server is on the 10.125.100.0/24 subnet, and I'm thinking that it has something to do with the multiple subnets on the vlan 1 svi.

Thanks,

John

HTH, John *** Please rate all useful posts ***

Edison Ortiz · ‎03-03-2009

They are all part of the same Layer2 broadcast domain thus you aren't doing any inter-vlan switching.

I believe the article you posted may be the reason why it isn't working for you.

__

Edison.