MAC forwarding table aging on UCS 6100 Fabric Interconnects

I was recently forwarded some information on the MAC table aging process in the UCS 6100 Fabric Interconnects that I thought was very valuable to share.

Prior to this information, I was under the impression (and various documentation had confirmed) that the Fabric Interconnect never ages MAC addresses – in other words, it understands where all the MAC addresses are within the chassis/blades, and therefore has no need to age-out addresses.   In the preferred Ethernet Host Virtualizer mode, it also doesn’t learn any addresses from the uplinks, again, so no need to age a MAC address.

So what about VMware and the virtual MAC addresses that live behind the physical NICs on the blades?

Well, as it turns out, the Fabric Interconnects do age addresses, just not those assigned by UCS Manager to a physical NIC (or a vNIC on a Virtual Interface Card – aka Palo).

On UCS code releases prior to 1.1, learned address age out in 7200 seconds (120 minutes) and is not configurable.

On UCS code releases of 1.1 and later, learned addresses age out in 7200 seconds (120 minutes) by default, but can be adjusted in the LAN Uplinks Manager within UCS Manager.

Why do we care?   Well, it’s possible that if a VM (from which we’ve learned an address) has gone silent for whatever reason, we may end up purging it’s address from the forwarding table after 120 minutes… which will mean it’s unreachable from the outside world, since we’ll drop any frame that arrives on an uplink to an unknown unicast MAC address.   Only if the VM generates some outbound traffic will we re-learn the address and be able to accept traffic on the uplinks for it.

So if you have silent VMs and have trouble reaching them from the outside world, you’ll want to upgrade to the latest UCS code release and adjust the MAC aging timeout to something very high (or possibly never).

8 thoughts on “MAC forwarding table aging on UCS 6100 Fabric Interconnects”

  1. Good info, I have seen this behavior and thought it was a pinning issue or Nexus 5k issue that the 6120 was connected to. Do you know the command to disable MAC aging?

  2. To disable MAC aging, go into the LAN Uplinks Manager within UCS Manager. You can find it by going to the Equipment tab, then selecting one of the Fabric Interconnects.

    In the content pane, under the Actions column, you should see a link for “LAN Uplinks Manager”.

    Once inside the LAN Uplinks Manager, select the “Global Policies” tab. From there, you can adjust the MAC-aging policy.

  3. i can configure the vmware using vsphere management to notify upstream switches about the max on the vm’s. isn’t it?

  4. I need to enable failover on UCSM using the Qlogic 72KR-Q Gen2 Card. I’ve been told that you can not use the failover like in the palo cards or malon cards. Can you please go over step by step how to ensure failover for FicA & FicB using the Gen2 Cards. I was considering creating 2 profiles vEth0 (with FicA primary) and vEth1 (with FicB as Primary). Also anyway to use the Bridge of 2 physical Nics on a B200 M2 blade (bare metal) OS W2K8R2 and have the UCSM see it as 1 nic. The goal in doing so would be both physical failover of the Nics but i also need failover of the Fics to the Fiber Interconnect switch. Hope I stated this clearly. Any feedback greatly appriciated. Rob

    1. Unfortunately, the M72KR-x cards cannot perform hardware failover of the vNICs the way the M71KR-x and M81KR can. You will need to perform any kind of teaming at the OS level… this will be complicated by the fact that your two vNICs are connected to separate L2 switching, without any kind of VPC between them. I’m not experienced in teaming NICs for bare metal Windows insatllations, so I can’t offer any advice on that front.

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