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).