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I have a situation where I need to place a static MAC on a switch as the server in question does not generate any outbound traffic. This static MAC seems as if it will work for hosts on the same switch but it will not likely work for hosts on other switches. Besides putting the static MAC in every switch is there a way to simply propagate static MAC addresses so the other switches can simply dynamically learn the MAC across the trunks just like any other. My topology is as follows.

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Host 1 should have no problem communicating with the static MAC at L2 but Host 2 will likely have problems.

For the sake of discussion let's just assume Cisco Nexus switches.

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  • This actually makes no sense as the server will need to respond to ARP, and the switches will learn the server's MAC address from the ARP response. Also, if you are using TCP on the server, then it will need to reply with TCP setup and acknowledgment messages, and those frames will also populate the switch MAC address tables.
    – Ron Maupin ♦
    Jul 29, 2020 at 21:52
  • By the way, any traffic sent to the server by Host 2 will reach the server because the switch connected to the server has the server in its MAC address table, whether you manually put it there, or it was automatically put there. The problem you have is that any host sending to the server will use ARP (IPv4) or NDP (IPv6) to relate the IP address to the MAC address, and the switches will see the frames with ARP/NDP replies from the server and put that in their MAC address tables.
    – Ron Maupin ♦
    Jul 29, 2020 at 21:56
  • @RonMaupin - This is all layer2. The interface in question on the green box will not hold an IP hence why it doesn't really generate frames. It will ingest packets that are destined to it's MAC. The goal is getting the packets there.
    – o8uijoipjasdfjiloasdfjl
    Jul 29, 2020 at 22:57
  • If it does noit have a network address, then you are sending frames, not packets, and it does not matter if the switches have the MAC address or not, the frames will get to it. Remember that switches flood unknown unicast frames to all interfaces.
    – Ron Maupin ♦
    Jul 29, 2020 at 23:00
  • On the Nexus platform take a look at MAC redirection features. If you want it pre-managed across multiple switches for you then check out Nexus Data Broker.
    – rnxrx
    Jul 31, 2020 at 1:53

3 Answers 3

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Based on your comments, you do not seem to understand that any frames sent to that MAC address will arrive on that switch interface, regardless of whether or not the switch has the MAC address in its MAC address table.

Switches will flood unknown unicast MAC addresses (those not in its MAC address table) to every switch interface A frame sent by Host2 will be delivered to the server, but it will do that by being sent to every switch interface, much like a broadcast or multicast frame.

What you want to do requires something connected to the server's switch interface to send a frame into the switches, but it will need to happen on a regular basis because entries in a switch MAC address tables are dynamic and time out. This prevents dead hosts from using switch resources, and it facilitates moving a host from one interface to another on the same of different switch.

In any case, a switch does not need an entry in its MAC address table to be able to get a frame to the destination; it is just much more efficient to do it that way.

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Switches learn MAC addresses from the frames they receive. If you could find a way to flood a frame with the MAC of your server, every switch would learn it.

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  • Ya that's my question. Can the switch flood 'learned MACs' to other switches.
    – o8uijoipjasdfjiloasdfjl
    Jul 29, 2020 at 20:33
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    You need some other device. Switches don’t generate frames.
    – Ron Trunk
    Jul 29, 2020 at 21:17
  • Not in a traditional switched network. In an "SDN", where there's a central, common controller, it may be possible.
    – Ricky
    Jul 30, 2020 at 2:07
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Unless you've disabled flooding of unknown unicast -- may not be an option with your switches -- the traffic will still get there, but it will go everywhere else, too. To avoid this defacto broadcast storm, the receiver needs to send a broadcast frame periodically so every switch will see it and know where it is. Doing this manually -- "static MAC" -- is such an unmanageable pain in the a** I won't go there.

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  • A broadcast storm is something different - when broadcasts circulate and accumulate through a bridge loop. Unknown unicast MAC addresses just cause broadcast-like flooding which is undesirable and may potentially even be harmful, but it's not remotely as destructive as a storm.
    – Zac67 ♦
    Jul 30, 2020 at 8:56
  • A broadcast storm is an abnormally high number of broadcast packets within a short period of time. It has nothing to do with loops, 'tho loops are usually what causes them. This can be a "storm" if the traffic rate is high enough. But because it's not broadcast, broadcast storm suppression will not kick in.
    – Ricky
    Jul 30, 2020 at 20:08

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