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Forum Replies

  1. Hi Chia,

    Switches will learn source MAC addresses from any Ethernet frame. The problem here, is that SW2 is unable to learn H1’s MAC address and SW1 is unable to learn H2’s MAC address.

    When H1 replies with an ICMP reply, it will send it to SW1 (its default gateway) in VLAN 10.

    SW1 receives it in VLAN 10, routes it and decides to forward it to VLAN 20. It builds a new Ethernet frame with its own MAC address as the source.

    That’s the key to this problem…SW2 will never learn the source MAC address of H1 since SW1 inserts its own MAC address as the source. It never sees the MAC address of H1. The only time it was able to learn it was the first ARP, since that is a broadcast frame…

    Does that make sense?


  2. Hi Stuart,

    This situation could also occur with HSRP yes. Cisco has a good example for this:

    HSRP Assymetric Routing

    Having only one SVI per switch would solve the problem but it’s not the underlying problem here :slight_smile: It’s fine to have multiple SVI interfaces on your switches, if you use one switch as the default gateway for both VLANs then you wouldn’t have this problem.


  3. Hi Rene,

    Thank you for the article. But I have a question. If I use SW1 multilayer switch as a gateway for all vlans, and sw2 as a L2 switch will the problem occur again? I think it will occur for the traffic from H2 to H1 because, SW2 will not learn the mac address of H1. Is it true ?If it is true, then for the design should we use just 1 multilayer switch and connect all hosts to it to avoid flooding?


  4. Aaahh!! That is an intense topic.
    Is it possible in a network/ LAN to have two default gateways? I am not sure how much stupid is this question!!! Just came to mind if redundancy can give a better result.

  5. Excellent explanation. I was thinking whole day and your answer helped me to realize the difference between an ARP message (inside Ethernet Frame) and Ethernet Frame.
    What currently in my mind is - Why the switch doesn’t learn MAC address from the ARP table if it is not present in MAC table? Too many confusion in this topic… !!!

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