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Forum Replies

  1. Hi, i lost you at this part

    SW2 will put the IP packet from host 2 in an Ethernet frame that has its own MAC address as the source. It will then be forwarded to SW1. The problem here is that SW1 will never learn the MAC address of host 2.

    When H1 actually replies with icmp reply packets, wouldn’t the switches learn of H1’s mac address in their vlan/trunk ?
    I can understand the initial broadcast to get H1 to reply, but what i dont understand is why this problem persists for the entire 1000 frames.

    Do the switches (mac-address-table) only get updated with ARP pac

    ... Continue reading in our forum

  2. 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 neve

    ... Continue reading in our forum

  3. 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.


  4. 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?


  5. 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.

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