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HELP on Unknown unicast and vlans!?!?

Discussion in 'Routing & Switching' started by mentman, Sep 18, 2008.

  1. mentman

    mentman Bit Poster

    hi, im having a funny moment and cant find the answer to this!

    if you have a switch right out the box. so no mac addys in its mac add table.

    put port 3 and 4 in a new vlan, say vlan 5

    and connect host 1 to port 1 vlan 1, host 2 to port 2 vlan 1, host 3 to port 3 vlan 5, and host 4 to port 4 vlan 5.

    if host 1 sends a frame to host 4, the switch will class it as a unknown unicast so it will flood the frame out all ports except port 1.

    BUT wil the fact that port 4 host 4 is in a different vlan affect the flood? will it still get there as i know that a ping would not arrive due to the vlan seperation.

    so in essence my question is, is a flood treated the same as a broadcast when i comes to vlans?

    im studying for ccna and its starting to bug me!


    Certifications: CCNA, CCENT, ITE, 642-812 BCMSN
    WIP: 642-901 BCSI
  2. Spice_Weasel

    Spice_Weasel Kilobyte Poster

    Each vlan is a seperate broadcast domain - traffic entering on a port in a vlan (vlan 1 in your example, above) will only be able to leave on ports that are members of vlan 1. In order to reach hosts on vlan 5 some device must route or bridge between the seperate vlans.

    Using your example, imagine host 1 has an ip address of /24 and host 5 an ip address of /24. If host 1 wants to send a packet to host 5, the first thing it does is check its arp cache for a mac address for If no mac address is found it will broadcast to find the mac address for The switch will receive the broadcast and forward it out all ports in vlan 1, except port 1. Host 5 will never see the arp request since it is on vlan 5.

    Even if you added a static mac address entry on host 1, the switch will still not send packets from host 1 to host 5, even though the mac address is correct, since the switch maintains a seperate table for each vlan.

    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP

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