VLAN / L2 / L3 / trunk questions.

Discussion in 'Cisco' started by Joel M. Baldwin, Nov 28, 2003.

  1. In the course of figuring out Layer2 vs. Layer3 and VLANs I've
    come up with the following questions.

    | Cisco 2950 L2 switch S2 |
    ------------------------- -------------
    | Cisco 3570 L3 switch S1 |----------| file server |
    ------------------------- -------------
    | |
    | |
    | |
    | |
    | -------------------------
    | | Cisco 2950 L2 switch S3 |
    | -------------------------
    | Cisco 2950 L2 switch S4 |

    VLAN 10 has ports on switches S2 and S3
    VLAN 20 has ports on switches S2 and S4
    i.e. S2 has ports on both VLANs
    The port between S1 and S2 has to be defined as a trunk because it
    has more than one VLAN.
    The ports between S1 and S3, and S1 and S4 don't HAVE to be defined
    as a trunks because they only have a single VLAN, but SHOULD be
    defined as trunks.
    Switch S1 does the routing between the two subnets
    The S1 switch has the port to the file server setup as a trunk.
    The file server has a NIC doing 802.1q trunking and has IP addresses
    defined for both of the two subnets, and VLANS defined on the interface.

    Questions given the above configuration.

    1.) Is the above configuration correct?

    2.) If a packet goes between two machines in the same VLAN, and both
    machines are attached to the same switch, does that packet get transmitted
    out of that switch on the trunk line to the other switches?

    i.e. Packet goes from port 2 to port 3 on S4
    Does that packet also get set up the trunk to S1 and thus to S2 and S3?

    3.) If port 2 on S4 is transferring a file from the file server, is S1
    smart enough to not send those packets out the trunk lines to S2 and S3?

    i.e. Is there any intelligence as to where packets in a VLAN are
    transmitted so as to reduce unneeded traffic?
    Joel M. Baldwin, Nov 28, 2003
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  2. Joel M. Baldwin

    PES Guest

    From my brief reading, it appears OK.

    Only if it is a broadcast or multicast, would it go through a trunk to
    another/all switches.
    This doesn't have that much to do with the server as it does the 802.1q
    trunk protocol and the switch architecture. The server should only send
    traffic to the port with the correct destination IP address/mac address on
    the dot.q interface that it received the request. The switches will send
    all unicast traffic only through the required switches and router as

    One other note that does not really apply but sounds a lot like your
    question is VTP Pruning. With it on, it prunes all vlans from a trunk port
    if there are no ports on the destination switch in a particular vlan. This
    prevents the unnecessary propogation of multicast and broadcast traffic.
    PES, Nov 29, 2003
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