Visibility: Open to anyone

    i set up a monitoring session between a 3550 and a 2960 to capture dot1d activity using wireshark... the interface of the pc is connected to g0/1 of the 2960 and set as the destination for monitoring from the g0/21 port on the 2960...the 3550 and 2960 are connected through fiber between g0/21 (2960) and g0/11 (3550)



    sw2960#sh run int g0/21

    Building configuration...


    Current configuration : 60 bytes


    interface GigabitEthernet0/21

    switchport mode trunk




    sw3550#sh run int g0/11

    Building configuration...


    Current configuration : 98 bytes


    interface GigabitEthernet0/11

    switchport trunk encapsulation dot1q

    switchport mode trunk



    nothing special there on the trunk... monitoring is set up on the 2960 as such:


    sw2960#sh run

    begin monitor

    monitor session 1 source interface Gi0/21

    monitor session 1 destination interface Gi0/1 encapsulation replicate



    sw2960(config)#monitor session 1 source int g0/21 both


    notice i used the "both" command in the argument for int g0/21... both is the default...


    do sh int g0/1 to ensure that it monitors:


    sw2960#sh int g0/1

    GigabitEthernet0/1 is up, line protocol is down (monitoring)

    Hardware is Gigabit Ethernet, address is 001e.79cd.9281 (bia 001e.79cd.9281)


    then i changed the root to be vlan 1 on the 2960... let's prove it through wireshark...


    in flags a topology change is noted...

    the root identifier is now that of the 2960...

    the extension id is 1 indicating vlan 1

    the port identifier is is 0x8015 15hex = 21 decimal or port 21


    sw2960#sh spann




    Spanning tree enabled protocol ieee

    Root ID Priority 20481

    Address 001e.79cd.9280

    This bridge is the root

    Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec


    Bridge ID Priority 20481 (priority 20480 sys-id-ext 1)

    Address 001e.79cd.9280

    Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

    Aging Time 15


    Interface Role Sts Cost Prio.Nbr Type

    ---------------- ---- --- --------- -------- --------------------------------

    Gi0/21 Desg FWD 4 128.21 P2p


    if you need a hex refresher go here: