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Created on: Apr 19, 2012 7:10 AM by arteq - Last Modified:  Apr 26, 2012 11:42 AM by arteq

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:

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