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network cisco ccna gns3 certification arteq

network cisco ccna gns3 certification arteq
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Tuesday, April 17, 2012

more bpdu analysis/monitoring...


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
end


sw3550#sh run int g0/11
Building configuration...

Current configuration : 98 bytes
!
interface GigabitEthernet0/11
switchport trunk encapsulation dot1q
switchport mode trunk
end

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
end

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


VLAN0001
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:
http://insearchofthecert.blogspot.com/search/label/hexadecimal

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