08-21-2008 09:36 AM - edited 03-06-2019 12:56 AM
I am experimenting STP in a lab and got confused by the output of the show spanningtree command on the second switch. I have the 2 switches connected via Gig interfaces and have VLANs on both switches configured. The output from SW1 is clear but not the one from SW2. In the output, it shows on VLAN1 and VLAN51 different MAC addresses on the Priority Root ID and Bridge ID and I am confused as why these two MAC addresses are different. The second thing I am confused about is VLAN39 which says âThis is the rootâ where the other switch is the Root Bridge!!! Attached are the two outputs from both switches.
Thanks in advance,
sK
08-21-2008 09:43 AM
vlan 39 is not on the first switch, onyl on your second switch. so there for it will be the root for the second switch.
08-21-2008 09:48 AM
Hello Sadik,
your switches are using the extension-id
Bridge ID Priority 32807 (priority 32768 sys-id-ext 39)
Address 001d.456e.dd80
in Vlan X Priority = Base_Pri +VlanX#
the base priorities are usually in steps of 4096 to accomodate all the possible Vlan-id values as per 802.1Q standard.
this feature is also called MAC address reduction: instead of using a different MAC for each Vlan they use a single MAC and modify priority.
This is thought to help troubleshooting.
2) are you sure that vlan39 is permitted on the link between switches ?
is it a trunk permitting vlans 1 and 39 ?
I think gi0/4 is permitting vlan 1, 51 on sw1 not vlan39
Hope to help
Giuseppe
08-21-2008 10:26 AM
Thanks for the prompt response. I am still not clear on why there two different MAC addresses on VLAN1 and VLAN51 on SW2!
Here is the trunking status:
sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/48 on 802.1q trunking 1
Gi0/1 on 802.1q trunking 1
Gi0/4 on 802.1q trunking 1
Port Vlans allowed on trunk
Fa0/48 1-4094
Gi0/1 1-4094
Gi0/4 1-4094
Port Vlans allowed and active in management domain
Fa0/48 1,39,51,56
Gi0/1 1,39,51,56
Gi0/4 1,39,51,56
Port Vlans in spanning tree forwarding state and not pruned
Fa0/48 1,39,51,56
Gi0/1 39
Gi0/4 1,39,51,56
08-21-2008 10:47 AM
Hello Sai,
let's look at this sample from your attachment:
VLAN0051
Spanning tree enabled protocol ieee
Root ID Priority 32819
Address 0015.c663.4f00
Cost 4
Port 28 (GigabitEthernet0/4)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32819 (priority 32768 sys-id-ext 51)
Address 001d.456e.dd80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
The reason you see two MAC addresses is:
root bridge : Sw'1 MAC = 0015.c663.4f00
bridge id = local switch MAC = 001d.456e.dd80
this is true for vlans : 1, 51
for vlan 39 you have
root bridge = this bridge
Hope to help
Giuseppe
08-21-2008 12:31 PM
It is clear now. Thanks so much!!!
sK
Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: