Root port

altjx · October 2011

Ok, I have 3 switches in a triangular shape. S1 at the top, S2 at the lower left, S3 at the bottom right. They're all connected to each using a rollover cable. I'm studying for CCNA and I just ran across the Designated Ports section.

Ok, S2 is the root switch. Here is the output of show-spanning tree on S1.

S1#sh span
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     0060.479A.2816
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     00D0.BA78.5980
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Root FWD 19        128.1    P2p
Fa0/2            Altn BLK 19        16.2     P2p

Why is Fa0/1 the root port when the costs are the same, but FA0/2 has a lower priority?

I think I may be misunderstanding something.

Also, S3 clearly has lower priority to the root than S1, so why isn't S1 using the interface between S1-S3 as the root port?

S3(config)#do sh span
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0060.479A.2816
             Cost        19
             Port        1(FastEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0090.21CC.9485
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1            Root LSN 19        16.1     P2p
Fa0/2            Desg FWD 19        16.2     P2p

altjx · October 2011

sorry, you misunderstood. I know how to get the root switch. I am speaking specifically of the root port between the switches. Why is S1 not using Fa0/2 as root port?

tex3030 · October 2011

The cost of 19 is being displayed on SW1 is because both of these ports are 100Mb ports.
SW1 -> root
fa0/1= 19
fa0/2= 19 + 19 = 38 (also have to add in the cost from the link of sw3-sw2)

it is the same calculation for sw2.

I don't understand why the priority for the root bridge is different on sw1 & sw3. 24577 on sw1, 32769 on sw3, both have the same mac.

altjx · October 2011

tex3030 wrote: »

The cost of 19 is being displayed on SW1 is because both of these ports are 100Mb ports.
SW1 -> root
fa0/1= 19
fa0/2= 19 + 19 = 38 (also have to add in the cost from the link of sw3-sw2)

it is the same calculation for sw2.

I don't understand why the priority for the root bridge is different on sw1 & sw3. 24577 on sw1, 32769 on sw3, both have the same mac.

Ah, I didn't even think about that (19 + 19 = 3

. Forgot about that.

That clears everything up. I can't change the interface cost in Cisco Packet Tracer though, so blah. >_<

angel.oa · October 2011

altjx wrote: »

Ok, I have 3 switches in a triangular shape. S1 at the top, S2 at the lower left, S3 at the bottom right. They're all connected to each using a rollover cable. I'm studying for CCNA and I just ran across the Designated Ports section.[/code]

i think you meant crossover cables

altjx · October 2011

angel.oa wrote: »

i think you meant crossover cables

Ah, yeah, I'm using crossover cables. Typo.

Root port

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