A port on the root switch went into blocking state

[chandra_rc16]

A port on the root switch went into blocking state in my network with all the default values.

I was just building a topology to practice STP and VLANs in that process i just established the connections as i like, but i observed one thing here is

the port Gig1/2 on the root switch went into blocking state.

As far as i know all the ports on the root switch will be designated forwarding.

Here is my topology and o/p of show spanning-tree from root switch. I haven't created any configuration just made connections between switches that's it.

Switch0>show spa

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     0004.9A8C.3415

             This bridge is the root

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

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     0004.9A8C.3415

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

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

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

Gi1/2            Altn BLK 19        128.26   Shr

Fa0/1            Desg FWD 19        128.1    Shr

Fa0/2            Desg FWD 19        128.2    Shr

I'm still trying to trace out how it happened

Thanks in advance.

Regards,

Chandu

[chandra_rc16]

[milan.kulik]

Hi,

if the root switch is connected to a hub via two ports (Fa0/2 and Gi1/2), it's the same like it had those two port connected each to the other via a cross cable.

So it has to break the loop by putting one of the ports to a blocking stage!

Best regards,

Milan

[chandra_rc16]

Thanks milan, but may i know why it didn't block its FastEthernet port.

REgards,

Chandu

[riyaaz000]

Hi Chandra,

I guess in packet tracer the link costs are the same, so to break the tie, the highest port number will be blocked.

Hope that helps

Regards

[chandra_rc16]

But before we compare with the port numbers.. there is one more thing ahead right i.e, cost.

[chandra_rc16]

I'm saying about Gig bit  port. It has the lowest cost.

[riyaaz000]

In packet tracer Fe and Ge have the same cost, I guess its 19.

[chandra_rc16]

No it is not like that, because i tried connecting two switches with two of there ports one with gig port and another with FE port. The gig port goes to forwarding and FE port goes into blocking.

This means that they don't have same cost in PT.

[riyaaz000 · Accepted solution]

yes I agree, because this time you are using two switches.

let me explain:

1. In your first scenario you have linked Fe0/1 and Ge1/1 through a hub. Hubs are not intelligent like switches. Using the hub in this case, is like you have taken the Fe0/1 cable and connected it directly to the Ge1/1 port. Now, Fe cost is 19 and Ge cost is 4, what cost would the link be??? of course 19 because it is a high number an is the bottleneck. So the link cost is 19, now the switch send out a BPDU from Fe0/1 and baaam!! it receives the same BPDU on port Ge1/1, the switch detects a loop and the only way to break the loop is to block the highest port number.

2. In your second scenario you have used two switches, in this case there is two links, because switches are intelligent and it knows how many ports it have. So Ge is 4 and Fe is 19, which is best... I guess you got the answer 4, STP will choose the lowest number, so Fe is block.

Read this:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml