spanning-tree issue

Answered Question
Aug 4th, 2008

Hi, I have a problem with spanning-tree protocol on two interface on 2801.

Two interface in bridge are connected on the same switch but are all two in forwarding state.

My conf:

interface FastEthernet0/0

description nn

no ip address

duplex auto

speed auto

!

interface FastEthernet0/0.1

description LAN_STI_if1

encapsulation dot1Q 1 native

bridge-group 1

bridge-group 1 path-cost 150

bridge-group 1 priority 16

!

interface FastEthernet0/0.2

description SGRT_if1

encapsulation dot1Q 2

bridge-group 2

bridge-group 2 path-cost 150

bridge-group 2 priority 16

!

interface FastEthernet0/0.3

description SCC_if1

encapsulation dot1Q 3

bridge-group 3

bridge-group 3 path-cost 150

bridge-group 3 priority 16

!

interface FastEthernet0/1

no ip address

duplex auto

speed auto

!

interface FastEthernet0/1.1

description LAN_STI_if2

encapsulation dot1Q 1 native

bridge-group 1

bridge-group 1 path-cost 100

bridge-group 1 priority 8

!

interface FastEthernet0/1.2

description SGRT_if2

encapsulation dot1Q 2

bridge-group 2

bridge-group 2 path-cost 100

bridge-group 2 priority 8

!

interface FastEthernet0/1.3

description SCC_if2

encapsulation dot1Q 3

bridge-group 3

bridge-group 3 path-cost 100

bridge-group 3 priority 8

!

bridge 1 protocol ieee

bridge 1 route ip

bridge 2 protocol ieee

bridge 2 route ip

bridge 3 protocol ieee

bridge 3 route ip

Now the log of sh spanning-tree:

gw-boav-002#sh spAnning-tree 3

Bridge group 3 is executing the ieee compatible Spanning Tree protocol

Bridge Identifier has priority 32768, address 0000.0c4b.c400

Configured hello time 2, max age 20, forward delay 15

We are the root of the spanning tree

Topology change flag not set, detected flag not set

Number of topology changes 11 last change occurred 00:38:06 ago

from FastEthernet0/0.3

Times: hold 1, topology change 35, notification 2

hello 2, max age 20, forward delay 15

Timers: hello 0, topology change 0, notification 0, aging 300

Port 40 (FastEthernet0/0.3) of Bridge group 3 is forwarding

Port path cost 150, Port priority 16, Port Identifier 16.40.

Designated root has priority 32768, address 0000.0c4b.c400

Designated bridge has priority 32768, address 0000.0c4b.c400

Designated port id is 16.40, designated path cost 0

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

BPDU: sent 253064, received 66

Port 43 (FastEthernet0/1.3) of Bridge group 3 is forwarding

Port path cost 100, Port priority 8, Port Identifier 8.43.

Designated root has priority 32768, address 0000.0c4b.c400

Designated bridge has priority 32768, address 0000.0c4b.c400

Designated port id is 8.43, designated path cost 0

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

BPDU: sent 171659, received 42

gw-boav-002#

In attach all log of sh spanning-tree, as you can see other if are in blocking state correctly.

Anyone have some ideas?

Thanks Enrico.

Attachment: 
I have this problem too.
0 votes
Correct Answer by lee.reade about 8 years 3 months ago

Hi,

The reason both ports are forwarding is because;

"We are the root of the spanning tree "

Your router is the STP root for vlan 3, whereas vlans 1 & 2, are using an external STP root, as shown by

"Current root has priority 24578, address 001e.132b.6080"

So whatever device this is, probably the switch that the router is connected to either has its STP root for vlan3 higher than the router does, or perhaps doesnt even have vlan 3 configured.

Check the STP for vlan 3 on the device upstream from router.

HTH

LR

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Correct Answer
lee.reade Mon, 08/04/2008 - 02:22

Hi,

The reason both ports are forwarding is because;

"We are the root of the spanning tree "

Your router is the STP root for vlan 3, whereas vlans 1 & 2, are using an external STP root, as shown by

"Current root has priority 24578, address 001e.132b.6080"

So whatever device this is, probably the switch that the router is connected to either has its STP root for vlan3 higher than the router does, or perhaps doesnt even have vlan 3 configured.

Check the STP for vlan 3 on the device upstream from router.

HTH

LR

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