stp question

[henrybb]

Network topo is attached. Pls see it.

My question is :

After I connected f0/14 and f0/16 which is belong to different vlan,why pc1 still can't communicate with pc2? And why root id of vlan 2 is different with root id of vlan 5 ?

In my opinion,root id of vlan2 and vlan 5 will be same after I interconnect vlan2 and vlan5.

any comment is appreciated.thanks!

[henrybb]

l2_switch#sh spanning-tree vlan 2

VLAN0002

Spanning tree enabled protocol ieee

Root ID Priority 32770

Address 0014.6acb.bd80

Cost 3019

Port 9 (FastEthernet0/9)

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

Bridge ID Priority 49154 (priority 49152 sys-id-ext 2)

Address 000d.290c.ca00

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

Aging Time 300

Uplinkfast enabled

Interface Role Sts Cost Prio.Nbr Type

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

Fa0/9 Root FWD 3019 128.9 P2p

Fa0/14 Desg FWD 3019 128.14 P2p

Fa0/16 Back BLK 3019 128.16 P2p

Fa0/21 Altn BLK 3019 128.21 P2p

l2_switch#sh spanning-tree vlan 5

VLAN0005

Spanning tree enabled protocol ieee

Root ID Priority 32773

Address 0014.6acb.bd80

Cost 3019

Port 9 (FastEthernet0/9)

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

Bridge ID Priority 49157 (priority 49152 sys-id-ext 5)

Address 000d.290c.ca00

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

Aging Time 300

Uplinkfast enabled

Interface Role Sts Cost Prio.Nbr Type

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

Fa0/9 Root FWD 3019 128.9 P2p

Fa0/14 Desg FWD 3019 128.14 P2p

Fa0/16 Back BLK 3019 128.16 P2p

Fa0/21 Altn BLK 3019 128.21 P2p

[tdrais]

This is a very bad thing to do but I think what happens is that spanning tree will block fa 0/16 for both vlans. You have a loop and it has to block someplace. Since it is blocking the connection between the vlans the pc can't talk. If you would move the root to the layer 2 switch it may block one of the trunks and it may work. Not that I recommend you do that.

[Francois Tallet]

Actually, if the two ports were in different vlans, they would both be forwarding. One would be a root port and the other a designated port. My guess is that the ports that you connected together are actually trunking, which means that you did not connect vlan 2 to vlan 5 but vlan 2 to vlan 2 and vlan 5 to vlan 5 on the same box;-)

How I'm guessing that? Because one port is backup (which means that it is blocked receiving bpdus from the same bridge). And also because the default priority for a bridge running extended sysid is 32768 + vlan #. So I can see that vlan 2 has a root ID with a priority of 32770 and vlan 5 priority 32773.

If you configure your port as access, you will merge vlan 2 and vlan 5 and vlan 2's root will become vlan 5's root (because 32770 < 32773).

regards,

Francois

[henrybb]

Hi tdrais,

I have changed STP ROOT for vlan 2,5. Now the l2 switch is the root for them. But pc1 still can't communicate with pc2.

Hi ftallet,

thanks,you are right. I config port like this,so port become trunk via negotiate. After I config 'sw mode acc',it works normal like you said.

l2_switch#sh run int f0/14

Building configuration...

Current configuration : 75 bytes

!

interface FastEthernet0/14

switchport access vlan 2

no ip address

end

l2_switch#sh run int f0/16

Building configuration...

Current configuration : 75 bytes

!

interface FastEthernet0/16

switchport access vlan 5

no ip address

end