Per-Vlan Spanning-Tree Issue

[Tauer Drumond]

Hi all,

I have a topology like on the attachment. Please check it out...

I have four VLANs (10,20,30,40). The Core 1 is the root primary for the 10 and 20 VLAN.

The Core 2 is the root primary for the 30,40 VLAN.

On the SW1, the output of command "show spanning-tree root" shows me that the port g0/49 is the root port for all VLANs.

Root Hello Max Fwd

Vlan Root ID Cost Time Age Dly Root Port

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

VLAN0010 24584 0019.55f2.f000 8 2 20 15 Gi0/49

VLAN0020 24585 0019.55f2.f000 8 2 20 15 Gi0/49

VLAN0030 24586 0019.55f2.edc0 4 2 20 15 Gi0/49

VLAN0040 24587 0019.55f2.edc0 4 2 20 15 Gi0/49

The output on SW2 shows me that the port 0/52 is the root port for all VLANs.

Root Hello Max Fwd

Vlan Root ID Cost Time Age Dly Root Port

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

VLAN0010 24584 0019.55f2.f000 4 2 20 15 Gi0/52

VLAN0020 24585 0019.55f2.f000 4 2 20 15 Gi0/52

VLAN0030 24586 0019.55f2.edc0 8 2 20 15 Gi0/52

VLAN0040 24587 0019.55f2.edc0 8 2 20 15 Gi0/52

Does it mean that the traffic is flowing through this ports?

The traffic wouldn't be balanced bettween the uplinks, on both switches?

I mean, on SW 1, wouldn't the VLAN 10 and 20 traffic flow through the interface G0/49 and the VLAN 30 and 40 traffic flow throught the interface G0/52?

Thanks

Tauer

[Marwan ALshawi]

u welcome :)

[Mohamed Sobair]

Tauer,

As Edison pointed out, Since all connection are Giga, then the Cost toward the root bridge is equal for both direction. The (Tie breaker) for spanning-tree path selection would be choosing the lease port-ID which is G0/49 to reach the root.

HTH

Mohamed