SG300 Spanning-Tree is Always Root?

pashdown1 · ‎10-16-2012

I have a couple new SG300's with software version 1.2.7.76 connected to a pair of 6500s. They have only a couple of VLANs trunked to them. However, each of them are insisting they are the root of the spanning tree, even though both of the 6500s have lower spanning tree priorities. Look:

#show spanning-tree

Spanning tree enabled mode RSTP

Default port cost method: long

Root ID Priority 61440

Address c8:f9:f9:46:a9:68

This switch is the root

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

Is this a bug, or is there something else I need to do to the SG300 to configure its priority properly?

srikanth ath · ‎10-17-2012

strange..

Can you please issue sh spanning-tree in 6500 pairs..

sh spanning-tree vlan-X ------> for the two vlans you have.

Thanks

pashdown1 · ‎10-17-2012

6500 A:

VLAN0020

Spanning tree enabled protocol rstp

Root ID Priority 8212

Address 0019.0732.e000

Cost 4

Port 275 (GigabitEthernet3/19)

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

Bridge ID Priority 28692 (priority 28672 sys-id-ext 20)

Address 0019.07f1.1c00

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

Aging Time 14440

Interface Role Sts Cost Prio.Nbr Type

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

Te1/4 Altn BLK 2 128.4 P2p

Gi3/16 Desg FWD 4 128.272 P2p

Gi3/19 Root FWD 4 128.275 P2p <--- where the SG300 above is connected

Gi3/20 Altn BLK 4 128.276 P2p

Gi7/10 Desg FWD 19 128.778 P2p

Gi7/35 Desg FWD 4 128.803 P2p Edge

Gi7/38 Desg FWD 4 128.806 P2p Edge

Interface Role Sts Cost Prio.Nbr Type

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

Te9/1 Altn BLK 5 128.1025 P2p

Po11 Desg FWD 3 128.1667 P2p

Po12 Altn BLK 3 128.1668 P2p

Po13 Altn BLK 3 128.1669 P2p

Po21 Altn BLK 4 128.1672 P2p

6500 B:

VLAN0020

Spanning tree enabled protocol rstp

Root ID Priority 8212

Address 0019.0732.e000

This bridge is the root <-- the proper root

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

Bridge ID Priority 8212 (priority 8192 sys-id-ext 20)

Address 0019.0732.e000

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

Aging Time 14440

Interface Role Sts Cost Prio.Nbr Type

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

Te1/4 Desg FWD 2 128.4 P2p

Gi3/9 Desg FWD 4 128.265 P2p Peer(STP)

Gi3/16 Desg FWD 4 128.272 P2p

Gi3/19 Desg FWD 4 128.275 P2p

Gi3/20 Desg FWD 4 128.276 P2p

Gi7/1 Desg FWD 4 128.769 P2p

Gi7/35 Desg FWD 4 128.803 P2p Edge

Gi7/38 Desg FWD 4 128.806 P2p Edge

Interface Role Sts Cost Prio.Nbr Type

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

Gi7/42 Desg FWD 19 128.810 P2p Peer(STP)

Te9/1 Desg FWD 3 128.1025 P2p

Po12 Desg FWD 3 128.1669 P2p

Po13 Desg FWD 3 128.1670 P2p

Po21 Desg FWD 3 128.1673 P2p

eric.long · ‎10-17-2012

Both Switch A and B believe that Switch B is the root for this VLAN. Note that the Root ID priority and address matches on both switches. Is the SG300 connected to both 6500s?

pashdown1 · ‎10-17-2012

Why is "Root" appearing on Gi3/19 on A? This all started when I noticed a mac address that is connected to another SG300 was being shown as being sourced from the wrong SG300 (but on the same VLAN) on A.

Both SG300s have the same VLAN20 and are connected to both 6500s for redundancy.

eric.long · ‎10-17-2012

Switch A is using the SG300 to reach Switch B (the root) for VLAN20. The path cost over the A->SG300->B path is lower than A->B for this VLAN. You don't say how A and B are connected, but if it is over port ten9/1, you will notice that port is Blocking and the cost is 5.

pashdown1 · ‎10-17-2012

Thank you Eric.

Each SG300 is connected directly to A & B, SG300 #1 on Gig3/19 and SG300 #2 on Gig3/20. Why would A use SG300 #2 to get to SG300 #1 if #1 is directly connected?

The cost on ten9/1 is higher because I'd rather that was a fallback than a preferred route. The costs to both SG300s on both devices are listed as 4, but still the SG300 on A is listed as root in the spanning-tree.

eric.long · ‎10-17-2012

Switch B is the spanning tree root based on the output you provided, not SG300. The MAC address of the root is indicated in in the address line of the output. Both switches agree that switch B is the root.

Root ID Priority 8212

Address 0019.0732.e000

Switch A is using Gig3/19 as the root port, which just means that it is using that port to reach the root. It does not necessarily mean that the directly connected switch is the root. Based on the output you have provided Switch A is using SG300#1 to reach Switch B.

pashdown1 · ‎10-17-2012

Thanks for outlining it for me. As a result, I've removed the cost of the TenGig link between A&B.