So I've ran across a STP issue that i'm having difficulty explaining the behavior. The problem is that I don't have a cable matrix but based on the show output, there's a clear L2 loop that STP isn't catching for some reason. Basically i have two switches...SAS-01 and SAS-02. These two switches are trunked together via a PortChannel (Po1). SAS-01 is configured as the root bridge.The trouble that i'm seeing is on ports g1/0/7 and g1/0/14 on SAS-02. When i do a 'sh cdp nei' on SAS-02, it see itself. I suspect that both g1/0/7 & 14 are plugged into a hub or something because I am seeing multiple mac addresses on their interfaces but what i cant figure out is why STP isn't putting one of the two ports into blocking since this is a loop and/or because the cost to the root via g1/0/14 is alot higher (21000) than via Po1 (1000). See the below show commands. Also notice that although SAS-01 is the root bridge, it's not seeing itself as the root bridge, instead it thinks the path to the root is via the Po1 to SAS-02. I have someone going onsite tomorrow to provide me a cable matrix but in the meantime, any ideas?
SAS-02#sh cdp ne
Device ID Local Intrfce Holdtme Capability Platform Port ID
SAS-02.asurion.com
Gig 1/0/7 147 R S I WS-C3850- Gig 1/0/14
SAS-02.asurion.com
Gig 1/0/14 157 R S I WS-C3850- Gig 1/0/7
SAS-02#sh span vlan 32
VLAN0032
Spanning tree enabled protocol rstp
Root ID Priority 16385
Address c025.5c15.b500
Cost 21000
Port 14 (GigabitEthernet1/0/14)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24608 (priority 24576 sys-id-ext 32)
Address c025.5c15.aa80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/14 Root FWD 20000 128.14 P2p
Gi1/0/16 Desg FWD 20000 128.16 P2p
Gi1/0/35 Desg FWD 20000 128.35 P2p Edge
Gi1/0/36 Desg FWD 20000 128.36 P2p Edge
Po1 Desg FWD 1000 128.2027 P2p
Po13 Desg FWD 10000 128.2039 P2p Edge
SAS-02#sh span int g1/0/7 d
Port 7 (GigabitEthernet1/0/7) of VLAN0001 is designated forwarding
Port path cost 20000, Port priority 128, Port Identifier 128.7.
Designated root has priority 16385, address c025.5c15.b500
Designated bridge has priority 24577, address c025.5c15.aa80
Designated port id is 128.7, designated path cost 1000
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 5713059, received 26
SAS-02#sh span int g1/0/14 d
Port 14 (GigabitEthernet1/0/14) of VLAN0032 is root forwarding
Port path cost 20000, Port priority 128, Port Identifier 128.14.
Designated root has priority 16385, address c025.5c15.b500
Designated bridge has priority 24577, address c025.5c15.aa80
Designated port id is 128.7, designated path cost 1000
Timers: message age 15, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 36, received 5713060
SAS-01#sh span vlan 32
VLAN0032
Spanning tree enabled protocol rstp
Root ID Priority 16385
Address c025.5c15.b500
Cost 22000
Port 2027 (Port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 16416 (priority 16384 sys-id-ext 32)
Address c025.5c15.b500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/14 Desg FWD 20000 128.14 P2p
Gi1/0/16 Desg FWD 20000 128.16 P2p
Gi1/0/35 Desg FWD 20000 128.35 P2p Edge
Gi1/0/36 Desg FWD 20000 128.36 P2p Edge
Po1 Root FWD 1000 128.2027 P2p
Po12 Desg FWD 10000 128.2038 P2p Edge
