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New Member

SpanningTree Head Scratcher

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

2 REPLIES
New Member

I was able to get the issue

I was able to get the issue resolved and as suspected, it was a cabling issue. Someone had plugged up port 1/0/7 & 1/0/14 into the same segment on the IPS devices therefore it was behaving like a hub. Once the cabling was corrected spanning tree corrected itself and all is well. I would still like to know why the behavior was seen...perhaps a bug? I've never seen a port with a higher cost become the root port.

Purple

   We have seen this many

   We have seen this many times when someone loops the cable into the same or different subnet and it buries the cpu and bpdu's are not received the way they should be .   This slows the network to a crawl , we have all  been there.   As you have seen if you see a cdp neigh as its self there is almost always a loop .

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