Switch learning mac through rstp blocking port

 

Why would a switch learn mac through a blocking port? doesn't it it defeat the purpose of STP unless something else is going on?

SW4 is learning mac of SW3 though e1/0 which is a member of blocking Po46 of Sw4. I would expect Sw4 to learn the mac through Po45. SW3 is root of all vlans. The result is that pings are failing between SW3 and SW4 but they are intermittently passing because for a split second Sw4 is learning the mac through Po45 like it is supposed to.

If I shutdown Po46 all pings will be successful. Please see output and Layer 2 designs below and let me know your thoughts if someone else came across something similar. I am using IOU-web.

Thank you,

W3#show etherchannel sum

Group  Port-channel  Protocol    Ports

------+-------------+-----------+-----------------------------------------------

35     Po35(SU)        LACP      Et1/0(P)    Et1/1(P)    

36     Po36(SU)        LACP      Et1/2(P)    Et1/3(P)  

SW4#show etherchannel sum

Group  Port-channel  Protocol    Ports

------+-------------+-----------+-----------------------------------------------

45     Po45(SU)        LACP      Et1/2(P)    Et1/3(P)    

46     Po46(SU)        LACP      Et1/0(P)    Et1/1(P)    

SW5#show etherchannel sum

Group  Port-channel  Protocol    Ports

------+-------------+-----------+-----------------------------------------------

35     Po35(SU)        LACP      Et1/0(P)    Et1/1(P)    

45     Po45(SU)        LACP      Et1/2(P)    Et1/3(P)   

SW6#show etherchannel sum

Group  Port-channel  Protocol    Ports

------+-------------+-----------+-----------------------------------------------

36     Po36(SU)        LACP      Et1/2(P)    Et1/3(P)    

46     Po46(SU)        LACP      Et1/0(P)    Et1/1(P)    

SW3#show spanning-tree vlan 34


VLAN0034

  Spanning tree enabled protocol rstp

  Root ID    Priority    34

             Address     aabb.cc00.2100

             This bridge is the root

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


  Bridge ID  Priority    34     (priority 0 sys-id-ext 34)

             Address     aabb.cc00.2100

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

             Aging Time  300 sec


Interface           Role Sts Cost      Prio.Nbr Type

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

Po35                Desg FWD 56        128.514  Shr 

Po36                Desg FWD 56        128.515  Shr 




SW4#show spanning-tree vlan 34


VLAN0034

  Spanning tree enabled protocol rstp

  Root ID    Priority    34

             Address     aabb.cc00.2100

             Cost        112

             Port        514 (Port-channel45)

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


  Bridge ID  Priority    4130   (priority 4096 sys-id-ext 34)

             Address     aabb.cc00.2200

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

             Aging Time  300 sec


Interface           Role Sts Cost      Prio.Nbr Type

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

Po45                Root FWD 56        128.514  Shr 

Po46                Altn BLK 56        128.515  Shr 




SW5#show spanning-tree vlan 34


VLAN0034

  Spanning tree enabled protocol rstp

  Root ID    Priority    34

             Address     aabb.cc00.2100

             Cost        56

             Port        514 (Port-channel35)

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


  Bridge ID  Priority    32802  (priority 32768 sys-id-ext 34)

             Address     aabb.cc00.2300

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

             Aging Time  300 sec


Interface           Role Sts Cost      Prio.Nbr Type

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

Po35                Root FWD 56        128.514  Shr 

Po45                Desg FWD 56        128.515  Shr 




SW6#show spanning-tree vlan 34


VLAN0034

  Spanning tree enabled protocol rstp

  Root ID    Priority    34

             Address     aabb.cc00.2100

             Cost        56

             Port        514 (Port-channel36)

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


  Bridge ID  Priority    32802  (priority 32768 sys-id-ext 34)

             Address     aabb.cc00.2400

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

             Aging Time  300 sec


Interface           Role Sts Cost      Prio.Nbr Type

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

Po36                Root FWD 56        128.514  Shr 

Po46                Desg FWD 56        128.515  Shr 




SW3#show ip arp

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  10.2.0.14              47   aabb.cc80.2200  ARPA   Vlan34




SW4#show ip arp

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  10.2.0.13              47   aabb.cc80.2100  ARPA   Vlan34



E1/2 is the correct port where SW4 should learn the mac address of SW3, it actually does but then it falls back to learning through E1/0 thus pings failing...why??



SW4#show mac ad dyn vlan 34

          Mac Address Table

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


Vlan    Mac Address       Type        Ports

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

  34    aabb.cc80.2100    DYNAMIC     Et1/2

Total Mac Addresses for this criterion: 1


SW4#show mac ad dyn vlan 34

          Mac Address Table

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


Vlan    Mac Address       Type        Ports

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

  34    aabb.cc80.2100    DYNAMIC     Et1/0

  34    aabb.cc80.2200    DYNAMIC     Et1/0

Total Mac Addresses for this criterion: 2










Layer 2




Comments

  • From the operational point of view, there is no difference between a port on LISTENING state or a port in BLOCKING state: in both states we don't send BPDUs out but we expect and accept BPDUs in, in both states we don't do MAC learning on the port. So what you see is not expected.

    Is ths IOU? Because you either hit a bug, especially this being IOU, or there is something missconfigured and the active STP topology constantly changes, which could make a port to rapidly (because you run RSTP) change between BLK and FW states, thus MAC's are learned on different ports. So except basic RSTP and port-channels configuration, do you have anything else configured in there? If not, reload IOU or remove and re-apply the configuration.

    Regards,
    Cristian. 

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