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
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.