[c-nsp] MAC loop in REP network

[Garry]

Hi,

I've had a problem on a pair of 4500 switches with a MAC address. We
first noticed the CPU being at 99%, and upon investigating, noticed one
switch complained about a flapping MAC address. Further examination
showed that the two switches showed the MAC being advertised from the
other's TenGB interface - they're running with dual TenG in a REP loop.
Now while there are lots of VLANs and devices connected to the two
switches, it only happened with one single MAC on one VLAN. Examining
the REP structure resulted in this output:

switch1#show rep topology
REP Segment 1
BridgeName       PortName   Edge Role
---------------- ---------- ---- ----
switch1.fd3      Te4/1      Pri  Alt
switch2.fd3      Te4/1           Open
switch2.fd3      Te3/1           Open
switch1.fd3      Te3/1      Sec  Open
(same for both)

DIsplaying the detailed version showed this:

REP Segment 1
switch1.fd3, Te4/1 (Primary Edge)
  Alternate Port, some vlans blocked
  Bridge MAC: 0023.5ef0.d2c0
  Port Number: 0100
  Port Priority: 050
  Neighbor Number: 1 / [-4]
[..]
switch1.fd3, Te3/1 (Secondary Edge)
  Open Port, all vlans forwarding
  Bridge MAC: 0023.5ef0.d2c0
  Port Number: 0C0
  Port Priority: 010
  Neighbor Number: 4 / [-1]
 
(same for both switches)

Switch1 is the one that has the MAC that was flapping, between a
portchannel that is physically connected to the device sourcing that
mac, and the Ten3/1 interface. The second switch showed the MAC being
sourced on Ten4/1.

I temporarily "fixed" this flapping as well as the high CPU load by
blocking the VLAN in question on one of the TenG interfaces ...

Here's the port configs:

interface TenGigabitEthernet3/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 rep segment 1 edge preferred
 rep preempt delay 15
interface TenGigabitEthernet4/1
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 1-212,214-4094  <- did this to mitigate
the loop problem
 switchport mode trunk
 rep segment 1 edge preferred
 rep preempt delay 15

Switch2:

interface TenGigabitEthernet3/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 rep segment 1
 rep preempt delay 15
interface TenGigabitEthernet4/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 rep segment 1
 rep preempt delay 15

Any idea what's going wrong here? This only started when we added a port
with access to VLAN 213 on switch2 ...

Tnx, -gg