The same mac address has flag "+" on both vPC peers

[devinliu1991]

Hi, guys

We have 2 Nexus 92160s running as vPC peers in our network, the topology's like below:

We found that the flags in "show mac address-table" output for one server's mac address are both "+" which means "primary entry using vPC Peer-Link". We think if both vPC peers learn this mac address from vPC peer-link, there might be some problems in network connection, however, the server is working properly.

Output as below:

N92160-1# show mac address-table
Legend:
* - primary entry, G - Gateway MAC, (R) - Routed MAC, O - Overlay MAC
age - seconds since last seen,+ - primary entry using vPC Peer-Link,
(T) - True, (F) - False, C - ControlPlane MAC
VLAN MAC Address Type age Secure NTFY Ports
---------+-----------------+--------+---------+------+----+------------------

+  105     xxxx.xxxx.d43c   dynamic  0         F      F    Po3032

N92160-2# show mac address-table
Legend:
* - primary entry, G - Gateway MAC, (R) - Routed MAC, O - Overlay MAC
age - seconds since last seen,+ - primary entry using vPC Peer-Link,
(T) - True, (F) - False, C - ControlPlane MAC
VLAN MAC Address Type age Secure NTFY Ports
---------+-----------------+--------+---------+------+----+------------------

+  105     xxxx.xxxx.d43c   dynamic  0         F      F    Po3032

So am I wrong with the meaning of "+" flag in "show mac address-table" output? When this both "+" would happen?

(For most mac addresses in address-table, there is flag "*" in at least one vPC peer)

[Paul Chapman · Accepted solution]

Hi -

I don't have a lot of different switches to compare on right now, but I don't think that this is an issue.  I see equivalent behavior on the systems I do have access to though.  In your case CAMs on both switches are pointing to Po3032, which is the downstream switch, to get to the target MAC.

Page 51 on this CiscoLive! presentation provides a good idea of the likely cause.  Since you are connected via Port-Channel to the downstream switch, you don't know which link the downstream switch will choose when sending data.  In this case each switch is learning the MAC on a regular basis from the downstream partner, then informing the other via CFS.

PSC

[devinliu1991]

Hi, Paul

Really thanks for your help, I misunderstood the meaning of flag "+".

We've checked this issue with Cisco TAC and he told us that it is a bug which doesn't have any business impact. So it doesn't need any workaround.

As you said, when the switch sees flag "+" in the mac address table, the frame will be forwarded downstream through Po3032 in this case rather than the peer link.

[Prasanna Damodaran]

Hi Paul,

could you please help me with Bug ID details.