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Wednesday, April 3, 2013

Cisco 3750 does not learn MAC address from LLDP multicasts

Does the 3750 switch learn MAC address from LLDP multicasts generated every 30 seconds by end device connected to the port .
We are experiencing issues, not sure if this is expected behavior or not. It is not dependent on if port security is set or not.

For the issue above we had a Polycom IP phone connected to the port. It generates LLDP multicast every 30 seconds, but does not do anything else.The switch sees the phone as LLDP neighbour, but does not learn its MAC address.
We reproduced this scenario later on with Cisco switches only, and it 'works' the same way.

Does this mean that the switch does not forward the frame and processes it just within the switch it does not learn the MAC address. We have not found any explicit document that would say LLDP multicast are not used for learning process.

We compared those to LOOP protocol (  used by keepalive ) frames. These frames are also not ( normally ) forwarded, but the switch learns MAC address from them. But maybe  this comparison is not good, as in case of loops LOOP frames would be forwarded, so there is probably no reason why they should not be used for learning.

Are there an explicit list of all protocols that are not used for learning process?


See 802.1Q-2011 section 7.5: "Frames that carry control information to determine the active topology and current extent of each VLAN, i.e., spanning tree and MVRP PDUs, and frames from other link constrained protocols, such as EAPOL and LLDP, are not forwarded".

"Are not forwarded" means not relayed from a port to another port, through the MAC 802.3 switching function. So, the learning process is not invoked.

you are referring to a specific function of the link OAM sub family included in the Slow protocols family. This loop function inverts SA and DA in the MAC frame received, and returns the frame to the same port. So, there is no need of forwarding between ports!

If it is a loop through another port, it is not a link OAM protocol but a network OAM protocol with other characteristics.

There is not a exhaustive list, it depends on the implementation.

For example, for EVPLAN in a WAN bridge, you have systematically PAUSE ad LLDP discard: see G.8011.2 (01/2009) table 8-2.
Generally, this question concerns (1) the SA addresses when the DA addresses are with OUI = 00-80-C2, (2) the configuration of the filtering database.
 

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