Hi Carl,
The simple answer would be on the Sup module if the line cards doesnt have the DFC on it.
Please find the explanation below:
Catalyst switches maintain several types of tables that are tailored for Layer 2 switching or multilayer switching (MLS), and are kept in very fast memory so that many fields within a frame or packet can be compared in parallel.
- ARP—Maps an IP address to a MAC address in order to provide IP communication within a Layer 2 broadcast domain. For example, Host B wants to send information to Host A but does not have the MAC address of Host A in its ARP cache. Host B generates a broadcast message for all hosts within the broadcast domain to obtain the MAC address associated with the IP address of Host A. All hosts within the broadcast domain receive the ARP request, and only Host A responds with its MAC address.
- CAM—All Catalyst switch models use a CAM table for Layer 2 switching. As frames arrive on switch ports, the source MAC addresses are learned and recorded in the CAM table. The port of arrival and the VLAN are both recorded in the table, along with a timestamp. If a MAC address learned on one switch port has moved to a different port, the MAC address and timestamp are recorded for the most recent arrival port. Then, the previous entry is deleted. If a MAC address is found already present in the table for the correct arrival port, only its timestamp is updated.
- Ternary Content Addressable Memory (TCAM)—In multilayer switches, all the processes that access control lists (ACLs) provide in traditional routing, such as matching, filtering, or control specific traffic, are implemented in hardware. TCAM allows a packet to be evaluated against an entire access list in a single table lookup. Most switches have multiple TCAMs so that both inbound and outbound security, as well as QoS ACLs, can be evaluated simultaneously, or entirely in parallel with a Layer 2 or Layer 3 forwarding decision.
In distributed switching, each Distributed Feature Card (DFC) is responsible for maintaining each own CAM table. This means that each DFC learns the MAC address and ages them, which depends on the CAM aging and traffic matching that particular entry. With distributed switching, it is normal that the supervisor engine does not see any traffic for a particular MAC address for a while, so the entry might expire. There are currently two mechanisms available to keep the CAM tables consistent between the different engines, such as DFC (present in line modules) and Policy Feature Card (PFC) (present in supervisor modules):
- Flood to Fabric (FF)
- MAC Notification (MN)
When a MAC address entry is aged out on the PFC, the show mac-address address all command displays the DFC or PFC that holds this MAC address.
In order to prevent the age out of an entry on a DFC or PFC, even if there is not traffic for that MAC address, enable the MAC address synchronization. Issue these commands in order to enable the synchronization:
Hope this helps.
Regards
Inayath.
*Plz rate if this helps.
