Section 7.2: MLS-RP Advertisements

When an MLS-RP is enabled in the campus network, MLS-RP advertisements begin. The MLS-RP sends out multicast Hello messages every 15 seconds to all switches in the network. The advertisement message consists of the MAC addresses used by the MLS-RP on its interfaces that are participating in MLS, access list information, and additions and deletions of routes. MLSP uses the Cisco Group Management Protocol (CGMP) multicast address, which ensures interoperability with the Cisco switches in the network, as the destination address of the Hello message. Although this address is the same as that used by CGMP, the message contains a different protocol type so the switch can distinguish these messages from other multicast packets.

All switches in the network receive the Hello message, although only Layer 3 switches process the message. Any switches that are not Layer 3 capable passes the frames on to any downstream switches. When an MLS-SE receives the frame, the device extracts the MAC addresses in the frame, along with the associated interface or VLAN ID for that address. The MLS-SE records the addresses of the MLS-RPs in the MLS-SE content-addressable memory (CAM) table.

The MLS-SE the assigns XTAGs to each and every MLS-RP attached to a switch. The XTAG is a one-byte value attached to the MAC address of each attached MLS-RP. These values are used to differentiate between MLS-RPs when there are more than one available MLS-RP. The XTAG is also used to delete entries from the Layer 3 table when an MLS-RP fails or exits the network.

The Switching Engine (SE) is involved in the MSL-RP advertising process to maintain the cache for MLS flows. MLS caching is a process that occurs based on individual flows. Packets in a flow are compared to the cache, which is based on one-way flows with a flow in the reverse direction being regarded as another flow. If the cache has an entry that is a match for the packet, the SE switches the packet instead of passing it to the router. If it does not match an entry in the cache, a process occurs that is designed to create an entry in the cache.