7.1 MLS Processes
7.1.4
Step 1: MLS hello packets are sent by the MLS-RP
When the router first boots, it begins sending MLSP hello packets every 15 seconds. These packets contain information on the VLANs and MAC addresses in use on the router. By listening for these hello packets, the MLS-SE can learn the attributes of any MLS-capable routers in the Layer 2 network. The MLS-SE associates a single XTAG value with every MLS router that it identifies. Because the MLSP hellos are periodic in nature, they allow routers and Catalyst Switches to boot at random times while also serving as a router keepalive mechanism for the MLS-SE (if a MLS-RP goes off line, associated cache entries are purged).

Note: XTAGs are used by the MLS-SE to help in its Layer 3 switching process. Whenever the MSL-SE updates information it received from the MLS-RP, it includes the unique XTAG for that MLS-RP. For example, if an MLS-SE has multiple MSL-RPs in its MLS Cache, and one of the MLS-RPs fail, the MLS-SE will only remove the layer 3 information associated with the MLS-RP that failed, identifying it by its unique XTAG.

As shown in the Figure, the MLSP packets are sourced from subinterface Fast Ethernet1/0.1 on the router (this is a configurable option; the router commands are presented later). These packets are then used to populate the Layer 2 CAM table (a form of bridging table commonly used in modern switches) with special entries that are used to identify packets going to or coming from a router interface. The show cam Catalyst command places an R next to these entries. Each router is also assigned a unique XTAG value. If a second router were present in the Figure, it would receive a different XTAG number than the value of 1 assigned to the first router.

Although it is not illustrated, the MLSP hello packets flow throughout the Layer 2 network. Because they are sent using a multicast address (01-00-0C-DD-DD-DD), non-MLS-aware switches simply flood the hello packets to every segment in VLAN 1. In this way, all MLS switches learn about all MLS-capable routers.