Tip

There is one XTAG per MLS-capable router. The XTAG serves as a single handle for a router's multiple MAC addresses (each interface/VLAN could be using a different MAC address). XTAGs are locally significant (different NFFCs can refer to the same router with different XTAGs).

Figure 11-5 illustrates the MLS hello process.

Figure 11-5 MLS Hello Process

Figure 11-5 MLS Hello Process

As shown in Figure 11-5, 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 Figure 11-5, it would receive a different XTAG number than the value of 1 assigned to the first router. However, notice that all MAC addresses and VLANs for a single router are associated with a single XTAG value.

Although it is not illustrated in Figure 11-5, the MLSP hello packets flow throughout the Layer 2 network. Because they are sent using a multicast address (01-00-0C-DD-DD-DD, the same address used by CGMP), 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.

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