| 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.
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