Specifications

Multilayer Switching Overview
MLS Implementation
4
Note The MLS-RPs shown in the figures represent either a RSM or an externally attached Cisco router.
The MLSP informs the Catalyst 5000 series switch of the MLS-RP MAC addresses used on different
VLANs and the MLS-RP’s routing and access list changes. Through this protocol, the MLS-RP
multicasts its MAC and VLAN information to all MLS-SEs. When the MLS-SE hears the MLSP hello
message indicating an MLS initialization, the MLS-SE is programmed with the MLS-RP MAC address
and its associated VLAN number (see Figure 63).
Figure 63 MLS Implementation
In Figure 64, Host A and Host B are located on different VLANs. Host A initiates a data transfer to
Host B. When Host A sends the first packet to the MLS-RP, the MLS-SE recognizes this packet as a
candidate packet for Layer 3 switching because the MLS-SE has learned the MLS-RP’s destination
MAC address and VLAN through MLSP. The MLS-SE learns the Layer 3 flow information (such as the
destination address, source address, and protocol port numbers), and forwards the first packet to the
MLS-RP. A partial MLS entry for this Layer 3 flow is created in the MLS cache.
The MLS-RP receives the packet, looks at its route table to determine how to forward the packet, and
applies services such as Access Control Lists (ACLs) and class of service (COS) policy.
The MLS-RP rewrites the MAC header adding a new destination MAC address (Host B’s) and its own
MAC address as the source.
Figure 64 MLS Implementation
MLS-RP multicasts its
MAC addresses and
VLAN number to all
MLS-SEs…
… all MLS-SEs
program the NFFC
with the MSLP
hello
message information
MLS-RP
(MLS-SE)
12000
MLS-RP
(MLS-SE)
12001 Host A Host B
Because the Catalyst switch has learned
the MAC and VLAN information of the MLS-RP,
the switch starts the MLS process for the Layer 3
flow contained in this packet, the
candidate packet
Candidate packet