This section provides a step-by-step description of MLS implementation.
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 the figure below).
|Figure 1 ||MLS Implementation |
In the figure below, 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 2 ||MLS Implementation |
The MLS-RP routes the packet to Host B. When the packet appears back on the Catalyst 5000 series switch backplane, the MLS-SE recognizes the source MAC address as that of the MLS-RP, and that the packet's flow information matches the flow for which it set up a candidate entry. The MLS-SE considers this packet an enabler packet and completes the MLS entry (established by the candidate packet) in the MLS cache (see the figure below).
|Figure 3 ||MLS Implementation |
After the MLS entry has been completed, all Layer 3 packets with the same flow from Host A to Host B are Layer 3 switched directly inside the switch from Host A to Host B, bypassing the router (see the figure below). After the Layer 3-switched path is established, the packet from Host A is rewritten by the MLS-SE before it is forwarded to Host B. The rewritten information includes the MAC addresses, encapsulations (when applicable), and some Layer 3 information.
The resultant packet format and protocol behavior is identical to that of a packet that is routed by the RSM or external Cisco router.
MLS is unidirectional. For Host B to communicate with Host A, another Layer 3-switched path needs to be created from Host B to Host A.
|Figure 4 ||MLS Implementation |
See the Catalyst 5000 Series Multilayer Switching User Guide for additional network implementation examples that include network topologies that do not support MLS.