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| 5.1 |  | Multilayer Switching |
| 5.1.5 |  | MLS operation |

MLS operation consists of four distinct steps. They include:

Step 1
MLSP multicast hello packets are sent every 15 seconds to inform the MLS-SE(s) of the MLS-RP MAC addresses used on different VLANs and the routing and ACL information of the MLS-RP when the router boots or after changes in ACLs. When an MLS-SE hears the MLSP hello message indicating an IP MLS initialization, the MLS-SE is programmed with a locally unique XTAG for the MLS-RP and with the MLS-RP MAC addresses associated with each VLAN. The XTAG is a 1-byte value that the MLS-SE attaches to each VLAN to all MAC addresses learned from the same MLS-RP via MLSP.

Step 2
In Figure , 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 destination MAC addresses and VLANs of the MLS-RP through MLSP. The MLS-SE learns the Layer 3 flow information (such as the destination IP address, source IP address, and protocol port numbers) and forwards the first packet the MLS-RP. A partial MLS entry for this Layer 3 flow is created in 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 and class of service (CoS) policy. The MLS-RP rewrites the MAC header, adding a new destination MAC address of Host B and its own MAC address, associated with Host VLAN, as the source.

Step 3
The MLS-RP routes the packet to the destination host. When the switch receives the packet, the MLS-SE recognizes that the source MAC address belongs to the MLS-RP, that the XTAG matches that of the candidate packet, and that the flow information of the packet matches the flow for which the candidate entry was created. The MLS-SE considers this packet an enable packet and completes the MLS entry in the MLS cache.

Step 4
After the MLS entry has been completed, all Layer 3 packets in the same flow from the source host to the destination host are Layer 3/4 switched directly by the switch, bypassing the router. IP MLS is unidirectional. A separate Layer 3 switched path is created for traffic from Host B to Host A.

Finally, some Catalyst 5000 family switching line cards have onboard hardware that performs the rewrite (as opposed to rewrites on the Supervisor Engine), maximizing IP MLS performance. This performance enhancement is also used on the Catalyst 2926G series switch ports. When the line cards or individual ports perform the PDU header rewrites, this is called inline rewrite. With inline rewrite, frames traverse the switch bus only once.

The Catalyst 5000 NetFlow Feature Card (NFFC) is a daughter card for the Supervisor module that enables intelligent network services, such as high-performance multilayer switching and accounting and traffic management. The NFFC contains central rewrite engines (one per bus) to handle PDU header rewrites when inline rewrite is not an option. If the central rewrite engines are used, this means that a frame must traverse the bus twice, first with a VLAN tag for the source, on its way to the NFFC, and second with a VLAN tag for the destination, on its way to the egress port. To determine whether a port supports packet rewrite, use the show port capabilities command. If the port does not support inline rewrite, the packet rewrite is done in the Supervisor Engine.

In summary, the basic MLS ingredients are the MLS-RP, MLS-SE, and MLSP. The MLS-SE creates flows in the MLS cache based on candidate and enable packets and ACLs configured on the MLS-RP. The result is wire-speed routing of packets based on criteria such as destination IP address, source IP address, and protocol port numbers.