MLS (Multi-layer Switching) provides hardware based layer 3 high-performance switching for switches. It uses advanced application specific integrated circuit (ASIC) switching components to complete the IP packet switching between subnets, which can greatly reduce the excessive system overhead caused by the router when processing data packets.

MLS is a technology that uses hardware processing packet switching and rewrites frame headers to improve IP routing performance. Cisco Multi-Layer Switching technology supports all traditional routing protocols, while the frame forwarding and rewriting functions previously completed by the router have now been completed by the hardware of the switch. MLS migrates the packet switching function of traditional routers to layer 3 switches. Of course, this first requires that the exchanged path must exist.

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Composition of MLS

1. The multi-layer routing processor (MLS-RP), which is equivalent to a router in the network, is responsible for processing the first packet of each data stream and assisting the MLS-SE in establishing a Shortcut Entry in the CAM (Content-Addressable Memory) of the third layer. MLS-RP can be an external router or implemented by the Route Switching Module (RSM) of the layer 3 switch.

2. Multi-layer switching engine (MLS-SE) is a switching entity responsible for forwarding and rewriting packets.

3. Multi-layer switching protocol (MLSP) is a lightweight protocol used to initialize the switching engine of multi-layer switching through the multi-layer routing processor (MLS-RP).

Processing Steps of Multi-layer Switching

1: Send MLSP Hello information. When the router is activated, the multi-layer routing processor sends an MLSP Hello packet every 15 seconds, which contains the VLAN ID and MAC address information used by the router interface. MLS-SE uses this information to master the second layer attributes of routers with multi-layer switching capabilities. If the switch is connected to a few MLS-RPs, MLS-SE distinguishes the MAC address entries of each MLS-RP by assigning XTAG values to their MAC addresses. If the MLSP frame gets all MAC addresses from the same MLS-RP, MLS-SE appends the same XTAG value to them. These associated records are stored in CAM. Because the Hello packets are sent periodically, this method can ensure that the correlation value dynamically tracks the changes of the network, and can implement a certain elimination mechanism. The Hello package is published in layer 2. It uses the multicast address 01-00-0c-dd-dd-dd.

Step 2: Identify Candidate Packets. After knowing the relevant addresses of routers with multi-layer switching capabilities, MLS-SE can match and judge the data packets entering the switch. For a packet in a stream, if the MLS cache contains a matching shortcut entry, the MLS-SE will bypass the router and directly forward the packet. If the MLS does not contain a shortcut entry matching the packet, MLS-SE classifies it as a candidate packet and establishes a partial shortcut in the cache. Such packets are processed in the traditional layer 2 switch processing mode and sent to the router interface (Gateway) connected to them. It should be noted here that the candidate packets (frames) must meet two criteria: the target address passes through a MAC address of the router interface listed in the MLSP and there is no shortcut entry.

Step 3: Identify the Enable Packet. The router receives and forwards data packets in the traditional way. It is known from the target address routing table of the packet that this packet should be transferred from the second interface of FastEthernet1/0, and the packet is encapsulated as VLAN 2 frame and sent back through the ISL link. At this point, the router has rewritten the frame header of the layer 2 frame. At the same time, the router not only rewrites the VLAN number of the ISL header, but also modifies two MAC address domains: the source MAC is changed to the MAC address of the router outlet, and the target MAC is changed to the MAC address of host B. Although the IP address of the packet is not overwritten, the TTL value of the IP packet header is reduced by 1, so the checksum of the IP packet header needs to be modified accordingly. This modified packet is called Enable Packet. When the packet is sent from the router and reaches the destination host computer B through the switch, the following five functions should be performed: the layer 2 switch knows that the packet should be forwarded from the PORT 3/1 port according to the destination MAC address of the enabling packet; MLS-SE knows that the source address on the frame header of the enable packet is one of the address records established through the Hello process.

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