Four steps for multi-layer Switching

MLSMultiLayer Switching provides a hardware-based layer-3 high-performance switch for the switch. It uses advanced dedicated ic asic) switching components to complete IP packet switching between subnets, which can greatly reduce the overhead of the router when processing data packets.

MLS is a technology that uses hardware processing packages to exchange and override frame headers to improve IP routing performance. Cisco multi-layer switching technology supports all traditional routing protocols, and the Frame Forwarding and rewriting functions originally completed by routers are now completed by the hardware of the switch. MLS migrates the packet switching function of a traditional router to a layer-3 switch. Of course, this requires that the switched path must exist first.

MLS consists of the following three parts:
1. multi-tier routing processor (MLS-RP) it is equivalent to a router in the network, responsible for processing the first packet of each data flow, assisting the MLS switching engine (MLS-SE) in the third layer of the CAM (Content-Addressable Memory) ). The MLS-RP can be an external router or a layer-3 Switch Routing Switching Module RSM.

2. Multi-layer switching engine (MLS-SE) It is the switching entity responsible for processing the forwarding and rewriting of data packets.

3. Multi-layer switching protocol (MLSP) is a lightweight protocol used to initialize a multi-layer switching engine through a multi-layer routing processor (MLS-RP.
The following describes how to implement multi-layer Switching Based on the network shown in Figure 1.
　
Step 1: Send the MLSP Hello Message

After the router is activated, the multi-layer router processor sends a MLSP Hello packet every 15 seconds. These packets contain the vlan id and MAC address information used by the router interface. The MLS-SE uses this information to understand the Layer 2 properties of a router with multi-layer switching capability. If the switch is connected to multiple MLS-RP, The MLS-SE differentiates the MAC address entries for each MLS-RP by assigning an XTAG value to their MAC addresses. If the MLSP frame gets all MAC addresses from the same MLS-RP, The MLS-SE attaches the same XTAG value to it, as shown in 2. These associated records are stored in the CAM. Because the Hello packet is sent cyclically, this method can ensure that the relevant values dynamically track network changes and implement a certain elimination mechanism. The Hello package is released on the second layer. It uses the multicast address 01-00-0C-DD-DD-DD.
　
Step 2: Identify the Candidate Packet)

After understanding the IP address of the vro with multi-layer switching capability, the MLS-SE can judge the packet entering the vswitch. For a packet in a stream, if the MLS cache contains a matched shortcut entry, the MLS-SE forwards the packet directly through the router; if the MLS does not include a shortcut entry that matches the packet, the MLS-SE classifies it as a candidate package and creates part of the shortcut Partial cut in the cache ). Such a package is processed by a traditional Layer 2 switch and sent to the connected router interface (GATEWAY), as shown in figure 3.
　
Note that the candidate packet frame must meet two criteria: the target address goes through a MAC address of the router interface listed in MLSP; there is no shortcut entry.

Step 3: Identify Enable Packet)

The router receives and forwards data packets in the traditional way. According to the route table of the destination address of the packet, the packet should be transferred from the second interface of Fast Ethernet1/0, and the packet is encapsulated as VLAN2 frame and sent back through the ISL link. The specific process is shown in step 4.
　
At this time, the router has overwritten the frame header of the second frame. At the same time, the vro not only modifies the ISL header VLAN number, but also modifies two MAC address domains: the source MAC address is changed to the MAC address at the router exit, and the target MAC address is changed to the MAC address of host B. Although the IP address of the packet is not changed, the TTL value of the IP address header is reduced by 1, so the checksum of the IP Address Header also needs to be modified accordingly.

The modified Packet is called the Enable Packet). When the Packet is sent from the router and passes through the switch to host B, the following five functions must be performed:

The second layer switch knows that the packet should be forwarded from Port 3/1 Based on the destination MAC address of the enable packet;
The MLS-SE knows that the source address of the enable packet is one of the address records established through the Hello process;
The MLS-SE searches for some shortcut entries created in step 2 based on the destination IP address of the Enable package;
The MLS-SE compares the XTAG value associated with the enable packet source MAC address with the XTAG value corresponding to some shortcuts, if matched, it indicates that the Enable package and the candidate package in step 2 come from the same vro;
The MLS-SE completes the creation of the shortcut entry, which contains all the information required to override the subsequent packet frame header in the data stream.

Step 4: direct exchange and forwarding) subsequent packets in the data stream

When subsequent packets are sent by host A, the MLS-SE uses the destination IP address in the packet to find the complete shortcut established in step 3. After address match, the MLS-SE uses the rewrite engine to modify the frame header information, and then directly forward the packet to host B not to the router ). The rewrite operation modifies the frame header field. The value is the same as the domain value of the first packet modified by the router. See Figure 5. Here, we need to explain that NFFC (NetFlow Feature Card) is a network flow performance Card equipped with a layer-3 switch, which maintains the MLS Cache of the layer-3 Exchange data packet stream ), as part of a multi-layer exchange engine.

The above process is called "One route, multiple switches ". The switch uses specialized hardware ASIC to process data packets. The speed can reach 1000 Mbps or even Mbps.