4 steps to realize multi-tier exchange

MLS (multilayer switching, multi-layer switching) provides hardware-based third-tier high-performance switching for switches. It uses the advanced special-purpose integrated circuit (ASIC) Exchange parts to complete the IP packet exchange between subnets, which can greatly reduce the overhead caused by routers in processing packets.

MLS is a technology that uses hardware to exchange and rewrite frame headers to improve IP routing performance. Cisco multilayer switching technology supports all traditional routing protocols, while the frame forwarding and rewriting capabilities originally performed by routers are now completed by the hardware of the switch. MLS migrates the packet switching capabilities of traditional routers to a third tier switch. Of course, this first requires that the path of the exchange must exist.

MLS consists of the following three parts:

1. Multi-layer routing processor (MLS-RP) it is equivalent to routers in the network, responsible for processing the first packet of each data flow, to assist the MLS switching engine (MLS-SE) in the third layer of CAM (content-addressable Memory) To create a shortcut entry (shortcut Entry). The MLS-RP can be an external router, or it can be implemented by the routing switching Module (RSM) of the three-tier switch.

2. Multi-tiered switching engine (MLS-SE) It is the Exchange entity responsible for handling forwarding and rewriting of packet functions.

3. Multi-layer Switching protocol (MLSP) it is a lightweight protocol that is used to initialize a multi-layer switched Exchange engine via a multi-tiered routing processor (MLS-RP).

The following example of the network shown in Figure 1 illustrates the steps of Multi-layer interchange implementations.

The first step: send MLSP Hello information

When the router is activated, the multi-tier routing processor sends a MLSP Hello package every 15 seconds, which contains the VLAN identification and MAC address information used by the router interface. Mls-se uses this information to master the second-tier properties of routers with multi-layer switching capabilities. If the switch is connected to multiple Mls-rp,mls-se, the MAC address entry for each MLS-RP is distinguished by a method that assigns xTAG values to their MAC addresses. If the MLSP frame obtains all MAC addresses from the same MLS-RP, Mls-se attaches the same xTAG value, as shown in Figure 2. These associated records are stored in cam. Since the Hello packet is sent periodically, this method can ensure that the correlation value dynamically tracks the change of the network and can achieve a certain elimination mechanism. The Hello package is published on the second level, and it uses the multicast address 01-00-0C-DD-DD-DD.

Step two: Identify candidate packages (candidate Packet)

After understanding the related addresses of routers with multilayer switching capabilities, Mls-se can match the data packets that enter the switch. For a packet in a stream, if the MLS cache contains a shortcut entry to match, Mls-se forwards the packet directly on the bypass router, or if the MLS does not contain a shortcut entry that matches the packet, mls-se it as a candidate package and establishes a partial shortcut to the cache (Partial Shortcut). Such packages are handled in a traditional second-tier switch, and are connected to the Router interface (gateway), as shown in Figure 3.

It is important to note that the candidate package (frame) must meet two criteria: the destination address passes through a MAC address of the router interface listed in MLSP; there is no shortcut entry.