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THE MULTILAYER SWITCH We have already described multilayer switching as an intelligent combination of switching and routing technologies which provides a complete and integrated solution for all kinds of LAN infrastructures. Now let's get into some more detail. A multilayer switch is a device which has multiple LAN ports, over which stations can communicate either by means of Layer 2 packet forwarding (as in conventional LAN switching), or by means of Layer 3 packet forwarding (as in conventional routing). The type of packet forwarding that is used in each case is whichever is needed for any given pair of stations to intercommunicate. In practice, this depends on whether they are members of the same subnetin which case Layer 2 forwarding is usedor if they are members of different subnets, in which case Layer 3 forwarding comes into play. MULTILAYER SWITCHING DEVICES A multilayer switching device can be logically viewed as a Layer 2 switching fabric which has a Layer 3 forwarding function attached to it by a high capacity connection. A number of LAN port interfaces are attached directly to the Layer 2 switching fabric. Just like a conventional router, the Layer 3 forwarding function has one or more IP addresses and MAC addresses associated with it that end stations use to send IP packets to it, for forwarding on to different subnets. Similarly, if the Layer 3 forwarding function supports other protocols such as IPX, it would look like an IPX router from the point of view of the end stations. Let's consider how a pair of stations using IP would communicate across the multilayer switch, via, say, Ethernet segments on each side. The sending station starts out by knowing the IP address of the destination station, but not the MAC address which is needed to send the packet out on the Ethernet. It uses the Address Resolution Protocol (ARP) to determine this. The sending station compares its own IP address with the IP address of the target station and uses the subnet mask which is configured in its software to determine whether the destination station is a member of the same subnet as itself or not. If the two stations are on the same subnet, the sending station will broadcast an ARP request identifying the destination IP address and requesting the station that owns this address to respond with its MAC address. On receipt of the response, the sending station will cache this address and use it to send the Ethernet packets to the destination. When these packets arrive at the multilayer switch, the Layer 2 switching fabric will look up the destination MAC address to establish which port to forward the packet on, and send it on its way. If the two stations are on different subnets, the sending station will expect to transmit packets via the default gateway (which, confusingly, means a router), the IP address of which is configured into its software. This IP address will actually be the address that refers to the Layer 3 forwarding function in the multilayer switch. So when the station broadcasts an ARP request for the default gateway IP address, the multilayer switch responds with the MAC address which corresponds to its Layer 3 forwarder. Then when the sending station starts transmitting Ethernet packets with this destination MAC address, the Layer 2 switching fabric will send these packets directly to the Layer 3 forwarder. At this point, the Layer 3 forwarder may need to broadcast an ARP request to obtain the MAC address of the final destination station, which it will store in a cache. As each packet is forwarded, the original MAC destination address (which refers to the Layer 3 forwarder itself) is stripped off and replaced by the new MAC address, which refers to the final destination station. The packet is then sent back into the Layer 2 switching fabric where the MAC address tables are used to direct the packet to the correct output port. (See Exhibit 2-2-3.)
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