Three-layer exchange realizes communication between different VLANs

The three-tier switch swaps and routes the packet through hardware. To complete high-level information on the processing of packets in hardware, Cisco switches use the traditional MLS (multi-layer switching) architecture or the CEF (Fast forward) MLS architecture.

1, the traditional MLS

MLS allows the ASIC (applied ASIC) to perform a second-level rewrite operation on the routed packet, and the second layer overrides the source and destination MAC address, and the cyclic Redundancy check code (CRC) that is written to recalculate.

With traditional MLS, the switch forwards the first packet in the data stream to the third-tier engine, which processes the packet in a software-switched manner. After the first packet in the data flow is routed, the third tier engine processes the hardware Exchange components to select routes for subsequent packets.

2, based on the CEF MLS

CEF is a topology-based forwarding model that can be used to add all routing information to the forwarding repository (FIB) in advance. This allows the switch to quickly find routing information.

CEF mainly contains the following two information tables for forwarding.

Forwarding information Base (FIB): CEF uses FIB to make forwarding decisions based on the destination IP prefix. FIB is similar to the routing table, which contains a mirror of the forwarded information in the routing table. When the topology of the network changes, the routing table will be updated and the FIB will change. The fib contains the next hop address information, which is based on the information in the routing table. When using CEF MLS, the third tier engine and the hardware switching component maintain a fib.

adjacency table: In a network, if two nodes have only one hop in the data link layer, they are adjacent to each other. In addition to FIB, CEF uses an adjacency table to store 2-tier compilation information, which contains the corresponding second-level address for each FIB entry adjacency table. As with FIB, the third-tier engine and hardware Exchange components maintain an adjacency table when using CEF MLS.

3, the introduction of virtual interface

You may not notice that the VLAN1 in the switch is actually a virtual interface, the default all interfaces belong to this VLAN, so no matter from which port access to the switch, you can connect to the switch remotely, provided that the configuration IP address can be related to the password. From this point we can analyze that we only need to open the relevant VLAN on the three-tier switch and configure the IP address, which belongs to all the gateways of the host within that VLAN.

Three-tier switching configuration

1. Turn on routing function

Switch (config) #ip routing

2. Create VLAN and configure address

Columns such as: Create VLAN 10 and VLAN 20 on three-tier exchange and configure IP addresses to enable communication between different VLANs.

The topology map is as follows:

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