Layer 3 switch principle and configuration

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Layer 3 switch principle and configuration

1. Overview of Layer 3 Switching Technology

Layer 3 switches use hardware to switch and route packets
Implement inter-VLAN communication using Layer 3 switching technology
Layer 3 switching = Layer 2 switching + Layer 3 forwarding

2. Traditional MLS

Layer 2 needs to be re-encapsulated during Layer 3 forwarding.
On a Layer 3 switch, the Layer 3 engine processes the first packet of the data flow.
The switching ASIC learns the Layer 2 rewrite information from the Layer 3 engine and creates an MLS entry in the hardware
Responsible for rewriting and forwarding subsequent data in the data flow.

Simply put, after the host broadcasts the first ARP request, it sends the data to the Layer 3 switch. The switch learns the Layer 2 rewrite information from the Layer 3 engine, records it to the underlying hard disk, and forwards it. The next time it has the same request When the data flow of address information passes through again, forwarding is completed directly at the second layer (data link layer) according to this table, that is, "once routing, multiple exchanges", which effectively improves the efficiency of data packet forwarding. *Layer 2 must be re-encapsulated during Layer 3 forwarding.

3. MLS based on CEF

CEF concept: topology forwarding-based model

1. Forwarding Information Base (FIB)

2. Adjacency table

3. Working principle:

① Host A sends a unicast data packet to host B

②. The switch searches the FIB table and finds the next hop address.

③. Find the Layer 2 encapsulation information of the adjacency relationship corresponding to the next hop address.

④、Forward

4. Configuration of Layer 3 switch

1. Layer 3 switch configuration commands

SW1:

vlan batch 10 20 30: 建立vlan10 20 30

int g0/0/1: 进入接口模式

port link-type trunk: 选择端口类型（连接交换机）

port trunk allow-pass vlan all:允许所有vlan通过

int vlanif 10: 建立虚拟接口

ip add IP地址 子网掩码: 配置ip地址

SW2:

vlan batch 10 20 30: 建立vlan10 20 30

int e0/0/1: 进入接口模式

port link-type access: 选择端口类型（连接主机）

port default vlan 10: 将端口加入到vlan10中

int e0/0/4: 进入接口模式

port link-type trunk: 选择端口类型（连接交换机）

port trunk allow-pass vlan all:允许所有vlan通过