Factors for reference in the use of layer-3 switching Technical Features

Factors for reference in the use of layer-3 switching technology, such as line rate routing, IP routing, routing functions, route protocol support, and possible causes of route table changes will be covered in the following article. This article can be used as a reference for practitioners.

Three-layer switching technical features line rate routing:

Compared with traditional routers, the routing speed of layer-3 switches is generally 10 times faster or dozens of times faster, enabling line rate routing forwarding. Traditional routers use software to maintain the route table, while layer-3 switches use ASIC Application Specific Integrated Circuit) hardware to maintain the route table, so as to achieve line-rate routing.

Layer-3 switching technology features IP routing:

On a LAN, a layer-2 switch identifies the packet sender through the source MAC address and forwards the packet according to the destination MAC address. For a data packet whose destination address is not on the local area network, a layer-2 switch cannot directly send it to the destination. It must be forwarded through a routing device such as a traditional router, connect the vswitch to the routing device.

If you set the default gateway of A vswitch as the IP address of the router device, the layer-3 switch will send the packet to be forwarded by the route device. The routing device checks the destination address of the data packet and its route table. If the route table finds the forwarding path, the routing device forwards the data packet to other network segments. Otherwise, the data packet is discarded.

Vro is expensive, complex, and slow. It is easy to become a network bottleneck because it needs to analyze all broadcast packets and forward some of them, and exchange route information with other vrouters, and these processing processes are all handled by the CPU rather than the dedicated ASIC), so the speed is slow.

A layer-3 Switch identifies a packet forwarding through a MAC address like a layer-2 switch, and also performs route forwarding between two CIDR blocks as a traditional router. In addition, the layer-3 switch can implement line rate routing because it uses a dedicated chip to process route forwarding.

Layer-3 switching technology features Routing

Compared with traditional vrouters, layer-3 vswitches are not only fast but also easy to configure. In the simplest case, when the layer-3 switch enables the automatic discovery function by default), once the switch is connected to the network, you only need to set the VLAN and set a route interface for each VLAN.

The layer-3 switch automatically limits the data stream in the subnet to the subnet, and implements Packet Exchange Between subnets through routing. The administrator can also manually configure the routing mode: Set a port-based VLAN, and assign an IP address and subnet mask to each VLAN to generate a routing interface. Then, manually set the static route or start the dynamic routing protocol.

Layer-3 switching technology features routing protocol support:

The layer-3 switch can use the automatic discovery function to process the forwarding of local IP packets and learn the addresses of neighboring routers. It can also use the dynamic routing protocols RIP1, RIP2, and OSPF to calculate the routing path. The following describes the RIP and OSPF protocols.

Routing Information Protocol (RIP) is an Internal Gateway Protocol (IGP). It is mainly used in medium-scale networks. The RIP protocol uses the Distance Vector Algorithm and includes the route information to reach the destination IP vector) ), the path with the minimum number of hops is the optimal path.

The maximum number of hops allowed by RIP is 15. The destination address that requires 16 or more hops is considered inaccessible. The RIP Router exchanges route information with the neighboring RIP Router through periodic broadcast. The broadcast interval can be set. The broadcast content is the entire route table.

When the RIP Router receives the route table of the neighboring router, it determines whether to update its route table after calculation. If your route table needs to be updated, the router sends the updated content to the neighboring router immediately after the update is completed without waiting for the end of the broadcast interval.

The route table may change due to the following reasons:
◆ A new interface is started;
◆ The interface in use experienced a fault;
◆ The route table of the neighboring router is changed;
◆ The lifecycle of a record in the route table ends and is automatically deleted.

The RIP Router must receive the route information of the neighboring router within each broadcast period. If the router cannot receive the information, the router will discard the route: If the router does not receive the information within 90 seconds, the router will replace this route with other neighboring routes with the same HOP times. If the route is not received within 180 seconds, the neighboring router is considered inaccessible.

RIP divides routers into two types: active and passive. The active router can send its own route table or accept the route table of the neighboring router. A passive router can only accept route tables of neighboring routers. Once a port of the RIP Protocol has learned a route, it retains the route until it learns a better route.

Once a port broadcast means that a route fails, other ports that receive the message must process the route information obtained through RIP. If no route information is broadcast to the Internet within 180 seconds, the route will be considered invalid.

In addition, when the interface starts RIP, it creates a route table through the interface directly connected to it. Information loops may occur when you exchange route information with neighboring routers and establish a stable and optimal route table. Once the router receives a route that uses itself as the intermediate jump, there must be an information loop.

For example, R2 has a route to RA, which broadcasts this route to R1. However, there are also routes to RA in the routing information of R1 to R2, in addition, the information loop appears when R2 is used as the hop router. The horizontal segmentation technology can avoid the generation of such information loop.

Layer-3 switching features automatic discovery:

Some layer-3 switches have the automatic discovery function, which can reduce the configuration complexity. Layer-3 switches can learn route information by monitoring data streams. By analyzing inbound port data packets, layer-3 switches can automatically discover and generate a broadcast domain, VLAN, IP subnet, and update their members.

The automatic detection function improves network performance without changing any configurations. The layer-3 switch automatically has the IP packet routing function after it is started. It checks all inbound packets to learn the subnet and workstation address, it automatically sends route information to neighboring routers and layer-3 switches to forward data packets.

Once the layer-3 switch is connected to the network, it starts to listen to data packets on the Internet and build and update the route table according to the learned content. During the automatic discovery process, no additional management configurations are required for the vswitch, and no probe packets are sent to increase the network burden. You can use the automatic discovery function to obtain simple and efficient network performance, and then add other routing and VLAN functions as needed.

On the third layer, the automatic discovery process is as follows:

◆ Detect the original IP address of ARP, RARP, or DHCP response packet, and discover the IP subnet topology within several seconds.

◆ Establish a logical connection between different CIDR blocks of the same network, that is, route between CIDR blocks to achieve Information Communication Between CIDR blocks.

◆ Learning address: VLAN is configured based on IP subnet, network Protocol, or multicast address, and VLAN members are dynamically updated using IGMP Internet Group Management Protocol.

◆ Supports ICMP Internet Control Message Protocol) route discovery options.

◆ Store learned routes to hardware and forward data packets of these addresses at line rate.

◆ Send a packet whose destination address is not in the route table to another vro on the network.

◆ Listen to ARP requests to learn the address of each workstation.

◆ IP packet switching within the subnet.

The second layer automatically discovers the following process:

◆ Hardware address MAC) learning to find the network structure based on the hardware address MAC.

◆ Create a route table Based on ARP requests.

◆ Exchange various non-IP packages.

◆ View the destination address of the received packet. If the destination address is known, forward the packet to a known port. Otherwise, broadcast the packet to all the members of the VLAN where it is located.