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S2989G-24TX Operation Manual

Chapter 4 Routing Protocol Related Configuration

4-1

Chapter 4 Routing Protocol Related

Configuration

4.1

Routing Protocol Overview

To communicate with a remote host over the Internet, a host must choose a proper

route via a set of routers or Layer3 switches.

Both routers and layer3 switches calculate the route using CPU, the difference is that

layer3 switch adds the calculated route to the switch chip and forward by the chip at wire

speed, while the router always store the calculated route in the route table or route buffer,

and data forwarding is performed by the CPU. For this reason, although both routers and

switches can perform route selection, layer3 switches have great advantage over routers

in data forwarding. The following describes basic principle and methods used in layer3

switch route selection.

In route selection, the responsibility of each layer3 switch is to select a proper midway

route according to the destination of the packet received; and send the packet to the next

layer3 switch until the last layer3 switch in the route send the packet to the destination

host. A route is the path selected by each layer3 switch to pass the packet to the next

layer3 switch. Route can be grouped into direct route, static route and dynamic route.

Direct route refer to the path directly connects to the layer3 switch, and can be

obtained with no calculation.

Static route is the manually specified path to a network or a host; static route cannot

be changed freely. The advantage of static route is simple and consistent, and it can limit

illegal route modification, and is convenient for load balance and route backup. However,

as this is set manually, it is not suitable for mid- or large-scale networks for the route in

such conditions are too huge and complex.

Dynamic route is the path to a network or a host calculated by the layer3 switch

according to the routing protocols enabled. If the next hop layer3 switch in the path is not

reachable, layer3 switch will automatically discard the path to that next hop layer3 switch

and choose the path through other layer3 switches.

There are two dynamic routing protocols: Interior Gateway Protocol (IGP) and

Exterior Gateway protocol (EGP). IGP is the protocol used to calculate the route to a

destination inside an autonomous system. IGP supported by switch include RIP and

OSPF, RIP and OSRF can be configured according to the requirement. Switch supports