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Chapter 33 Routing Protocol Overview
To communicate with a remote host over the Internet, a host must choose a proper route via a set of rout ers or
Layer3 switches. Both routers and layer3 switches calculate the rout e using CP U, 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 s election, lay er3 switches have
great advantage over routers in data forwarding. The following describes basic principle and methods used in
layer3 switch route selection.
In route s election, 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 rout e 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 lay er3 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 prot ocol used to calculate the route to a destination inside an autonomous system. IGP
supported by switch include RIP and OSPF, RIP and OSRF c an be configured according to the requirement.
Switch supports running several IGP dynamic routing protocols at the same time. Or, other dynamic routing
protocols and static route can be introduced to a dynamic routing protocol, so that multiple routing protocols
can be associated.
EGP is used to exchange routing information among different autonomous systems, such as BGP protocol.
EGP support ed by switch include BGP-4, BGP-4+.
33.1 Routing Table
As mentioned before, layer3 switch is mainly used to establish the route from the current layer3 switch to a
network or a host, and to forward packets according to the route. Each layer3 switch has its own route table
containing all routes used by that switch. Each route entry in the route table specifies the physical port should
be used for forwarding packet to reach a destination host or the next hop layer3 switch to the host.
The route table mainly consists of the following:
Destination address: used to identify the destination address or destination net work of an IP
packet.
Network mask: used t ogether with destination address to identify the destination host or t he
33-1
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