38-3
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter38 Configuring IP Unicast Routing
Understanding IP Routing
Default routing refers to sending traffic with a destination unknown to the router to a default outlet
or destination.
Static unicast routing forwards packets from predetermined port s through a single path into and out of a
network. Static routing is secure and uses little bandwidth, but does not automatically respond to
changes in the network, such as link failures, and therefore, might result in unreacha ble destinations. As
networks grow, static routing becomes a labor-intensive liability.
Dynamic routing protocols are used by routers to dynamically calc ulate the best route for forwarding
traffic. There are two types of dynamic routing protocols:
•Routers using distance-vector protocols maintain routing tables with distance values of networked
resources, and periodically pass these tables to their ne ighbors. Distance-vector protocols use one
or a series of metrics for calculating the best routes. Th ese protocols are easy to configure and use.
•Routers using link-state protocols maintain a complex database of network topology, based on the
exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in
the network, which speeds up the convergence time or time required to respond to these changes.
Link-state protocols respond quickly to topology changes, but require greater bandwidth a nd more
resources than distance-vector protocols.
Distance-vector protocols supported by the switch are Routing Information Protocol (RIP), which use s
a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which
adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state
protocol and Enhanced IGRP (EIGRP), which adds some link-stat e routing features to traditional
Interior Gateway Routing Protocol (IGRP) to improve efficiency.
Note On a switch or switch stack, the supported protocols are deter mined by the software running on the
switch or stack master. If the switch or stack master is running the IP base feature set, only default
routing, static routing and RIP are supported. All other routing protocols require the IP service s feature
set.
IP Routing and Switch Stacks
A Catalyst 3750-E switch stack appears to the network as a single ro uter, regardless of which switch in
the stack is connected to a routing peer. For additional information ab out switch stack operation, see
Chapter5, “Managing Switch Stacks.”
The stack master performs these functions:
•It initializes and configures the routing protocols.
•It sends routing protocol messages and updates to other rout ers.
•It processes routing protocol messages and updates received from peer routers.
•It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database
to all stack members. The routes are programmed on all switches in the stack bases on this database.
•The MAC address of the stack master is used as the router MAC address for the whole stack, and
all outside devices use this address to send IP packets to the stack.
•All IP packets that require software forwarding or processing go through the CPU of the stack
master.