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Cisco Catalyst Switch Module 3110 and 3012 for IBM BladeCenter Software Configuration Guide
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Chapter 38 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 ports through a single path into and out of a
network. Static routing is secure and uses little bandwidth, but it does not automatically respond to
changes in the network, such as link failures. Therefore, network changes might result in unreachable
destinations. As networks grow, static routing becomes a labor-intensive liability.
Routers use these dynamic routing protocols to dynamically calculate the best route for forwarding
traffic:
• Routers that use distance-vector protocols maintain routing tables with distance values of networked
resources and periodically pass these tables to their neighbors. Distance-vector protocols use one or
a series of metrics for calculating the best routes.
• (Only on the Catalyst Switch Module 3110) 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 and more resources than distance-vector protocols.
Distance-vector protocols supported by the Catalyst Switch Module 3110 use Routing Information
Protocol (RIP), a single-distance metric (cost) that determines 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-state routing
features to traditional Interior Gateway Routing Protocol (IGRP) to improve efficiency.
The Catalyst Switch Module 3012 supports only RIP.
Note On a switch or switch stack, the supported protocols are determined 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 services feature
set.
IP Routing and Switch Stacks
A switch stack appears to the network as a single router, regardless of which switch in the stack is
connected to a routing peer. For additional information about switch stack operation, see Chapter 5,
“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 routers.
• 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.
