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Chapter 6: InterVLAN and Basic Module
Configuration
In Depth
One of the first things you will discover in this chapter is that switch is merely a marketing term. When we
think of a switch, we think of a device that operates at Layer 2. Well, in this chapter we’ll walk through the
process of configuring Cisco switch−swappable cards and modules, and you’ll find that today’s switches have
modules and cards that allow them to operate not just at Layer 2 but at Layers 3 and 4, as well. Although this
chapter does not include the new Cisco 11000 series Web switches, they operate at Layer 5. This makes the
term switch very blurry, doesn’t it?
Normally, regardless of the vendor, routing is not considered a switch function. As you will learn, today’s
Cisco switches have plenty of features that involve routing. Today’s switches can also run routing protocols
that can be used for path determination and building routing tables; more to the point, they use Routing
Information Bases (RIBs). (A RIB is what you see when you use the show ip route command on a router.)
You can add many modules to a Cisco switch. In fact, we’d need this book and three others like it to
completely cover every module that can be placed in the switches. This chapter will focus on configuring
three internal route processors: the Route Switch Feature Card (RSFC), the Route Switch Module (RSM), and
the Multilayer Switch Module (MSM). It will also supplement what you have already learned in the book
about configuring the Supervisor Engine and Ethernet module interfaces.
Internal Route Processors
An internal route processor can be thought of as a router on a card. In a typical situation, the first packet to a
destination must go through the RIB to see if a route has been discovered by matching the destination address
field of the packet header. Routing protocols are used to learn the topology of the network and place the
information the protocols learn in a topology table called the Forwarding Information Base (FIB). Based on
information contained in the FIB, routes are calculated based on metrics used by the routing protocol and the
best route (and sometimes a feasible successor) is placed in the RIB. The RIB examines an incoming packet to
select the outgoing interface to which the packet is to be sent. The forwarding decision can be based on a
minimal amount of information, such as the destination address.
Cisco defines the FIB as a forwarding table that has an entry for every entry in the RIB. When Cisco speaks of
a forwarding cache, it means the forwarding table that contains the most recently used subset of the routes in
the RIB.
In a device using the Cisco Express Forwarding (CEF) Application−Specific Integrated Circuit (ASIC), each
forwarding element has its own copy of the FIB, which contains every route contained in the RIB. One of the
advantages of having the CEF ASIC, in comparison with other switching ASICs, is a one−to−one
correspondence between the RIB and FIB entries, thus making it unnecessary for the switch to maintain a
cache. When a destination address is received and is not present in the cache, the cache is invalidated and a
new FIB is generated. Depending on the platform, routing may slow or come to a stop during the cache
reconstruction.
The switch creates a routing table first and then forwards the information from the routing table to the FIB.
The FIB uses a highly optimized routing lookup algorithm. By prefix−matching the destination address, the
FIB can look up the destination in a large routing table much more quickly than it could using the
line−by−line lookup of a traditional routing table.
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