Evaluating an Existing Network

If you are building an enterprise network from scratch, you might find that it is fairly straightforward to build it in a hierarchical fashion. After all, you can begin with switches in the core layer and fan out into lower layers to meet the users, server farms, and service providers.

In the real world, you might be more likely to find existing networks that need an overhaul to match the hierarchical model. Hopefully, if you are redesigning your own network, you already know its topology and traffic patterns. If you are working on someone else's network, you might not know about its structure.

This section provides some basic information on two tasks:

■ Discovering the existing topology

■ Planning a migration to a better campus model

Discovering the Network Topology

Whether or not a diagram of a network is available, you should consider tracing out the topology for yourself. For one thing, network documentation tends to become out-of-date or isn't drawn to show the type of information you need.

Some network administrators draw up a diagram that shows only the physical cabling between network devices. That might benefit someone who is working with the cabling, but it might not show any of the logical aspects of the network. After all, switched networks can be cabled together and then configured into many logical topologies.

As you discover or trace out a network, you might end up building several diagrams. One diagram might show all the network devices and only the physical cabling between them. Further diagrams might show Layer 2 VLANs and how they extend through the network.

To discover an existing network, you can connect a computer to any switch as a starting point and begin to "walk" the topology. Cisco devices periodically send information about themselves to any neighboring devices. This is done with the Cisco Discovery Protocol (CDP).

TIP The information exchanged in CDP messages includes the device type, software version, links between devices, and number of ports within each device.

By default, CDP runs on each port of a Catalyst switch, and CDP advertisements occur every 60 seconds. CDP communication occurs at the data link layer so that it is independent of any network layer protocol that might be running on a network segment. This means that CDP can be sent and received using only Layer 2 functionality. CDP frames are sent as multicasts, using a destination MAC address of 01:00:0c:cc:cc:cc.

Cisco Catalyst switches regard the CDP address as a special address designating a multicast frame that should not be forwarded. Instead, CDP multicast frames are redirected to the switch's management port and are processed by the switch supervisor alone. Cisco switches become aware only of other directly connected Cisco devices.

CDP is enabled by default on all switch interfaces. To manually enable or disable CDP on an interface, use the following interface configuration command:

Switch(config-if)# [no] cdp enable If a switch port connects to a non-Cisco device or to a network outside your administrative control, consider disabling CDP on that port. Add the no keyword to disable CDP.

CDP is enabled by default on all Cisco switches and routers, so, chances are, you will be able to make use of it right away. With CDP, a switch becomes aware of only the devices that are directly connected to it. Therefore, you walk the topology one "hop" at a time: connect to one switch, find its neighbors, and then connect to them one at a time.

Figure 2-6 shows this process being used to discover a sample network. (The arrows in the sequence illustrated in Figure 2-6 point out where you are positioned as the topology is discovered.) A laptop PC has been connected to the console connection of an arbitrary switch, Switch-A. Here, Switch-A is a Catalyst 3550, determined either by inspection or from the show version command.

Figure 2-6 Network Discovery with CDP

Switch-A

show cdp neighbors [detail] Catalyst 3550

192.168.254.3

Figure 2-6 Network Discovery with CDP

Switch-A

show cdp neighbors [detail] Catalyst 3550

192.168.254.3

Telnet 192.168.254.17 Switch-A

show cdp neighbors [detail] Catalyst 3550

192.168.254.3

Switch-B Catalyst 4500 192.168.254.17

Telnet 192.168.254.17 Switch-A

show cdp neighbors [detail] Catalyst 3550

192.168.254.3

Switch-B Catalyst 4500 192.168.254.17

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