T

Telnet 192.168.254.199

Switch-B Catalyst 4500

show cdp neighbors [detail] Catalyst 3550 192.168.254.17

Gig 3/1

Switch-A

192.168.254.3

Gig 1/1

Gig 0/1

Gig 1/1

Gig 2/1

Gig 2/1

Router 192.168.254.1

Switch-C Catalyst 3550 192.168.254.199

Router 192.168.254.1

Switch-C Catalyst 3550 192.168.254.199

Gig 0/1

Gig 0/1

At the top of the figure, you don't know whether Switch-A is in the core, distribution, or access layer. Actually, you don't even know whether this network has been built in layers.

When you are connected and in the privileged EXEC or enable mode, you can begin looking for CDP information by using the show cdp neighbors command. At Switch-A, suppose the command had the output in Example 2-1.

Example 2-1 show cdp neighbors Command Output Reveals CDP Information

Switch-A#

show cdp neighbors

Capability

Codes: R - Router, T

Trans Bridge, B - Source Route Bridge

S - Switch, H

Host, I -

IGMP, r - Repeater, P -

Phone

Device ID

Local Intrfce

Holdtme

Capability Platform

Port ID

Switch-B

Gig 0/1

152

R S I WS-C4506

Gig 1/1

Switch-A#

Based on the neighbors listed, you should be able to draw the connections to the neighboring switches and detail the names and model of those switches. Notice that the CDP neighbor information shows the local switch interface as well as the neighbor's interface for each connection. This is helpful when you move to a neighbor and need to match the connections from its viewpoint.

From the output in Example 2-1, it's apparent that Switch-A has a neighbor called Switch-B on interface GigabitEthernet 0/1. Switch-B is a Catalyst 4506.

Now you can use a variation of the command to see more detail about each neighbor. The show cdp neighbors [interface mod/num] detail command also shows the neighbor's software release, interface settings, and its IP address, as demonstrated in Example 2-2.

Example 2-2 show cdp neighbors detail Command Output Reveals Detailed Information About Neighboring Switches

Switch-A# show cdp neighbors detail

Device ID: Switch-B

Entry address(es): 192.168.254.17

Platform: cisco WS-C4506, Capabilities: Router Switch

IGMP

Interface: GigabitEthernet0/1, Port ID (outgoing port)

: GigabitEthernet1/1

Holdtime : 134 sec

Version :

Cisco Internetwork Operating System Software

IOS (tm) Catalyst 4000 L3 Switch Software (cat4000-I9S-

M), Version 12.2(18)EW, EARLY

DEPLOYMENT RELEASE SOFTWARE (fc1)

TAC Support: http://www.cisco.com/tac

Copyright 1986-2004 by cisco Systems, Inc.

Compiled Fri 30-Jan-04 02:04 by hqluong

advertisement version: 2

VTP Management Domain: 11

Duplex: full

Management address(es):

Switch-A#

When you know the IP address of a neighboring device, you can open a Telnet session from the current switch to the neighboring switch. (This assumes that the neighboring switch has been configured with an IP address and a Telnet password on its vty lines.) Choose a neighbor and use the telnet ip-address command to move to the neighbor and continue your discovery. At Switch-B (the middle of Figure 2-6), you might see the CDP neighbor output in Example 2-3.

Example 2-3 show cdp neighbors Command Output Display for Switch-B

Switch-B#

show cdp neighbors

Capability

Codes: R - Router, T

Trans Bridge, B - Source Route Bridge

S - Switch, H

Host, I -

IGMP, r - Repeater, P - Phone

Device ID

Local Intrfce

Holdtme

Capability Platform Port ID

Switch-A

Gig 1/1

105

S I WS-C3550-4Gig 0/1

Switch-C

Gig 2/1

139

S I WS-C3550-4Gig 0/1

Router

Gig 3/1

120

R Cisco 2610Fas 0/0

Next, the show cdp neighbors detail command reveals that Switch-C has the IP address 192.168.254.199, so you can open a Telnet session there. Switch-C might show only one neighbor (Switch-B), so you have reached the end of the switched network topology. At the bottom portion of Figure 2-6, the physical network has been discovered and drawn.

TIP You should assess the utilization or bandwidth used over various connections in the network. This is especially true of switch-to-switch links—if they are heavily used, you might want to plan for expansion. You also might want to get an idea of the total traffic being passed to and from individual server or user connections.

You can do this by using a network or protocol analyzer that is set up to monitor specific switch interfaces. However, you can get a quick snapshot of average traffic volumes with the show interfaces command. A switch maintains a running 5-minute average of traffic rates into and out of each interface. The output from show interfaces displays this information along with a host of other interface statistics.

To see only the interfaces that are in use and only the input and output data rates, you can add a filter to that command:

show interfaces I include (is up I rate)

This produces output similar to the following:

Switch# show interfaces I include (is up I rate)

GigabitEthernet2/1 is up, line protocol is up (connected) 5 minute input rate 63000 bits/sec, 34 packets/sec 5 minute output rate 901000 bits/sec, 168 packets/sec

GigabitEthernet2/2 is up, line protocol is up (connected) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 194000 bits/sec, 80 packets/sec

GigabitEthernet2/3 is up, line protocol is up (connected) 5 minute input rate 219000 bits/sec, 103 packets/sec 5 minute output rate 1606000 bits/sec, 265 packets/sec

You can discover many more detailed aspects of a network. For example, you might want to know the extent of various VLANs across the switches, which interfaces are acting as trunks, the spanning tree topology for various VLANs, and so on.

These are all important things to consider in a network design and in troubleshooting a network, but they are beyond the scope of this chapter. These topics and the appropriate commands are presented in later chapters of this book.

Migrating to a Hierarchical Design

After you have discovered the topology of a network, you might find that it doesn't resemble the overall design goals that were presented earlier in this chapter. Perhaps it doesn't have a hierarchical layout with distinct layers. Or maybe you aren't able to see a modular layout with distinct switch blocks.

To move toward the campus hierarchical model, you also need to gather information about the traffic patterns crossing the network. For example, you should try to find answers to these questions:

■ Where are the enterprise resources (corporate email, web, and intranet application servers) located?

■ Where are the end user communities located?

■ Where are the service provider connections to the Internet, remote sites, and VPN users located?

Following the example of Figure 2-6, these have been identified by interviewing system administrators and network staff. Figure 2-7 shows the locations of user groups and server resources. Notice that these seem to be scattered across the entire network and that there is no clear picture of a modular network.

Figure 2-7 Identifying User and Enterprise Resources

Figure 2-7 Identifying User and Enterprise Resources

Now, you should add some structure to the design. Try to identify pieces of the network as specific modules. For example, the end user communities eventually will become switch block modules, containing both distribution- and access-layer switches. Redraw the network with the users and their switches toward the bottom.

Any resources related to connections to service providers, remote sites, or the Internet should be grouped and moved to become a service provider module or switch block. Enterprise servers, such as those in a data center, should be grouped and moved to become server farm switch blocks.

As you do this, a modular structure should begin to appear. Each module will connect into a central core layer, completing the hierarchical design. To see how the example of Figures 2-6 and 2-7 can be transformed, look at Figure 2-8. The existing switches have merely been moved so that they resemble the enterprise composite model. Without adding switches, the existing network has been migrated into the modular structure. Each module shown ultimately will become a switch block.

Figure 2-8 Migrating an Existing Network into a Modular Structure

Service Provider Module Core Module

Remote

Switch-B Catalyst 4500

Router j \

Server Farm Module i i

Servers

Switch-A Catalyst 3550

Switch-A Catalyst 3550

Switch-C Catalyst 3550

Switch-C Catalyst 3550

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