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A major part of modeling the internetwork consists of modeling LANs. Each lab in this book has a special section entitled "Equipment Needed," This section lists the minimum hardware requirements needed to complete that particular lab. Some labs might require only a crossover cable or two to connect two routers. Other labs might require a host connection, so you need to use either a hub or a switch when modeling your LANs. Four ways exist in which to model and simulate LANs:
Modeling LANs by Using Hubs and MAUsIn a controlled environment, it is necessary to model Layer 1 technologies. The two LAN technologies that we are focusing on are Ethernet and Token Ring. The easiest way to model Layer 1 is by the use of hubs and MAUs. Most of the labs in this book require multiple hubs of multiple types. Physically, the number of network segments are limited by the number of Ethernet or Token Ring interfaces present on the routers in the lab. The type of hubs that you use is not significant; what's important is that the hub is fully operational and has two or more ports. Some hubs are manageable and send IPX SAPs, which can be a useful feature for testing IPX filtering. Other times, it is preferable to have a nice, quiet MAU running in the lab. MAUs generate less heat, no electricity, and no noise. What type you chose to use in your model is up to you. Modeling LANs by Using SwitchesThe cleanest way to simulate many LANs is with the use of switches. One switch can be configured to accommodate many virtual LANs (VLANs). A good way to think of a VLAN is as a standalone hub. So, instead of having many hubs to accommodate the routers, you need only one switch with a few VLANs. Chapter 2, "LAN Protocols: Configuring Catalyst Ethernet and Token Ring Switches," provides a detailed explanation of VLANs and a switching overview. Using switches to model your LANs also saves on rack space and power requirements, although they are more expensive than hubs. Simulating LANs by Using Route Generators or Backbone RoutersTwo other quick ways to simulate LANs are by using a loopback interfaces and using a no keepalive command on the router's Ethernet interface. These two methods can be useful in creating a route generator or backbone router. A route generator is a device connected to a test network for the purpose of sending and receiving routing updates. A router configured with many loopback addresses with routing protocols will appear as an entire network of routers to a downstream neighbor. We use this functionality in the lab to help simulate networks for route filters and route maps. Examples 1-1 and 1-2 use a Cisco 2501 as a route generator. Notice that no keepalive has been added to the Ethernet to spoof it as up. Also note that the default keepalive value of 10 seconds is now replaced with not set. When you disable the keepalives on the Ethernet segment, you will notice that packets output, output errors, and lost carrier counts all increment together. Do not forget to add a keepalive when you reconnect your Ethernet port back to a real hub or switch. The default value of 10 will automatically be set. Example 1-1 Configuring a Two-Route Generator; Simple Loopback AddressRouter# conf t Enter configuration commands, one per line. End with CNTL/Z. Router(config)# int loopback 20 Router(config-if)# %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback20, changed state to up Router(config-if)#ip address 172.16.16.1 255.255.255.0 Router(config-if)# exit Router(config)# int loopback 21 %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback21, changed state to up Router(config-if)#ip address 172.16.17.1 255.255.255.0 Router(config-if)# exit Router(config)#router eigrp 2001 Router(config-router)# network 172.16.0.0 Router(config-router)# ^Z Router# Example 1-2 Configuring a Route Generator; Spoofing EthernetRouter# conf t Enter configuration commands, one per line. End with CNTL/Z. Router(config)#int ethernet 0 Router(config-if)# no keepalive Router(config-if)# ^Z Router# Router#show int e0 Ethernet0 is up, line protocol is up Hardware is Lance, address is 0000.0c8d.54ac (bia 0000.0c8d.54ac) MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 235/255, load 1/255 Encapsulation ARPA, loopback not set, keepalive not set ARP type: ARPA, ARP Timeout 04:00:00 Last input never, output 00:00:18, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 0 packets input, 0 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 0 input packets with dribble condition detected 21 packets output, 3030 bytes, 0 underruns 21 output errors, 0 collisions, 2 interface resets 0 babbles, 0 late collision, 0 deferred 21 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out Router# Modeling LANs by Using an Ethernet Crossover CableA commonly known way to connect two Ethernet hosts is to use an Ethernet crossover cable. An Ethernet crossover cable is simply an RJ-45 “to “RJ-45 patch cable, pinned out in a crossover pattern. The obvious limitation is that the crossover cable can be used to connect only two devices. In the lab environment, you can use this cable to connect two routers or to connect a router to one host. Figure 1-2 shows the pinouts for Ethernet crossover cable. Figure 1-2. Pinouts for an Ethernet Crossover Cable
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