Cisco 2501 - Router - EN Manual de usuario descargar pdf (Pagina 53)

Viewing, Saving, and Erasing Configurations

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the bandwidth is 100000Kbit, which is 100,000,000 (Kbit means to add three zeros), which

is 100Mbits per second, or FastEthernet. Gigabit would be 1000000Kbits per second.

The most important statistic of the show interface command is the output of the line and

data-link protocol status. If the output reveals that FastEthernet 0/0 is up and the line protocol

is up, then the interface is up and running:

Router#sh int fa0/0

FastEthernet0/0 is up, line protocol is up

The first parameter refers to the Physical layer, and it’s up when it receives carrier detect.

The second parameter refers to the Data Link layer, and it looks for keepalives from the con-

necting end. (Keepalives are used between devices to make sure connectivity has not dropped.)

Here’s an example of where the problem usually is found—on serial interfaces:

Router#sh int s0/0/0

Serial0/0 is up, line protocol is down

If you see that the line is up but the protocol is down, as shown above, you’re experiencing

a clocking (keepalive) or framing problem—possibly an encapsulation mismatch. Check the

keepalives on both ends to make sure that they match; that the clock rate is set, if needed, and

that the encapsulation type is the same on both ends. The output above would be considered

a Data Link layer problem.

If you discover that both the line interface and the protocol are down, it’s a cable or inter-

face problem. The following output would be considered a Physical layer problem:

Router#sh int s0/0/0

Serial0/0 is down, line protocol is down

If one end is administratively shut down (as shown next), the remote end would present as

down and down:

Router#sh int s0/0/0

Serial0/0 is administratively down, line protocol is down

To enable the interface, use the command no shutdown from interface configuration mode.

The next show interface serial 0/0/0 command demonstrates the serial line and the max-

imum transmission unit (MTU)—1,500 bytes by default. It also shows the default bandwidth

(BW) on all Cisco serial links: 1.544Kbps. This is used to determine the bandwidth of the line for

routing protocols such as EIGRP and OSPF. Another important configuration to notice is the kee-

palive, which is 10 seconds by default. Each router sends a keepalive message to its neighbor every

10 seconds, and if both routers aren’t configured for the same keepalive time, it won’t work.

Router#sh int s0/0/0

Serial0/0 is up, line protocol is up

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

47020.book Page 231 Wednesday, January 2, 2008 4:31 PM

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Cisco 2501 - Router - EN Manual de usuario Pagina 53