blackhawk

I have recently purchased the Cisco CCNA training package. ONe of the labs instructs one to execute a "no shut" command on a serial port. After doing so the line goes up; however 30 sec later it goes back down.

heatwave

Im assuming you are connecting the serial interface to another cisco serial interface so look at the cable one end says DTE and the other says DCE now on the one that says DCE type

config t
int s1
ip address 192.168.0.1 255.255.255.252
clock rate 64000
no shut


only do this on the DCE one as it needs to supply timing for the line. (Its emulating the ISP)

on the other side (DTE) type

config t
int s1
ip address 192.168.0.2 255.255.255.252
no shut

also remember serial 0 is the first interface and serial 1 is the second interface Thye always start with 0.

vega55

Basically there are two things that need to be brought up for the interface to be operational. The interface itself, and the line protocol. If you have nothing connected to the other end of the interface it will be up, but not really because the line protocol won't be up. This just means that it is waiting for a connection to another device.

As mentioned you need to have two routers connected together back to back with a DCE-DTE cable. A clock rate must be set on the DCE side ONLY. Then the interface will come up and stay up. Actually even when it says down as you are experiencing it will pop up as soon as you connect it to another up interface. The only time an interface is down and will never come up is when it is administratively down, meaning you issue a shutdown command.

blackhawk

how can you tell whether its DCE or DTE? Also remeber this is a simulator, there are no physical connections. I took your advice and it still shuts down after 30 sec.

vega55

No, you shouldn't be setting clock rates on both routers. In real life you would be connecting a router to a DCE device, provided by the ISP. This is your WAN connection. When you set a clock rate on a router you are simulating this. So your setup sould be the DCE device (incoming internet) connected to a router (DTE device). Both interfaces wil need IP addresses and subnet masks, and the no shutdown command.

heatwave

OH! I thought you purchased a CCNA lab kit that had real routers and cables (like off e-bay) . what simulator package did you buy?

blackhawk

thanks guys. it worked. i had to set ip address and clock rates on both routers and it works now. thanks

heatwave

Love those simulators hehe, I truly recommend getting real routers. Basicly in the real world only one device can keep the timing on the line (otherwise you get two different timing signals) by default the router will use the "line timing" meening it is letting the DCE (ISP) provide the signal and it is listening for that. There are different timing methods the most common is B8ZS (bipolar 8-zero substitution) which means every 8 bits reverse the signal so now zeroes are positive volts (or 1's), then 8 more bits they are now zeroes again, (bipolar) this way if a long lines of zeros come through it can determin exactly how many zeros there where, as a digital zero is usualy no signal. The only time you should be setting timing on the router is in simulation or if the telco does not provide it like over "dark fiber", from my experience.

blackhawk

ok,so i started off with a new simulator and its not working again. it did the same steps as before, like configuring both routers' serial ports to be on the same network and same ip address and it is still going down 30 sec later

heatwave

I can only tell ya how to do real routers without knowing what simulator you are using.