CCNA exam success depends largely on noticing the details, and this is especially true of configurations involving directly connected serial interfaces. And of course, it's not enough to notice these details - you've got to know what to do about them!
A Cisco router is a DTE by default, but directly connecting two DTEs with a DCE/DTE cable is not enough. In the following example, R1 and R3 are directly connected at their Serial1 interfaces. The line goes up briefly after being opened, but the line protocol goes down after about 30 seconds.
R3(config-if)#int s1
R3(config-if)#ip address 172.12.13.3 255.255.255.0
R3(config-if)#no shutdown
2d18h: %LINK-3-UPDOWN: Interface Serial1, changed state to up
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
R3(config-if)#
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down
The problem is that one of the routers needs to act as the DCE in order for the line protocol to come up and stay up. If this were your CCNA / CCNP home lab, you could just go over and look at the DTE/DCE cable to see which router had the DCE end of the cable attached. In this example, though, we don't have physical access to the routers. How can we tell which router has the DCE end of the cable attached?
R3#show controller serial 1
HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8
buffer size 1524 HD unit 1, V.35 DCE cable
The show controller command gives us this information. (There's a lot more output that this with this command, but it's unimportant for our purposes.) The router with the DCE end of the cable needs to supply a clock rate to the DTE, and we'll do just that with the interface-level clockrate command.
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#int serial1
R3(config-if)#clockrate 56000
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
In just a few seconds, the line protocol goes up and stays up.
When troubleshooting a connection, always run show interface first. If you see the combination shown below, the connection is physically fine but logically down. That's generally the result of a needed keepalive not being present. With Frame Relay, it's probably an LMI issue, but with directly connected serial interfaces the issue is most likely the DCE end of the connection not supplying clockrate.
R3#show interface serial 1
Serial1 is up, line protocol is down
Troubleshooting is a big part of the job, and it's a big part of the Cisco CCNA and CCNP programs as well. Know your show and debug commands and you're on your way to passing the CCNA!
Showing posts with label dce. Show all posts
Showing posts with label dce. Show all posts
Thursday, December 25, 2008
Cisco CCNA Exam Tutorial: Directly Connected Serial Interfaces
To pass the CCNA exam, you've got to master quite a few services and routing protocols that may be new to you. Between RIP, IGRP, EIGRP, OSPF, and switching, there are hundreds of details you've got to absorb! It's easy to spend all your time on those topics and not pay proper attention to "easier" technologies, and then all of a sudden on exam day you can't quite remember the details of those particular services.
One setup you've got to be more than familiar with is directly connecting serial interfaces on Cisco routers. This is also a valuable skill to have in your home lab, since it allows you to add segments to your network setup.
A Cisco serial interface is operating as a DTE by default. The problem is that when you take a cable and connect two routers directly by their serial interfaces (with a DTE/DCE cable, that is!), they're both waiting for the other to send them a clock rate. One of the interfaces must act as the DCE and that interface must send the clock rate.
If you can see the DTE/DCE cable, you can tell by looking which router has the DCE interface connected to it - the letters "DTE" or "DCE" will either be molded into the connector itself, or if it's an older cable there should be a little piece of tape on the cable that tells you what the interface type is. But what if you have no access to the cable, or there are other cables all around it and you can't see what type it is?
Run the command "show controller serial x", with x representing the interface number the cable's connected to. There will be quite a bit of output from this command, but the information you need is right at the top:
R1#show controller serial 1
HD unit 1, idb = 0x1DBFEC, driver structure at 0x1E35D0
buffer size 1524 HD unit 1, V.35 DTE cable
I left off the 16 or so rows of information that comes after this, but this is the information we need right now. If R1's got the DTE cable end, the other router should have the DCE end:
R3#show controller serial 1
HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8
buffer size 1524 HD unit 1, V.35 DCE cable
We know now that R3 needs to supply a clock rate to R1. There's a hint of a problem in just that little bit of command output - do you see what it is? Let's run show interface serial1 to get more information.
R3#show int s1
Serial1 is up, line protocol is down
The line protocol is down because there is no clockrate being supplied by R3. If there has been, we would have seen that in the output of show controllers serial 1.
This is simple enough to fix, though! We'll use the command clockrate 56000 on R3's serial1 interface, and the line protocol will soon come up.
R3(config)#int s1
R3(config-if)#clockrate 56000
1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
This is a simple concept, but there are a few details you must keep in mind! For a home lab configuration, you'll need a DTE/DCE cable to make this work. If you cannot see the cable connectors, run show controllers serial x to see if the router has the DTE or DCE end of the cable attached. On the interface with the DCE attached, use the clockrate command to bring the line protocol up. It's just that simple!
One setup you've got to be more than familiar with is directly connecting serial interfaces on Cisco routers. This is also a valuable skill to have in your home lab, since it allows you to add segments to your network setup.
A Cisco serial interface is operating as a DTE by default. The problem is that when you take a cable and connect two routers directly by their serial interfaces (with a DTE/DCE cable, that is!), they're both waiting for the other to send them a clock rate. One of the interfaces must act as the DCE and that interface must send the clock rate.
If you can see the DTE/DCE cable, you can tell by looking which router has the DCE interface connected to it - the letters "DTE" or "DCE" will either be molded into the connector itself, or if it's an older cable there should be a little piece of tape on the cable that tells you what the interface type is. But what if you have no access to the cable, or there are other cables all around it and you can't see what type it is?
Run the command "show controller serial x", with x representing the interface number the cable's connected to. There will be quite a bit of output from this command, but the information you need is right at the top:
R1#show controller serial 1
HD unit 1, idb = 0x1DBFEC, driver structure at 0x1E35D0
buffer size 1524 HD unit 1, V.35 DTE cable
I left off the 16 or so rows of information that comes after this, but this is the information we need right now. If R1's got the DTE cable end, the other router should have the DCE end:
R3#show controller serial 1
HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8
buffer size 1524 HD unit 1, V.35 DCE cable
We know now that R3 needs to supply a clock rate to R1. There's a hint of a problem in just that little bit of command output - do you see what it is? Let's run show interface serial1 to get more information.
R3#show int s1
Serial1 is up, line protocol is down
The line protocol is down because there is no clockrate being supplied by R3. If there has been, we would have seen that in the output of show controllers serial 1.
This is simple enough to fix, though! We'll use the command clockrate 56000 on R3's serial1 interface, and the line protocol will soon come up.
R3(config)#int s1
R3(config-if)#clockrate 56000
1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up
This is a simple concept, but there are a few details you must keep in mind! For a home lab configuration, you'll need a DTE/DCE cable to make this work. If you cannot see the cable connectors, run show controllers serial x to see if the router has the DTE or DCE end of the cable attached. On the interface with the DCE attached, use the clockrate command to bring the line protocol up. It's just that simple!
Wednesday, December 24, 2008
Cisco CCNA / CCNP Home Lab Tutorial: The (Many) Cable Types And Their Purposes
One of the most common questions I get from CCNA and CCNP candidates who are setting up their own home labs is "What cables will I need?" The answer is "It depends." As you know from your exam studies, the physical layout of your lab is what determines the cables you'll need. Let's take a look at the most common home lab cable types and when you will need them.
Straight-through cables have quite a few uses in a CCNA / CCNP home lab. You'll need them to connect a switch port to an AUI port on a router (and you'll need a transceiver for that as well). If you have an ISDN simulator, straight-through cables can be used to connect a router's BRI port to the simulator.
Crossover cables are used to connect switches and allow them to trunk. If at all possible, get two switches in your home lab. This will allow you to gain valuable experience in manipulating root bridge election, working with STP, and creating EtherChannels.
DTE/DCE cables are used to connect two routers via their serial cables. If you are planning on using a frame relay switch in your lab, you'll need several of these. You can also get some great practice in by directly connecting two routers and bringing the connection up (and making sure it stays up!). This is valuable practice for your CCNA exam.
Octal cables are used to connect an access server to each of the other routers and switches in your lab.
Finally, there's that precious blue cable, the rollover cable. Rollover cables (sometimes called "rolled cables") allow you to connect a host device directly to a router or switch's console port. These cables have a way of disappearing around an IT shop, so make sure to take one home - and leave it there!
Straight-through cables have quite a few uses in a CCNA / CCNP home lab. You'll need them to connect a switch port to an AUI port on a router (and you'll need a transceiver for that as well). If you have an ISDN simulator, straight-through cables can be used to connect a router's BRI port to the simulator.
Crossover cables are used to connect switches and allow them to trunk. If at all possible, get two switches in your home lab. This will allow you to gain valuable experience in manipulating root bridge election, working with STP, and creating EtherChannels.
DTE/DCE cables are used to connect two routers via their serial cables. If you are planning on using a frame relay switch in your lab, you'll need several of these. You can also get some great practice in by directly connecting two routers and bringing the connection up (and making sure it stays up!). This is valuable practice for your CCNA exam.
Octal cables are used to connect an access server to each of the other routers and switches in your lab.
Finally, there's that precious blue cable, the rollover cable. Rollover cables (sometimes called "rolled cables") allow you to connect a host device directly to a router or switch's console port. These cables have a way of disappearing around an IT shop, so make sure to take one home - and leave it there!
Tuesday, December 23, 2008
Cisco CCNA / CCNP Certification Exam: Troubleshooting Direct Serial Connections
A prime topic of your CCNA and CCNP CIT exams will be connecting Cisco routers directly via their Serial interfaces, and while the configuration is straightforward, there are some vital details and show commands you must know in order to pass the exams and configure this successfully in production and home lab networks. Let's take a look at a sample configuration.
Connecting Cisco routers directly via their Serial interfaces works really well once you get it running - and getting such a connection up and running is easy enough. You can use show controller serial x to find out which endpoint is acting as the DCE, and it's the DCE that must be configured with the clockrate command.
R3#show controller serial 1
HD unit 1, idb = 0x11B4DC, driver structure at 0x121868
buffer size 1524 HD unit 1, V.35 DCE cable
R3(config)#int serial1
R3(config-if)#ip address 172.12.13.3 255.255.255.0
R3(config-if)#clockrate 56000
R3(config-if)#no shut
Failure to configure the clockrate has some interesting effects regarding the physical and logical state of the interfaces. Let's remove the clockrate from R3 and see what happens.
R3(config)#int s1
R3(config-if)#no clockrate 56000
R3(config-if)#
18:02:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down
The line protocol doesn't drop immediately, but it does drop. Let's run show interface serial1 to compare the physical and logical interface states.
R3#show int serial1
Serial1 is up, line protocol is down
Physically, the interface is fine, so the physical interface is up. It's only the logical part of the interface - the line protocol - that is down. It's the same situation on R1.
R1#show inter serial1
Serial1 is up, line protocol is down
While a router misconfiguration is the most likely cause of a serial connection issue, that's not the only reason for clocking issues. Cisco's website documentation mentions CSU/DSU misconfiguration, out-of-spec cables, bad patch panel connections, and connecting too many cables together as other reasons for clocking problems. Still, the number one reason for clocking problems in my experience is simply forgetting to configure the clockrate command!
Connecting Cisco routers directly via their Serial interfaces works really well once you get it running - and getting such a connection up and running is easy enough. You can use show controller serial x to find out which endpoint is acting as the DCE, and it's the DCE that must be configured with the clockrate command.
R3#show controller serial 1
HD unit 1, idb = 0x11B4DC, driver structure at 0x121868
buffer size 1524 HD unit 1, V.35 DCE cable
R3(config)#int serial1
R3(config-if)#ip address 172.12.13.3 255.255.255.0
R3(config-if)#clockrate 56000
R3(config-if)#no shut
Failure to configure the clockrate has some interesting effects regarding the physical and logical state of the interfaces. Let's remove the clockrate from R3 and see what happens.
R3(config)#int s1
R3(config-if)#no clockrate 56000
R3(config-if)#
18:02:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down
The line protocol doesn't drop immediately, but it does drop. Let's run show interface serial1 to compare the physical and logical interface states.
R3#show int serial1
Serial1 is up, line protocol is down
Physically, the interface is fine, so the physical interface is up. It's only the logical part of the interface - the line protocol - that is down. It's the same situation on R1.
R1#show inter serial1
Serial1 is up, line protocol is down
While a router misconfiguration is the most likely cause of a serial connection issue, that's not the only reason for clocking issues. Cisco's website documentation mentions CSU/DSU misconfiguration, out-of-spec cables, bad patch panel connections, and connecting too many cables together as other reasons for clocking problems. Still, the number one reason for clocking problems in my experience is simply forgetting to configure the clockrate command!
Cisco CCNA / CCNP Certification Exam: Cabling Your Home Lab
More CCNA and CCNP candidates than ever before are putting together their own home labs, and there's no better way to learn about Cisco technologies than working with the real thing. Getting the routers and switches is just part of putting together a great CCNA / CCNP home lab, though. You've got to get the right cables to connect the devices, and this is an important part of your education as well. After all, without the right cables, client networks are going to have a hard time working!
For your Cisco home lab, one important cable is the DTE/DCE cable. These cables have two major uses in a home lab. To practice directly connecting Cisco routers via Serial interfaces (an important CCNA skill), you'll need to connect them with a DTE/DCE cable. Second, if you plan on having a Cisco router act as a frame relay switch in your lab, you'll need multiple DTE/DCE cables to do so. (Visit my website's Home Lab Help section for a sample Frame Relay switch configuration.)
If you have multiple switches in your lab, that's great, because you'll be able to get a lot of spanning tree protocol (STP) work in as well as creating Etherchannels. To connect your switches, you'll need crossover cables.
You'll need some straight-through cables as well to connect your routers to the switches.
Finally, if you're lucky enough to have an access server as part of your lab, you'll need an octal cable to connect your AS to the other routers and switches in your lab. The octal cable has one large connector on one end and eight numbered RJ-45 connectors on the other end. The large connector should be attached to the async port on your AS, and the numbered RJ-45 connectors will be connected to the console ports on your other routers and switches.
Choosing and connecting the right cables for your Cisco CCNA / CCNP home lab is a great learning experience, and it's also an important part of your Cisco education. After all, all great networks and home labs all begin at Layer One of the OSI model!
For your Cisco home lab, one important cable is the DTE/DCE cable. These cables have two major uses in a home lab. To practice directly connecting Cisco routers via Serial interfaces (an important CCNA skill), you'll need to connect them with a DTE/DCE cable. Second, if you plan on having a Cisco router act as a frame relay switch in your lab, you'll need multiple DTE/DCE cables to do so. (Visit my website's Home Lab Help section for a sample Frame Relay switch configuration.)
If you have multiple switches in your lab, that's great, because you'll be able to get a lot of spanning tree protocol (STP) work in as well as creating Etherchannels. To connect your switches, you'll need crossover cables.
You'll need some straight-through cables as well to connect your routers to the switches.
Finally, if you're lucky enough to have an access server as part of your lab, you'll need an octal cable to connect your AS to the other routers and switches in your lab. The octal cable has one large connector on one end and eight numbered RJ-45 connectors on the other end. The large connector should be attached to the async port on your AS, and the numbered RJ-45 connectors will be connected to the console ports on your other routers and switches.
Choosing and connecting the right cables for your Cisco CCNA / CCNP home lab is a great learning experience, and it's also an important part of your Cisco education. After all, all great networks and home labs all begin at Layer One of the OSI model!
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