Network Troubleshooting Practice For Your CCNA, CCENT, And Network+ Certification Exams:
"The Mystery Of The Unroutable Packet"
By Chris Bryant, CCIE #12933
When you're studying for your Cisco CCNA, CCNP, CCENT, or Network+ certification exam - really, when you're studying for any certification exam - practicing your troubleshooting is an important part of that study!
To help you with that practice, I'm starting a new series of Network Troubleshooting Challenges! The first one concerns a connectivity issue that looks easily resolved... but is it?
Whether you're on a PC or a Cisco router, ping is generally the first troubleshooting tool you'll use. Ping is a basic IP connectivity test where five Internet Control Message Protocol (ICMP) packets to the destination we specify in the command, and hopefully we get five packets back!
We're first going to concentrate on a typical LAN scenario where two hosts are connected to two Cisco switches, as shown below.
Here's what we want to see:
R2#ping 172.23.23.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.23.23.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms
Five exclamation points = good.
Here's what we don't want to see:
R2#ping 172.12.23.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.23.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Five periods, dots, whatever you want to call them = bad.
Now we need a plan, a structured approach, to troubleshooting. Let's say you just walked into a client site and they report this kind of issue. Where to start?
When troubleshooting, always start at the physical layer.
You'll be surprised how often the problem is simply a loose cable or another physical problem. A great place to start troubleshooting a Cisco router is show interface, so let's run that on the ethernet interface and see what we can see.
R2#show interface ethernet0
Ethernet0 is administratively down, line protocol is down
Hardware is Lance, address is 00e0.1e68.91f0 (bia 00e0.1e68.91f0)
Well, that was easy! The interface is administratively down. Remember, Cisco router interfaces are shut down by default. Let's open the interface and see what happens.
R2(config)#inter ethernet0
R2(config-if)#no shutdown
R2(config-if)#^Z
02:26:13: %SYS-5-CONFIG_I: Configured from console by console
02:26:14: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up
02:26:16: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up
The line protocol comes up, and running show interface verifies that the interface is up.
R2#show interface ether0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 00e0.1e68.91f0 (bia 00e0.1e68.91f0)
Let's try that ping again.
R2#ping 172.23.23.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.23.23.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R2#
The ping still isn't working. We know the cable isn't the problem, since show interface told us that the physical interface and line protocol are both up.
You may have already spotted the reason, but let's run debug ip packet and see what's going on. Remember, do not run a debug on a production network unless you are sure of the result - this debug in particular can quickly overwhelm a router's available CPU resources.
R2#debug ip packet
IP packet debugging is on
R2#ping 172.23.23.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.23.23.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R2#
02:29:28: IP: s=2.2.2.2 (local), d=172.23.23.3, len 100, unroutable
02:29:30: IP: s=2.2.2.2 (local), d=172.23.23.3, len 100, unroutable
02:29:32: IP: s=2.2.2.2 (local), d=172.23.23.3, len 100, unroutable
02:29:34: IP: s=2.2.2.2 (local), d=172.23.23.3, len 100, unroutable
02:29:36: IP: s=2.2.2.2 (local), d=172.23.23.3, len 100, unroutable
How can that address be unroutable when they're on the same subnet? Take a look at the configuration, apply your troubleshooting skills, and check out the answer further down the page!
Five Minutes From Now, You Can Be Studying For Exam Success Just As Thousands Of Other Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!
If You Prefer Video Training, You Can Earn Your CCNA Using My Exclusive CCNA Video Boot Camp - The Only CBT That Brings You 25 Hours Of Instruction From Chris Bryant, CCIE #12933, AND Free Membership In My Exclusive Online Cisco Certification Forum, Where You Can The Instructor Questions Personally!
Just Click The Image For More Details And Video Samples!
Let's revisit the topology and figure out how the packet can be unroutable.
There's no router involved, and the two hosts are on the same subnet, so routing shouldn't be a problem. Unless.....
R2#show interface ethernet0
Ethernet0 is up, line protocol is up
Hardware is Lance, address is 00e0.1e68.91f0 (bia 00e0.1e68.91f0)
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load 1/255
The MAC address is shown, but not the IP address. The problem is that there is no IP address configured on the ethernet interface!
R2(config)#int e0
R2(config-if)#ip address 172.23.23.2 255.255.255.0
The interface is already open, and there's no need to close and reopen it after assigning an IP address. Let's send that ping again.
R2#ping 172.23.23.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.23.23.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms
Success!
If we were still not able to ping between those two hosts, I would check the VLAN port membership on the switches with show vlan brief. You'll see more of this in your CCNA studies, but remember - if hosts are in different VLANs, they will not be able to send IP packets to each other without a Layer 3 device being involved.
Just with that simple network, we ran into quite a few things that could have been the cause of the issue:
-
Ethernet interface not open
-
Ethernet interface had no IP address
-
-
Loose cable (show interface verified the physical cable was fine)
Troubleshooting is a true test of your analytical abilities as well as knowing the right command to use at the right time. To help you develop those attributes, join me on the Tutorials page for more Network Troubleshooting Challenges!
Five Minutes From Now, You Can Be Studying For Exam Success Just As Thousands Of Other Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!
If You Prefer Video Training, You Can Earn Your CCNA Using My Exclusive CCNA Video Boot Camp - The Only CBT That Brings You 25 Hours Of Instruction From Chris Bryant, CCIE #12933, AND Free Membership In My Exclusive Online Cisco Certification Forum, Where You Can The Instructor Questions Personally!
Just Click The Image For More Details And Video Samples!
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