Overview
During installation and configuration of Networking devices, so many things can go wrong especially when you are dealing with a large LAN installation, as an engineer, you need to be aware of these issues and the possibilities of them happening and how to check for them and fix them. You might see questions in the exam that will ask you specifics of these kind of errors and how to check for them.
In this section, we shall have some configuration performed.
We shall be taking a look at the following; collisions, errors, mismatch duplex, and/or speed
Collision
COAX Cable Bus Topology HUB in Star Topology
The first generation of Ethernet using the Coax cable in a bus topology and later moving to hubs using the early UTP was prone to collision. Both of these era operated in a half duplex Ethernet network, meaning, two devices on a shared media can NOT send packets at the same time else a collision will occur. Therefore, a collision is the result of two devices on the same Ethernet network attempting to transmit data at exactly the same time. The network detects the “collision” of the two transmitted packets and discards them both. Ethernet uses CSMA/CD as its collision detection method. It was after these two eras that the switches were introduced and this eliminated the collision issue as switches can operate in full duplex mode.
In todays, world, collisions should not occur in a properly designed switched network. Today, we have the ability to design full-duplex networks using switches that intelligently queue frames to prevent them from being sent simultaneously out an interface.
Causes of collision
- A network collision occurs when two or more devices attempt to transmit data over a network at the same time
- Duplex mismatch on a shared Ethernet,
- broken NICs
- too many stations on the shared medium.
How will I detect a collision?
- Collision detection is disabled in Full-Duplex
- Use the “show interfaces “command to view collision reports.
- In the show command, a high number of “CRC errors” (Cyclical Redundancy Count) is usually the result of collisions or a station transmitting bad data
- In the show command, the “frame” shows the number of packet s received incorrectly having a CRC error.
******** SAMPLE OUTPUT FROM A SHOW COMMAND*****
Switch# show interfaces f6/0/1
FastEthernet6/0/1 is down, line protocol is down (not connected)
Hardware is Fast Ethernet, address is 0013.804c.4683 (bia 0013.804c.4683)
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, media type is 10/100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of “show interface” counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
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 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
Switch#
How do I fix a collision?
- Hardcoding speed and duplex
Errors
Errors can be as a result of wide variety of reasons. In the case of electrical interference for example, where network cables and power cables are packed together somewhere or a bad network interface card that is not able to frame things correctly for the network. Checking the frame check sequence (FCS) is often the best way to catch these errors because each time a router forwards a packet on an Ethernet network, it replaces and rewrites the Layer 2 Ethernet header information and provides a new FCS.
-packet input: number of error-free packets received.
-bytes: Gives the total number of bytes in the error-free packets received.
-input errors: can include runts, giants, no buffer, CRC, frame, overrun, and ignored counts
-frame: shows the number of packets received incorrectly having a CRC error. On a LAN, this is usually the result of collisions or a malfunctioning Ethernet device
-input packets with dribble condition detected gives the dribble bit error, which indicates that a frame is slightly too long. This frame error counter: is incremented just for informational purposes; the router accepts the frame
-output errors: gives the sum of all errors that were prevented
-Runts are Ethernet frames that are less than 64 bytes and may be caused by excessive collisions. Of course, these frames have become rarer as networks have become nearly collision free.
<<>>
Mismatch Duplex and Speed
-Duplex and speed should match on both ends or else you will have problems.
-Traffic can still pass but you will experience re transmissions and reduced throughput.
-Speed mismatches will typically bring the interfaces down, you can see this with the “Show ip interface brief” command
-Duplex will typically stay up but performance will be bad because of collisions.
-CDP should detect a duplex mismatch.
Speed is another area where conflict can occur, but this is also becoming a less common problem as technologies advance. For example, 1 Gbps interfaces are quite common now and operate with each other seamlessly at 1 Gbps. However, older equipment might default to a slower speed, causing a speed mismatch.
*****SAMPLE OUTPUT FOR SHOW IP INTERFACE BRIEF COMMAND*****
Switch#sh ip inter brief
Interface IP-Address OK? Method Status Protocol
Vlan1 unassigned YES unset up down
FastEthernet6/0/1 unassigned YES unset down down
FastEthernet6/0/2 unassigned YES unset down down
FastEthernet6/0/3 unassigned YES unset down down
FastEthernet6/0/4 unassigned YES unset down down
!
NOTES:
‘administratively down’ – issue ‘no shutdown’ command
‘up/down’ – A a Layer 2 issue or speed mismatch. Check the interface configuration matches on both sides (maybe different VLAN settings on each side of the link).
1. Which of the following is not a valid error typically seen in show interface output?
A. Babble
B. Late collision
C. Ignored
D. Trickle
2. What counter increments if the number of frames transmitted is greater than 1518 bytes in size?
A. Babble
B. Late collision
C. Runt
D. Ignored
3. What process on a Cisco device attempts to dynamically resolve speed and duplex between two devices?
A. Negotiation
B. Autonegotiation
C. CDP
D. LLDP
ANSWERS
1. D is correct. There is no Trickle reported in the output as this is not a valid counter.
2. A is correct. Babble increments when the number of frames over 1518 bytes increases.
3. B is correct. Autonegotiation (Auto MDI-X) attempts to resolve a common duplex and speed between two Ethernet devices.