Friday, October 14, 2005

Understanding Data Link Errors

Understanding Data Link Errors
Many performance issues with NICs can be related to data link errors.
Excessive errors
usually indicate a problem. When operating at half-duplex setting,
some data link errors
such as Frame Check Sequence (FCS), alignment, runts, and collisions are normal.
Generally, a one percent ratio of errors to total traffic is
acceptable for half-duplex
connections. If the ratio of errors to input packets is greater than
two or three percent,
performance degradation may be noticed.
In half-duplex environments, it is possible for both the switch and
the connected device
to sense the wire and transmit at exactly the same time and result in
a collision.

Collisions can cause runts, FCS, and alignment errors due to the frame
not being completely copied to the wire which results in fragmented
frames.
When operating at full-duplex, FCS, Cyclic Redundancy Checks (CRC), alignment
errors, and runt counters should be minimal. If the link is operating
at full-duplex, the
collision counter is not active. If the FCS, CRC, alignment, or runt
counters are
incrementing, check for a duplex mismatch. Duplex mismatch is a
situation where the
switch is operating at full-duplex and the connected device is
operating at half-duplex, or
vice versa. The result of a duplex mismatch will be extremely slow performance,
intermittent connectivity, and loss of connection. Other possible
causes of data link errors
at full-duplex are bad cables, faulty switch port, or NIC
software/hardware issues.

Explanation of Port Errors Counter Description

Troubleshooting NIC Compatibility Issues on ISUnet

Alignment Errors
Alignment errors are a count of the number of frames received that don't end
with an even number of octets and have a bad CRC.

FCS(Frame Check Sequence)
FCS error count is the number of frames that were transmitted/received
with a bad
checksum (CRC value) in the Ethernet frame. These frames are dropped and not
propagated onto other ports.

Xmit-Err
This is an indication that the internal transmit buffer is full.

Rcv-Err
This is an indication that the receive buffer is full.

UnderSize
These are frames which are smaller than 64 bytes (including FCS) and have a
good FCS value.

Single Collisions
Single collisions are the number of times the transmitting port had
one collision
before successfully transmitting the frame to the media.

Multiple Collisions
Multiple collisions are the number of times the transmitting port had more than
one collision before successfully transmitting the frame to the media.

Late Collisions
A late collision occurs when two devices transmit at the same time and
neither side
of the connection detects a collision. The reason for this occurrence
is because the
time to propagate the signal from one end of the network to another is
longer than
the time to put the entire packet on the network. The two devices that cause the
late collision never see that the other is sending until after it puts
the entire
packet on the network. Late collisions are detected by the transmitter
after the first
"slot time" of 64 byte times. They are only detected during transmissions of
packets longer than 64 bytes. Its detection is exactly the same as for a
normal collision; it just happens late when compared to a normal collision.

Excessive
Excessive collision are the number of Collisions frames that are
dropped after 16 attempts
to send the packet resulting in 16 collisions.

Carrier Sense
Carrier Sense occurs every time an Ethernet controller wants to send data
and the counter is incremented when there is an error in the process.

Runts
These are frames smaller than 64 bytes with a bad FCS value.

Giants
These are frames that are greater than 1518 bytes and have a bad FCS value.

Possible Causes for Incrementing Port Errors
Counter Possible Cause

Alignment Errors
These are the result of collisions at half-duplex, duplex mismatch, bad hardware
(NIC, cable or port), or connected device generating frames that do
not end with on
an octet and have a bad FCS.

FCS (Frame Check Sequence)
These are the result of collisions at half-duplex, duplex mismatch, bad hardware
(NIC, cable, or port), or connected device generating frames with bad FCS.

Xmit-Err
This is an indication of excessive input rates of traffic. This is
also an indication
of transmit buffer being full. The counter should only increment in situations
where the switch is unable to forwarded out the port at a desired
rate. Situations
such as excessive collisions and 10 megabit ports will cause the transmit
buffer to become full. Increasing speed and moving link partner to full-duplex
should minimalize this occurance.

Rcv-Err
This is an indication of excessive output rates of traffic. This is
also an indication
of the receive buffer being full. This counter should be zero unless there is
excessive traffic through the switch. In some switches, the outlost
counter has a
direct correlation to the Rcv-Err.

UnderSize This is an indication of a bad frame generated by the
connected device.

Single Collisions
This is an indication of a half-duplex configuration.

Multiple Collisions
This is an indication of a half-duplex configuration.

Late Collisions
This is an indicationof faulty hardware (NIC, cable, or switch port) or duplex
mismatch.

Excessive Collisions
This is an indication of over-utilization of switch port at
half-duplex or duplex
mismatch.

Carrier Sense
This is an indication of faulty hardware (NIC, cable, or switch port).

Runts
This is an indication of the result of collisions, duplex mismatch, dot1q, or
ISL configuration issue.

Giants
This is an indication of faulty hardware, dot1q, or ISL configuration issue.

Additional Troubleshooting for 1000BaseX NICs
Gigabit Auto-Negotiation (No Link to Connected Device)
Gigabit Ethernet has an auto-negotiation procedure that is more
extensive than what is
used for 10/100 Mbps Ethernet (Gigabit Auto-negotiation spec: IEEE Std
802.3z-1998).
The Gigabit Auto-negotiation negotiates flow control, duplex mode, and
remote fault
information. You must either enable or disable link negotiation on
both ends of the link.
Both ends of the link must be set to the same value or the link will
not connect.
If either device does not support Gigabit auto-negotiation, disabling
Gigabit auto-
negotiation will force the link up. Disabling auto-negotiation "hides"
link drops and other
physical layer problems. Only disable auto-negotiation to end-devices
such as older
Gigabit NICs that do not support Gigabit auto-negotiation. Do not disable auto-
negotiation between switches unless absolutely required as physical
layer problems may
go undetected and result in spanning-tree loops. The alternative to
disabling auto-
negotiation is contacting the vendor for software/hardware upgrade for
IEEE 802.3z
Gigabit auto-negotiation support.

9 Comments:

Blogger Girija Manikandan said...

This blog is having a wonderful talk. The technology are discussed and provide a great knowledge toall. This helps to learn more details about technology. All this details are important for this technology. Thank you for this blog.
Android Training in Chennai

3:49 AM  
Blogger Girija Manikandan said...

Thank you for sharing like this information. This is the most easy way of learning. This helps me to get some idea regarding this and helps me to bring a creative thought.
Android Training in Chennai

6:46 AM  
Blogger Camellia Canan said...


All are saying the same thing repeatedly, but in your blog I had a chance to get some useful and unique information, I love your writing style very much, I would like to suggest your blog in my dude circle, so keep on updates.


SAP training in Chennai

10:13 AM  
Blogger Camellia Canan said...


These provided information was really so nice,thanks for giving that post and the more skills to develop after refer that post. Your articles really impressed for me,because of all information so nice.

SAP training in Chennai

2:52 AM  
Blogger Nainika Joseph said...


Great Article I love to read your articles because your writing style is too good, its is very very helpful for all of us and I never get bored while reading your article because it becomes more and more interesting from the starting lines until the end. So Thank you for sharing a COOL Meaningful stuff with us Keep it up..!

SAP training in Chennai

2:38 AM  
Blogger Sathya G said...

All are saying the same thing repeatedly, but in your blog I had a chance to get some useful and unique information, I love your writing style very much, I would like to suggest your blog in my dude circle, so keep on updates. Thanks for sharing and keep update more info for us. eagarly waiting for you tech service.
Software Testing Training

5:23 AM  
Blogger Jeffy said...

Thank you for sharing such a nice and interesting blog with us. Hope it might be much useful for us. keep on updating...!!
seo company in india
digital marketing company in india

1:51 AM  
Blogger Shalini said...

Superb. I really enjoyed very much with this article here. Really it is an amazing article I had ever read. I hope it will help a lot for all. Thank you so much for this amazing posts and please keep update like this excellent article.thank you for sharing such a great blog with us. expecting for your.

seo company in india

2:38 AM  
Blogger Lunameadow said...

This blog is the general information for the feature. You got a good work for these blog.We have a developing our creative content of this mind.Thank you for this blog. This for very interesting and useful.

Oracle Training in Chennai

10:34 PM  

Post a Comment

<< Home