duplex mismatch, and RCV-ERR counter

[sarahr202]

Hi everybody.

Rcv-err:  A receiver error occurs when a port's receive buffer overflows. Congestion on a switch's backplane could cause the receive buffer on a port to fill to capacity as frames await access to the switch's backplane. However most likely a Rcv-err is indicating a duplex mismatch.

I do not understand how a duplex mismatch will cause RCV-err .

I will appreciate your input.

thanks and have a great weekend.

[singhaam007 · Accepted solution]

hello Sarah,

Duplex mismatch is a condition where two connected devices operate in different duplex modes, that is, one operates in half duplex while the other one operates in full duplex. The effect of a duplex mismatch is a network that works but is often much slower than its nominal speed and will cause many issues. Duplex mismatch may derive from manually setting two connected network interfaces at different duplex modes, but also derives from connecting a device that performs auto negotiation to one that is manually set to a full duplex mode. Sometime duplex mismatch. As a result, the computer does not gain access to the network.

When both ends of the connection attempts to transfer data at the same time. This happens even if the channel is used in one direction only because of duplex mismatch (may be one end half duplex), in case of large data transfers it will drop some packets because This is an indication that the receive buffer is full.That’s way duplex mismatch cause rcv errors because the data side transfer in full speed but other side cant cope with that amount of data and end up dropping them.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00800a7af0.shtml

Hope this will helps.

thanks

[sarahr202]

Hi Singh

h1 (half)-----------------------full h2

Rcv-err:  A receiver error occurs when a port's receive buffer  overflows. Congestion on a switch's backplane could cause the receive  buffer on a port to fill to capacity as frames await access to the  switch's backplane.

However most likely a Rcv-err is indicating a duplex mismatch.

In above case, Rcv-err indicates a duplex mismatch on h1 . here is my reasoning

h2 is operating full duplex so when it has data to send , it just sends. H1 on the other hand, checks the medium and only sends data when the medium is available  . This means h2 has better chance sending more data than h1 which could cause receive buffer overflows at h1 thereby causing RCV-err to increment.

Is my understanding correct ?

thanks and have a great weekend.

[singhaam007 · Accepted solution]

hello,

yes you are correct.

The half-duplex end cannot accept the incoming data while it is sending because of duplex -- it will sense it as a collision. As a result, almost all of the packets sent by the full-duplex end will be lost because the half-duplex end. Some time TCP protocol perform error recovery for lost packets and try to retransmit them again but after trying again it will drop it again because the receive buffer is already full.

However most likely a Rcv-err is indicating a duplex mismatch.

Xmit-Err and Rcv-Err

This indicates that the internal port transmit (Tx) or receive (Rx) buffers are full. A common cause of Xmit-Err is traffic from a high-bandwidth link that is switched to a lower-bandwidth link, or traffic from multiple inbound links that is switched to a single outbound link. For example, if a large amount of bursty traffic comes in on a Gigabit port and is switched out to a 100-Mbps port, the Xmit-Err field might increment on the 100-Mbps port. This is because the port output buffer is overwhelmed by the excess traffic because of the speed mismatch between the incoming and outgoing bandwidths.

thanks

[michel.prevost]

Hi all, 

I Know that is a old post, but I am taking a chance to get a answer to that question.

If i understand correctly, If I get Rcv-err on a interface at full duplex, but there is no other counter that increment except the Rcv-err counter, I must be facing a backplane congestion problem here?

Thanks, have a nice day

[sarahr202]

Old post!!

Hi Michel,

This will be my understanding, receive buffer full error occurs when frames can not be emptied fast enough onto the back plane.  Receive buffer error can be caused frames faster into the receive buffer than they can be placed onto the back plane, it could be duplex issue or simply as you mentioned device is having internal congestion causing receiver buffer to take  a longer time to get empty.

I even forgot about this post:)

Have a good day!!