Collisions in VAX/VMS box

[Sk Noorul Hassan]

In my VAX server, collision light on Ethernet tranciever blinks continuously when running an application on it. But, it shows no collisions if we stop the application. Network group also find collisions on switch port to which this box is connected. Is there any way to know whether collisions are generated due to my VAX/VMS box problem or some external network problems ?

Please also suggest how to check collisions at VMS end .I have checked using "mcr ncp sh kn line counters"

[Wim Van den Wyngaert]

Use
$ mc lancp
show lan/char

to verify the settings of the LAN. Compare this with the setting on the switch. Should be the same.

$ mc lancp
show lan/count

to verify all kind of errors. Even if the interface is not configured for decnet (ncp).

Wim

[Volker Halle]

Hi,

what is the speed of the network card on your VAX server ? I don't think the VAX can do anything other than 10 mbit half-duplex. Is the switch port set to auto-negotiate ? Can you set it to fixed 10 mbit hdx ?

Do you see any increasing non-zero error counters in the NCP LINE COUNTERS ?

Volker.

[Ian Miller.]

Did you change anything recently? Previously did this report collisions?

[Sk Noorul Hassan]

In mcr ncp sh kn line counters, single & multiple collisions are showing 15000 & 10000 respectively as of now, but it is inceasing very fast. Do u want me to check any thing else ? Switch port is fixed to 10 MBPS.

MCR LANCP sho lan command does not exist in my vax. pls suggest

[Steven Schweda]

You could simply have a bad cable or other
hardware problem.

Also, you might get better advice if you would
reveal your VMS and DECnet versions.

[Sk Noorul Hassan]

VAX box has got VMS version 7.1 & DECNET Phase IV

[Volker Halle]

Could the switch possibly be set to 10 Mbps and FULL DUPLEX ?

A twisted pair piece of cable between a network card and a switch port is a really small LAN and you would not expect to see many collisions.

Volker.

[Bart Zorn_1]

Could this be the "heart beat", aka collision detect check?

Older DEC ethernet interfaces required this setting. Otherwise DECnet would complain.

AFAIK, for more recent ethernet cards, it doesn't make much of a difference if heart beat is on or off.

Anyway, it is likely a switch setting on the transceiver.

HTH,

Bart Zorn

[labadie_1]

Post the result of the command

$ mc ncp sh k line co
$ mc ncp zero k line co
$ wait 00:10:00
$ mc ncp sh k line co

and we will know if the heartbeat is badly positioned

[John Gillings]

Don't forget...

Collisions are part of the protocol. They're not necessarily "bad". Are you getting the expected network throughput? If so then cover the collision light with tape and forget about it.

If not, then start diagnosing what might be causing the problem. Don't assume it's the (apparent) collisions. As others have suggested, check that the ports match. The speeds must be right, or nothing would work, but there may be a duplex mismatch.

From an application and operating system perspective you have no control over collisions. There's NOTHING you can do to program them away. The only way to reduce collisions is to reduce the number of systems chattering on the same ethernet segment.

[Robert Gezelter]

Hassan,

I concur with John. As has been noted, mis-configuration of any hardware element, or a defect in the hardware, can easily cause this or similar problems.

If the "heartbeat" is set incorrectly, then the error counter will increase with EVERY packet. If there are mis-matches between the switch port and the NIC, all manner of errors can occur, from problematical collisions, to lost packets and timeouts.

- Bob Gezelter, http://www.rlgsc.com

[Sk Noorul Hassan]

After setting all the counters to 0, 10 mins observation shows 2360 multiple collisions & 3860 single collisions.

Pls suggest

[Richard Brodie_1]

It would would be much more useful if you could post or attach the full NCP counters, rather than just the collision statistics.

[Volker Halle]

What is the configuration of the LAN this node is connected to ? Which network card and media (AUI, thinwire, TWP) ? Direct connection to switch, if so is the switch port set to half-duplex ?

Has it always been like that ?

As John already said, collisions are part of the ethernet procotol. If you connect your VAXserver to a busy LAN segment and then start sending lots of packets, you will naturally see collisions...

Volker.

[Robert Gezelter]

Hassan,

Please post the output of the "NCP SHOW KNOW LINE COUNTER" command.

Citing a collision count of 3200/2200 (single/multiple) is not significant without the context of how many packets (and the time interval) are involved.

If the packet count is close to the number of multiple collisions, then you are getting a collision on essentially every packet, which is likely showing up as extremely poor performance. If the number of packets is in the tens or hundreds of thousands (10**5 or 10**6) or more, then the collision rate is actually very small, and this is, alas normal.

On non-trivial IEEE 802.3 networks (aka Ethernet), are fully natural and normal, a consequence of the underlying structure of the CSMA/CD (Carrier Sense Multiple Access/Collision Detect) unerlying network technology. To prevent an excessively long posting, I can direct you, depending on your personal reading preferences, to:

Text Books: Computer Networking (Tannenbaum, Prentice Hall)
Technical Aspects of Data Communication (McNamara, Digital Press)

Wikipedia: http://www.wikipedia.com

Published Paper:
The original Ethernet Paper (by Metcalfe et al.) published in Communications of the Association for Computing Machinery (and republished in the magazines 25th(?) anniversary issue is a exemplary paper. It is well written, discusses all the technical issues, and is quite accessible. It is available electronically in many cases through libraries and universities, and directly available from the ACM online archive (http://www.acm.org) for a modest fee.

In summary, some collisions (single and multiple) are to be expected on an active network. The problem occurs when collisions occur at too frequent a rate. If there is a high collision rate (relative to packets transmitted), it can signify (in no particular order):

- a need for modifications to the network topology to better route traffic
- an increase in the speed of the underlying network (10Mbps to 100 Mbps)
- a malfunction or incorrect configuration of one or more network components.

I hope that the above is helpful.

- Bob Gezelter, http://www.rlgsc.com

[Sk Noorul Hassan]

Thanks for your suggestions. Now the network team is looking into the problem. As sson as I get the root cause, I will update.