Thursday, February 07, 2008

Common problems with ethernet networks

The ugly face of the overdraft is stalking me and so I'm back onto freelancing in the evenings. I was at an interesting place last night - a film effects company who were having big trouble with their network. Windows shares that would sporadically not mount, name resolution that sometimes didn't work and file transfer speeds that often dropped to just a few kbits-1. They are running a mixed Linux/Windows/Mac network and so it seemed like the problems would be due to some underlying infrastructure issues. They have a Windows server running Active Directory with DHCP, DNS and WINS resolution - the reason for both WINS and DNS is that their render manager only talks NetBIOS names and won't resolve via DNS. Here are the problems I found;

  • Sharepoints is an OS-X app that makes sharing directories over Samba as easy as under Windows - it seems to add little extra value compared to using the Sharing control panel. One little gotcha is that there is a check box marked 'announce as master browser' which on all the machine I found was selected. Looking in the Windows server system error log showed numerous entries (many every minute) where the server was complaining about all the Macs trying to assume master-browse status. Unchecking sorted this and name resolution for the Windows machines improved markedly.
    Now, there is a well-established order for Windows browse master allocation - server, workstations etc. There will be at least one Master Browser on a workgroup/domain and one Backup Browser for every 32 systems in that workgroup/domain. This means that in a domain/workgroup with fewer than 32 systems, there will be one Master and one Backup Browser. One more Backup Browser will be added for every 32 systems. This is accomplished by the Master Browser telling a Potential Browser to become a Backup Browser. The browse service maintains an up-to-date list of domains, workgroups, and computers and provides this list to applications when requested. For example, a W2K user might see this list when they select a workgroup to which the user's computer does not belong from within the "My Network Places" application. "Browse lists" can also be displayed by using the "net view" command. The list can contain the names of domains, workgroups, and computers that run file and printer sharing services. These computers can be Windows for Workgroups (WFW) through XP machines, including both workstations and server, or any Samba shares that are in that subnet as well. Both domain and workgroup (if present) resources will be shown in these browse lists. So, having numberous Macs trying to bum-rush the show as browse masters is not a great idea!

  • The network seemed to have a good beginning - a couple of HP ProCurve 2800 and an 1800 all uplinked over 2gig fibre via SFPs. I discovered that one of the uplink ports was dead and so someone had 'got it going' with a cat6 between them. However - the failing port was intermitent and so periodically you'd get connectivity via two routes and a packet storm would ensue! I switched the SFP to a working port, re-established the fibre and removed the cat6 interconnect and it suddenly got a lot more solid! The HP ProCurve manager software is very good for identifying this kind of trouble.

  • This network had grown 'organically' (sic!) which means that where they needed more ports (an extra render-farm, for example) they had thrown in a Tottenham Court Road special - Netgear/DLink/etc. This had happened one time too often and so there were several pieces of equipment that were separated by four (or even five) ethernet switches - you run into the problem of the protocol retry period being less than the propigation delay through that many nodes. I nominated one of the ProCurves and made sure that all the switches in the place hung off that (and hence no two machines had more than three switches between them) and the servers all hung off that ProCurve.

So - there you go - I got home tired but really happy that I'd managed to sort some things out. I don't know how some of these little effects houses manage without engineers. I really like getting back to trouble-shooting and maintenance (and hopefully clearing down some debt!).

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