+ Post New Thread
Results 1 to 10 of 10
Network and Classroom Management Thread, Network Broadcast in Technical; I had a hell of a week last week; all network switches were going mad and lights flashing very fast ...
  1. #1

    Join Date
    Apr 2007
    Location
    Birmingham
    Posts
    174
    Thank Post
    13
    Thanked 2 Times in 2 Posts
    Rep Power
    15

    Network Broadcast

    I had a hell of a week last week; all network switches were going mad and lights flashing very fast and freezing up- I looked for a faulty device or network card but I had no success, I suspect it was a broadcasting problem or a loop somewhere on the network. The problem didn't go on for the whole day and it only happened during some hours (started at around 8:40 Am, stopped at lunch time and it started again after lunch and finally stopped at around 3 PM). I had the teachers to bring their laptops to me and we checked all of them but we didn't find faulty network card or wireless. The network seems to behave itself now and god knows when this is going to happen again.

    Thanks in advance for any advice and help you might offer.

  2. #2

    Geoff's Avatar
    Join Date
    Jun 2005
    Location
    Fylde, Lancs, UK.
    Posts
    11,800
    Thank Post
    110
    Thanked 582 Times in 503 Posts
    Blog Entries
    1
    Rep Power
    223

    Re: Network Broadcast

    Enable STP on your core switches.

  3. #3

    Join Date
    Jul 2005
    Location
    Corby
    Posts
    1,056
    Thank Post
    12
    Thanked 20 Times in 18 Posts
    Rep Power
    23

    Re: Network Broadcast

    STP is the way to go- but make sure there are no hubs around somewhere- makes for a headache :cry:

    http://cisco-engineer.com/new/index....id=27&Itemid=1

    Paul

  4. #4

    localzuk's Avatar
    Join Date
    Dec 2006
    Location
    Minehead
    Posts
    17,095
    Thank Post
    511
    Thanked 2,309 Times in 1,785 Posts
    Blog Entries
    24
    Rep Power
    803

    Re: Network Broadcast

    Do you have managed switches? If yes, what type? If no, I would propose them as something for purchase ASAP.

  5. #5

    Join Date
    Apr 2007
    Location
    Birmingham
    Posts
    174
    Thank Post
    13
    Thanked 2 Times in 2 Posts
    Rep Power
    15

    Re: Network Broadcast

    Thanks guys for your replies. We have two core HP switches a gigabit HP switches, a few DLinks and two 3Coms.

  6. #6

    Join Date
    Oct 2007
    Posts
    1
    Thank Post
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0

    Re: Network Broadcast

    Our students occasionally loop a network cable from one wall plate to another, affecting the same switch. spanning-tree does not seem to be able to control these problems

  7. #7

    Join Date
    Jan 2007
    Posts
    16
    Thank Post
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0

    Re: Network Broadcast

    I had the same problem students looping cable on two network points, took me while to catch them

  8. #8
    richard.thomas's Avatar
    Join Date
    Sep 2007
    Posts
    491
    Thank Post
    5
    Thanked 11 Times in 10 Posts
    Rep Power
    16

    Re: Network Broadcast

    Any chance you're running some sort of classroom management software that allows you to see what each student is doing? Something like vision perhaps? I've noticed vision sends out alot of broadcast packets... Could be something to look into...

  9. #9
    Butuz's Avatar
    Join Date
    Feb 2007
    Location
    Wales, UK
    Posts
    1,579
    Thank Post
    211
    Thanked 220 Times in 176 Posts
    Rep Power
    62

    Re: Network Broadcast

    Sudden broadcast storms tend to be a network loop caused by a little cherub plugging in a patch cable to two drops. As has been said above, if you have managed switches then spanning stree can prevent this from taking your network down.

    Good way to find your network loop would be to unplug each of your switches one at a time from your core switch until your core switch returns back to normal activity - at least then you've isolated the loop back to a partiuclar network cabinet / building, you can then further isolate the fault on that particular cabinet until you find the port that's actually looped.

    Andrew

  10. #10

    m25man's Avatar
    Join Date
    Oct 2005
    Location
    Romford, Essex
    Posts
    1,608
    Thank Post
    49
    Thanked 444 Times in 330 Posts
    Rep Power
    136

    Re: Network Broadcast

    Guy's,
    In STP, ports can be set for blocking, disabled, forwarding, learning and listening.

    STP & RSTP add an identifing Algorithm to the packet to ensure that it does not cross the same port twice. If the same packet appears on an STP controlled link twice it will be forwarded or discarded depending on the status of any other links.
    Some switches do this automatically others may need specific programming to tell the port when to fwd and when to block.

    STP is used to control Broadcast traffic but within a given context.

    Scenario,
    If you have 5 cabs and they all link together in a loop to provide link redundancy then STP is essential.
    If you had 2 switches in a Cab servicing a single classroom and the little toads put a patch across the wall outlets and it bridged the switches, STP will help you.
    If you have 8 GB links connected between two switches in a LAG configuration then STP is useful when a link becomes flakey or the LAG configuration is lost (like when you reset the switch without saving the config first!).

    The best one I ever had was two sites linked by a Gigabit Laser Bridge with a 108mbs Wireless Backup. STP keeps the Wifi link down until the sun shone directly into the Laser Reciever at 3.30 on an autum afternoon the switch realises that the GB link is down and unblocks the wifi link for the duration of the Sun Block once GB link is restored the STP reverts back to the faster link as priority has been configured to do so through the use of a "Cost" value.

    But when shorting 2 ports at random on the same switch, STP is unlikely to help. This just floods the switch, then all of the attached nodes join in and start to broadcast.

    Adding STP to a topology that does not actually contain any physical loops will simply add to the overall traffic and latency of the system.

    What I'm saying is, that STP may if not correctly implemented may make it even harder for you to identify the true cause of your problem.

    On all but the most expensive equipment the CPU simply wouldn't be able to cope with the demands for traffic control, on cheap switches they would probably struggle to deliver decent STP performance on a single port yet alone 48 at once!

    In TCP/IP networks broadcast packet is generated when there is nothing else to resolve the ARP request, if DNS or WINS are too busy or are being drowned out by other chatter, clients will begin to broadcast.
    At a lower level if the destination mac is unknown to a switch it will ask it's neighbours through a broadcast.
    Having lots of different switch makes on you LAN will undoubtebly result in higher broadcast levels as the switches will not be able to exchange switch table information as easily as if they were all from the same manufacturer and shared bespoke discovery protocols.
    More than 30% Broadcasts on a given segment and you are in trouble.

    Each broadcast packet is unique, therefore adding an STP header will not stop the broadcast only add to the overhead!

    What you need to do is identify the reason for the broadcast or agressive ARP (DOS attack) in the first place and stop it!

    A single virus or worm application that is scripted to connect the victims node to a non existent remote host can just take off (It's unlikely that the writer has coded in any reasonable fail the app on retry options) thus creating a broadcast beacon looking for an IP that either doesn't exist or is blocked by you own firewall. Half a dozen of these on your LAN you can kiss it goodbye until the user gets the hump and turns the node off! Broadcast stops network returns to normal without explaination.

    A misconfigured staff laptop or a students own device could all trigger a broadcast storm as can a loose or broken RJ45 connector.

    Gratuitous ARP requests due to an improptu IP address change can give switches a headache, duplicate IP addresses or DNS Cache pollution on your own DNS server can all be the catalyst for your nightmare day!

    If you want to know more here is one of many on the subject of STP
    http://www.wwp.com/technology/network-resiliency.asp

SHARE:
+ Post New Thread

Similar Threads

  1. Replies: 2
    Last Post: 15th February 2008, 04:22 PM
  2. HP 2650 broadcast storms (stp?)
    By jrubinstein in forum Wireless Networks
    Replies: 2
    Last Post: 19th August 2007, 08:51 AM
  3. terminating CAT5E network cables in network cabinets
    By broc in forum Network and Classroom Management
    Replies: 7
    Last Post: 10th July 2007, 11:54 AM
  4. Internet Radio Broadcast software?
    By CyberNerd in forum Educational Software
    Replies: 15
    Last Post: 15th March 2007, 02:09 AM
  5. live broadcast of poptech 2005 starts today
    By russdev in forum General Chat
    Replies: 3
    Last Post: 19th October 2005, 01:29 PM

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •