Network Loops

Unanswered Question
Sep 16th, 2010

Hi community,

On some desks we have shortage of data ports hence smaller switches have been installed

Is this likely to cause problems in Network Loops ?

Thanks

I have this problem too.
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Jernej Vodopivec Thu, 09/16/2010 - 03:22

Just be careful when connecting "smaller switches": one uplink to "bigger switches" (or better said access switches) and connect other ports on "smaller switches" directly to PCs, laptops...

Samir Shaikh Fri, 09/17/2010 - 06:02

Thank you for your reply

I have 3  smaller switches connected to Cisco Switch ports and workstations directly linked with those smaller switches

Is it OK? or might create any problem as looping is concerned.

Antonio Knox Tue, 09/21/2010 - 08:44

If you're using the unmanaged (or dumb) switches like linksys desktop switches or something like that, these shouldn't be a problem, as they don't send out BPDUs and won't attempt to claim a spot in the spanning tree.  If you're using managed Cisco switches, you should make sure that your primary and secondary root bridges are clearly defined for that given vlan before plugging these new switches in:

PriRootSwitch(config)#spanning-tree vlan 1 root primary

SecRootSwitch(config)#spanning-tree vlan 1 root secondary

This will eliminate the likelihood that your switches attempt to become root and unleash havoc on your network.  Also, as far as physical loops go, I'd assume you know how to uplink a switch to a switchport, so keep the physical loops out of the mix as well and you should be fine.

Please rate post if its helpful.

mohamedtag Tue, 11/23/2010 - 13:57

Althoguh that most documentations mentioned that the SOHO Non Managed Switches should not cause any Problem , BUT We used to face this Problem with the DLINK & NETGEAR Unmanaged Switches untill we configured under all the Cisco 3500 XL Switches FastEthernet Interfaces the " Root Guard " & " BPDU Filter " Commands. It helps to Decrease the Problem very much.

davy.timmermans Thu, 11/25/2010 - 08:10

mohamedtag wrote:

Althoguh that most documentations mentioned that the SOHO Non Managed Switches should not cause any Problem , BUT We used to face this Problem with the DLINK & NETGEAR Unmanaged Switches untill we configured under all the Cisco 3500 XL Switches FastEthernet Interfaces the " Root Guard " & " BPDU Filter " Commands. It helps to Decrease the Problem very much.

I think you mean BPDU guard. I never recommend BPDU filter to be configured

mohamedtag Fri, 11/26/2010 - 16:20

Dear Davy,

Well . When we faced this Problem ( I Believe in the Beginning of 2007 ) and we were just not able to identify why these Unmanaged SOHO Devices causing this troubles almost weekly , We Decided to escalate the Case with Cisco TAC. After some investigations they recommended us to configure the below Lines under ALL the FastEthernet Interfaces ( taking into consideration that the FastEthernet interfaces are only used within our Network for EndPoints ( PC's - IP Phones - SetUp Boxes - Printers ...etc ) , Here is a Copy of a Typical FastEthernet Ports that are currently Active :

=============================

Interface FastEthernet0/3

Switchport access vlan 120

spanning-tree portfast

spanning-tree rootguard

spanning-tree bpdufilter enable

==============================

Actually this did not solve the Problem 100 % , but it decreased the Number of repeating this Network Loop issue to alomost once every 2-3 Months instead of previously facing it almost once weekly.

davy.timmermans Sat, 11/27/2010 - 00:00

Don't we always suppose that only an endpoint is connected to an access port

With BPDUfilter enabled, you don't sent BPDU packets out these ports. So if due to miswiring (accidently or not) two ports on the same switch are connected via a cable, you'll have a loop in the network.

Without BPDUfilter, both ports will receive each others BPDU packet and block 1 port

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