High Latency has appeared on Cisco Switched network - 2 core routers in place which are swapping HSRP states

Posted on 2008-06-16
Medium Priority
Last Modified: 2008-10-06
We have a network consisting of 40 Cisco switches (ranging from 3560's to 2970's) with 10 vlans all doing the routing at 2 core Cisco 6513 routers (2 off for redundancy)

The last 2 days we have suddenly seen lots of latency on the network which has resulted in HSRP failures between the two routers.

Also Ciscoworks reports HighBroadcastRate & HighErrorRates on many switches
Switches physically look like they are passing large amounts of traffic, presumably broadcast traffic.
The switch logs do not appear out of the ordinary
Does anyone know where I could start to isolate this issue. It could be a broadcast storm, but how could I find where this started.  Any help or advice would be greatly appreciated.

Question by:Taylort79
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Expert Comment

by:Don Johnston
ID: 21792769
If the excessive traffic is broadcast then it certainly sounds like a broadcast storm. Unfortunately, there's no easy way to find the culprit. The first step is to diagram your network and ensure that any redundant paths are blocked. Use the "show spanning-tree" command to see which ports are blocked.

Author Comment

ID: 21793107
It appears to be just 1 of our VLans that is having problems. The latency appears when pinging to or from different vlans to this 1 vlan.
The config is fine as this worked before.  
I have tried running packet sniffers on the troublesome vlan but no unusual broadcasts were found?

Is there any other way of isolating broadcasts in a switched network

LVL 50

Accepted Solution

Don Johnston earned 1000 total points
ID: 21793280
Unfortunately, there's no easy answer.

And just because YOU haven't changed anything doesn't mean the network hasn't changed. All it takes is one cable connected where it wasn't connected before. It could be an end-user that brought in a small 4-port switch or a tech cleaning the wiring bird-nest in a closet.

You've already narrowed it down to one VLAN. Now just find out if you have any loops (redundant links not blocking).

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