Cisco Vlan Trunking

HI All,

  I have a core switch 4500 connected to 7 sites using fiber cables, I have configured my vlans for the 7 sites, I started latey getting some trunk ports disabled in the 7 sites, show log was showing that I have some loop pack, for example
 loopback error detected on Fa0/11, putting Fa0/11 in err-disable state.

can someone advise me what could be the reasons, is it some vkan configurations issues.

Core switch trunk example for one site

interface GigabitEthernet1/2
 switchport access vlan 4
 switchport trunk encapsulation dot1q
 switchport mode trunk

also in my core switch i get this messages host is flapping between port G2/6 and G1/3

kindly advise if i miss any commands.

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rfc1180Connect With a Mentor Commented:
You are seeing this issue  because the keepalive packet is looped back to the port that sent the keepalive. Keepalives are sent on the Catalyst switches in order to prevent loops in the network. Keepalives are enabled by default on all interfaces and cannot be disabled. Possible senarios would be a intermittent short on the cable (Assuming copper), NIC issue on the host, or someone installed a loopback adapter on the switchport. It is not a configuration issue on the chassis.

>also in my core switch i get this messages host is flapping between port G2/6 and G1/3
This is an indication of a switching loop or you are trying to bond to NICs on the host level.

Do you have a detailed network diagram?

ITMaster1979Author Commented:
Thanks Billy

  How can I discover  the cause of the looping and stop the trunk from getting disabled

rfc1180Connect With a Mentor Commented:
>How can I discover  the cause of the looping and stop the trunk from getting disabled

Keepalives are sent on all interfaces by default in Cisco IOS Software Release 12.1EA-based software. In Cisco IOS Software Release 12.2SE-based software and later, keepalives are not sent by default on fiber and uplink interfaces. For more information, refer to Cisco bug ID CSCea46385  ( registered customers only) .

The suggested workaround is to disable keepalives and upgrade to Cisco IOS Software Release 12.2SE or later.

hmm, I did not think you could disable keepalives, but apparently you can on uplink ports
'no keepalive'

Check Configuration of the switch, also make sure devices connected to the switch are not mirroring the traffic back to switch using SPAN.

I would start looking at all the spanning-tree states on the all switches and look for any signs of switching loops. Also, make sure that you have SPAN sessions being sent to a switchport

Additionally, ensure that you do not have portfast enabled on any trunk you have configured in the network.

Do you have a detailed network diagram
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BigBlakeConnect With a Mentor Commented:
Is the topology purely hub and spoke or are there links between any of the 7 sites?

Can you run a 'Show CDP Neighbor" command on each of the switches - this may help you to pinpoint any loops in your network.

Are you running any etherchannel or port channel configurations ?
ITMaster1979Author Commented:
I disabled keepalive in the trunks, is there software that shows any high traffic from any host?

BigBlakeConnect With a Mentor Commented:
Are you still having the original problem - trunk ports error-disabled?

To analyse what hosts are sending high volumes of traffic you will need to enable Netflow on your switch to gather flow information. You can then view the ip flow information via the command line or export the information to a third party analyser running on a server or PC somewhere.

Under each interface you want to monitor traffic from  you need to enable netflow   . This is done in two parts;

Enable Netflow globally using these commands at a configuration prompt
ip flow-top-talkers
 top 10

Then add the interfaces you wish to monitor
Interface gi 0/0/1
 ip route-cache flow

To view the top talkers at the cli issue the following command

Show ip flow top-talkers

Or you can export it to an external server for analysis

ip flow-export destination X.X.X.X Z  (Which is the ip address and port number of your netflow server)
ip flow-export source (Specify an interface - may be loopback or vlan but it needs an ip address)
ip flow-cache timeout active 1
ip flow-cache timeout inactive 15



ITMaster1979Author Commented:
Thanks, But I still have some looping, any idea
BigBlakeConnect With a Mentor Commented:
Can you do a "Show cdp neighbour" for each of the flapping ports ?

show cdp neighbor gi 2/6
show cdp neighbor gi 1/3

This should identify the remote switch for each port involved in the flapping and is easier than tracing the cables. Also can you post the config for the affected ports?
ITMaster1979Author Commented:
Thanks Billy and BigBlake

   Could you advise what is the best practice to configure STP in the core switch and in all the 2950 switches in the network to prevent the loops.

Thanks for the support
do you have a network diagram
BigBlakeConnect With a Mentor Commented:
There are a couple of different ways of setting up spanning-tree and a number of options. In order to work out which solution would help you (Rather than making things worse) we really need to understand the topology in detail, as well as the nature of the links between sites as spanning-tree is a layer two protocol and only works  between directly connected switches.
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