Monthly Archives: December 2010

Making “monitor session” work on Cisco 3750 switch

Posted on 2010/12/18 by

It may just be the version of IOS my customer is using, but today I came across a bug when trying to capture traffic on a Cisco Catalyst 3750 switch. The most frustrating part is that I’d seen this before, … Continue reading →

Posted in Cisco, IEEE, mst, portfast, rapid spanning-tree, rapid spanning-tree, session monitor, spanning-tree, spanning-tree, Wireshark

Remember – VTP is sent untagged (so ends up on the native VLAN)

Posted on 2010/12/14 by

When VTP sends updates, they are sent without tags. So (assuming the native VLAN is VLAN 1) if you have restricted your trunk port at the receiving end with a command like: switchport trunk allowed vlan 2,10,20 it will drop … Continue reading →

Posted in Cisco, VTP | 1 Comment

Command to show Cisco ACE contexts that use a particular resource-class

Posted on 2010/12/08 by

I had a customer who had many contexts running on a Cisco ACE module, and several resource-classes. She was getting frustrated at trying to to find which contexts had been allocated a particular resource-class – say resource-class XXX. The problem … Continue reading →

Posted in Cisco, Cisco ACE | 1 Comment

decoding vlan.dat

Posted on 2010/12/06 by

I was trying to find out where vtp transparent switches stored the vtp information that they pass on to other switches (I’m now of the opinion that they don’t – I think they simply read vtp frames, check them for … Continue reading →

Posted in Cisco, perl, Wireshark | Tagged , , , , ,

bad interpreter: No such file or directory when running perl script on OS X

Posted on 2010/12/05 by

I found a perl script that refused to run on Max OSX. I would consistently get: #: bad interpreter: No such file or directory This link set me on the right direction by suggesting that there were DOS CR characters … Continue reading →

Posted in Mac OS X, perl | 2 Comments

Cisco catalyst private-config.text file

Posted on 2010/12/05 by

I’ve often noticed the file private-config.text on a Catalyst switch, so I thought I’d have a look at it: Switch_x#more private-config.text %Error opening flash:private-config.text (Permission denied) so that really got my curiosity going. It took a couple of searches, but … Continue reading →

Posted in Cisco | Tagged , , , | 1 Comment

The Illusive Art of Acquiring A Good Idle-PC Value

Posted on 2010/12/05 by

One of the most difficult problems new users have to come to grips with when starting out with GNS3 is the concept of an Idle-PC. Get it right, and you will have a great GNS3 experience. Try to ignore it, … Continue reading →

Posted in Cisco, GNS3

bgp routes in bgp table but not in routing table

Posted on 2010/12/03 by

Had a strange problem on a router recently where the bgp routes would show up when the show ip bgp command was issued, but NOT when the show ip route command was issued. Turns out I was using an old … Continue reading →

Posted in Cisco | Tagged , , , ,

Solution to “TFTP copy: operation terminated due to bad checksum comparison”

Posted on 2010/12/02 by

In short: TFTP_CHECKSUM=0 will get you out of trouble I came across this error when trying to re-image a Cisco 2600 router.  I had set up another router as a tftp server, and was attempting to re-flash the problem router … Continue reading →

Posted in Cisco | Tagged , , , , ,