Monthly Archives: December 2010

Making “monitor session” work on Cisco 3750 switch

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)

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

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

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

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

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

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

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”

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 , , , , ,