All Cisco switches by default have PVST+ as their spanning-tree protocol (mode). PVST+ is Cisco proprietary and, in my humble opinion, should never be used in a production environment. The alternatives are: RPVST and MST. In a basic 1-3 VLAN network with little to no knowledge of spanning-tree you should run RPVST (802.1w) and be done with it. However, if you have a lot of VLANs and/or you need to ensure you’re not over utilizing the CPU resources, you should use MST (802.1s).Continue reading →
There is an issue I have noticed with VMware systems deployed with Nexus vPC technology that involve traffic only making it out of the vPC by disabling half the vPC or getting rid of the vPC completely. Initially you’re thinking this is a Cisco issue and I am here to tell you that you’re wrong.Continue reading →
It is a common mistake to assume X number of ports in an etherchannel equates to the common port speed * X; however, this is grossly incorrect and I’ll attempt to explain this behavior to you in layman terms, but just to make it quick too, it is not load balanced in a round-robin fashion where bandwidth is utilized evenly.Continue reading →
I recently was watching a CBTNuggets video when I heard mention that you could use a careful wildcard mask to select odd or even numbered subnets for route advertisement; however, I noticed there was something off about the comment and investigated a little deeper.Continue reading →
You can use tcpdump or wireshark but if you have a Linux box handy you can install: cdpr. It makes life easy like this:
Just follow the prompts for selecting the interface and wait for the cdp transmission to come through. Understand that some values, like Native VLAN, are in hexidecimal and you’ll need to convert it to decimal. Otherwise, happy hunting.
We’ll just do it the “quick and dirty” way. Just start capturing all traffic on an interface. Now, in the top portion by the word “Filter:” put this syntax in:
udp.port == 67 or udp.port == 68