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Fun With Wireshark: Packet Analysis and Ethical Hacking Part 3: Troubleshooting

Here is the part you want to see - Troubleshooting!

Question: Do trunks form between SW1 / SW2 and SW2 / S3?

Remember: VLAN trunks are the highway that every bit of information can pass upon, no matter what VLAN it came from. When the VLAN frame gets closer to its destination, it will travel on roads only avaliable for that VLAN.

The packets for SW1/2 so far show DTP and PAgP. Now, PAgP is Cisco-proprietary Etherchannel, but DTP is for trunking negotiation between switches.


Clicking the packet and reading the drop-down for Dynamic Trunk Protocol:

This is SW1

SW2 has similar output.

" Dynamic Auto" = Our side will negotiate with the other side, whatever that is.
"Access" = No VLAN tagging, trunk won't be formed.

Essentially: "We can talk about the possibility of trunking, but I'm waiting for you to make a move about trunking", except they say that to each other.

It's the IT equivalent of a middle school dance.

What about SW2/SW3? That's for you to buy the course and find out.

Next is CDP (Cisco Discovery Protocol) & LLDP (Link layer Discovery Protocol). I check the VTP domain, software version, and other things.

Fun fact: You can see what a device is capable of;

The entire troubleshooting section is looking at packets to glean information; If I can't figure out the information from the questions presented, I'll show you what I had to receive the answer to. Otherwise, I'm showing you most of the course.


Routing protocols are prioritized over other traffic types.

One question was "What protocol is OSPF using" It's an Interior Gateway Protocol

And protocol # 89!

There are more passwords in clear text. Don't do it!

DR and BDR can be found in the Hello packet information on said Hello Packet. DRs (Designated Router) and BDRs(Backup Designated Router) are dictated by a set OSPF Router priority. Highest number wins, 0 sits out of the game.

OSPF does not use TCP, instead it has it's own mechanism to communicate and makes sure that data gets through.

Make sure the area numbers are the same, or the OSPF routers will not become neighbors.

They must also share the same subnet mask on interfaces that wish to neighbor.

One of the routers sets itself as a backup designated does that work in the real world? Does it work? I feel it could if you had two NIC cards and one would just pick up the slack.


EIGRP has K values, used to scale numbers in the metric calculation of finding the best route.

They must match for routes to be shared.

K1 and K3 are defaults. The Authentication is in MD5 - Not plain text!

Check the routes being advertised:


BGP Identifiery = Router ID

Fun Fact - Autonomous Systems for BGP are reserved, much like port numbers. Each one used here is reserved for Private use.

Using the filters are especially helpful here. Type in
And check out the many options.

How was a certain route learned? Through IGP.


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