Cisco’s Traceroute Implementation
Given this topic is very basic and small but its allways good to have those informations handy, you never know when you need it 
Cisco’s Traceroute implementation uses a mix of UDP and ICMP packets. A Cisco devices sends 3 UDP packets (default) with an invalid port address and a TTL of 1. Since the TTL is set to one the first router in the path will send a time to live exceeded in transit message (ICMP Type 11, Code 0) back to the source, since its not the target of the packet but it cannot forward the packet to the next hope due to the TTL of 1.
After the device received the 3 ICMP packets it will send 3 UDP packets again but this time with a TTL of 2. The same process recurs until the destination is reached. The destination then will send destination unreachable (port unreachable) messages (ICMP type 3, code 3) instead of the time to live exceeded messages. The traceroute process then knows that it reached the destination and the process can be stopped.
The device can build the path to the destination based on the ICMP messages, since those messages use the interface towards the source device as their source address.
The following table shows output characters for the traceroute command:
| Character | Description |
| nn msec | Displays the round trip delay for each try and note in milliseconds (Default is 3 tries) |
| * | The try timed out (Default is 3 seconds) |
| A | Administratively prohibited, such as via an ACL |
| Q | Source Quench, the target is too busy to answer |
| I | User interrupted test |
| U | Port unreachable |
| H | Host unreachable |
| N | Network unreachable |
| P | Protocol unreachable |
| T | Timeout |
| ? | Unknown packet type |
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About
Im a network engineer working for an international medical company based in Basel, Switzerland. I began my professional life in 2000 after 4 years of an IT apprenticeship. I started as the Network Management System responsible with quite a lot other of other tasks inside the Cisco networking field. In 2005 I changed to my current employer where I changed my field of knowledge from part NMS part Networking to full time Networking. I also started to educate myself deeper into networking. After passing the CCNA and CCNP exams I wanted to go further with my education and took the CCIE R&S track instead of doing a school with lots of stuff Im not interested in 
I started the CCIE journey in August 2007 with the first of 5 workshops at the Cisco Network Academy in Rapperswil and passed the CCIE in Brussels at 07th of May 2008.
During that time I wrote a lot of documents to keep the stuff I learned and I thought it might be an idea to make it public so it might help other people on their path through the Cisco Certifications. I hope this site gets useful not only for CCIE candidates but also for other networking professionals on their search for knowledge.
If you have any questions or want to contact me, please drop me a line to pashtuk at gmail.com
