Layer 2 Traceroute - Another Cisco Baby To Help Operations Guys

Well it's been a while or probably years ago I wanted to talk and write about this interesting Cisco baby called " Layer 2 Traceroute ". I called it Cisco Baby because it requires Cisco Discovery Protocol (CDP) to be enabled in order to function.

Now Idea is simple, In traditional L2 Campus or Enterprise Network our Network Operations Team keep getting several requests such us tracing user port and change it's vlan to something else or maybe a troubleshooting call where we need to identify source port of user or server etc. The traditional approach usually was to first hop on to switch hosting SVI for given VLAN , Figure out Layer 2 address for given IP (User/Server IP) and do reverse engineering by tracing that Layer 2 Mac Address hop by hop until we hit the user port configured in access vlan or Server that might be using trunking in a virtualized network.

Here is a quick example of traditional approach:

Now Here is how this wonderful IOS tool helps you trace quickly

Further Readings:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/l2trace.html

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_13_ea1/command/reference/3550cr/cli3.pdf

http://packetpushers.net/tracing-a-layer-2-path-on-cisco-nexus-switches/


HTH...
Deepak Arora
Evil CCIE