It’s amazing what passes for “cool” when you spend your days and nights staring at router configurations. The last two nights I have been working on a very simple sparse-dense lab that had been causing me problems. I finally figured it out and thought I would document some of the interesting bits I found.

When I was working to pass my TSHOOT exam I figured out a few of the best commands for me to troubleshoot.

Can I ping the gateway? No, check layer 2 issues first:
sh vlan br
sh int trunk
sh int status

Can I ping past the gateway? No, check layer 3 routing and protocols:
sh ip route

sh ip ospf ne
sh ip ospf int

sh ip eigrp ne
sh ip eigrp int

You get the idea, just a few commands to learn the most about a protocol quickly. For debugging multicast routing my new favorite commands are:
debug ip mpacket
sh ip mroute
sh ip pim rpf

With my most favorite being debug ip mpacket, let me show you why. This ping was not working when it began, however, in the middle I changed the unicast routing protocol to advertise the RP address. Notice that the error actually states the RPF failed to find the route to the RP.

R6(config)#do debug ip mpacket

R6(config)#do ping 224.10.10.10 rep 100 

Type escape sequence to abort.
Sending 100, 100-byte ICMP Echos to 224.10.10.10, timeout is 2 seconds:

IP(0): s=155.1.146.6 (FastEthernet0/0.146) d=224.10.10.10 id=1463, ttl=254, prot=1, len=114(100), mroute olist null
Reply to request 0 from 155.1.108.10, 44 ms
IP(0): s=155.1.146.6 (FastEthernet0/0.146) d=224.10.10.10 id=1464, ttl=254, prot=1, len=114(100), mroute olist null
Reply to request 1 from 155.1.108.10, 28 ms
Reply to request 2 from 155.1.108.10, 28 ms
Reply to request 3 from 155.1.108.10, 28 ms
Reply to request 4 from 155.1.108.10, 28 ms
Reply to request 5 from 155.1.108.10, 28 ms