Hello Jelani,

I tried to look at this thread, that I think it is taken from Cisco Learning Network.

You don't need a routing protocol to be able to ping other connected IP addresses to the same router.

This is the key point.

From the point of view of CORP router there is no difference between Vlan subinterfaces connected to L2Switch3550 and its serial interfaces to other nodes.

So without a routing protocol you can from a PC in vlan 10 reach every other IP address on the SAME router.

>> However, I noticed it was able to "successfully" ping the CORP router's 192.31.7.6/30 and

172.31.1.5/30 interfaces...without provisions of layer 3 routing information.

if that lab still exists or if you build a similar one you will see that the difference is that when you try to ping another's router IP address you cannot that is you could reach 172.31.1.5 but not 172.31.1.6 without a routing protocol or static routes.

That's all

Hope to help

Giuseppe