you don't need a MIB on your router, but on your server where you are reading the MIB values.

SNMP always passess it's data as some number, like: 1.3.9.2.3.1.1.5:300

And MIB is the file that says (for example - not correct values - that's what MIB is for)

1.3.9.2 = Cisco

.3 = interface

.1 = Fast Ethernet

.1 = 0/1

.5 = OUT bytes per second

300 = 300

So SNMP is numbers and MIB is for you to understand it. So you load it where you want to understand it. On machine where you are reading SNMP. On your server, notebook, etc.