What is Autonomous System number?

Autonomous System number is numeric value with the range from 1 to 65535 is used to express a network or a group of network comes under single administration with same routing policy.

IGRP and EIGRP uses AS number, OSPF also uses AS number in the form of Process ID.

In IGRP and EIGRP different AS number will not communicate(exchange update) by default, to make them communicate we need to go for redistribution process.

whereas in OSPF no such issues different AS will communicate by default.

commands:

router(config)#router igrp 10

router(config)#router eigrp 100

router(config)#router ospf 20