Which of the following is not a classless routing protocol?

Which of the following is not a classless routing protocol?

• IGRP
• OSPF
• RIPv2
• EIGRP 

 

EXPLANATION

The Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary technology which has been superseded by the Enhanced Interior Gateway Routing Protocol (EIGRP).  Among its enhancements over IGRP is support for Variable Length Subnet Masks (VLSM), which is required for "classless routing".
Open Shortest Path First (OSPF) is a standard supported by most, if not all, router makers; it has always supported classless routing features.
Routing Information Protocol (RIP) version 1 did not include support for VLSM; version 2 added this support.