What are two major differences between OSPF and IS-IS routing protocols?
OSPF is a Layer 3 protocol that runs directly over IP, using IP protocol number 89 for its packets. IS-IS, by contrast, is an OSI-based protocol that operates directly over the data-link layer independent of IP; on Ethernet segments its PDUs are carried in IEEE 802.3/802.2 LLC frames rather than being encapsulated in IP. This fundamental encapsulation difference is well documented in comparisons of the two protocols.
Separately, the two protocols differ in how their designated-router role is maintained. OSPF's Designated Router (DR) election is non-preemptive — once a DR is elected on a segment, it keeps that role even if a router with a higher OSPF priority later joins the network; a new election only occurs if the existing DR fails. IS-IS's Designated Intermediate System (DIS) election, however, is preemptive — if a router with a higher interface priority joins the LAN at any point, it immediately takes over the DIS role, regardless of whether the current DIS is still functioning, purging the old pseudonode LSP and reflooding new LSPs.
The other options misstate actual protocol behavior: OSPF fully (not partially) refreshes its LSA database every 30 minutes while IS-IS's default LSP refresh interval is 15 minutes (not 60); an OSPF Area Border Router can belong to multiple areas while an IS-IS router belongs to only one area (the reverse of what is stated); and OSPF's default administrative distance is 110 while IS-IS's is 115 (also reversed in that option).
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