CCNP Route Practice Questions

IS-IS (Intermediate System in OSI) Questions

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Q5. OSPF is a routing protocol used in IP networks, where as IS-IS is used in OSI environment. Many features are common between OSPF and IS-IS. Identify two such features that are common to both IS-IS and OSPF? [Choose 2 correct answers].

A. Both OSPF and IS-IS use Area 0 as backbone network.

B. Both OSPF and IS-IS use Distance Vector to determine shortest path.

C. Both OSPF and IS-IS have authentication capability.

D. Both use SPF (Shortest Path First) algorithm to compute shortest path.

Correct Answer: C,D

Explanation:

The following are the features common to both OSPF and IS-IS:
1. SPF (Shortest Path First) algorithm is used by both OSPF and IS-IS for computing shortest route to the destination.
2. They both elect designated router in multi access environment. The term used for designated router in IS-IS environment is Designated Intermediate System (DIS). However, the DIS election in IS-IS is pre-emptive. If a new router boots on the LAN with a higher priority, it becomes the DIS replacing the old DIS. 
3. Both OSPF and IS-IS have authentication capability.
4. Both use Hello packets to establish adjacencies. 
5. Both use SPF (Shortest Path First) algorithm to compute the shortest path to the destination.
6. Both use Areas, however there is no ABR (Area Border Router) in IS-IS. The L2 router is analogous to ABR.
Some differences between OSPF and IS-IS are:
OSPF uses IP whereas IS-IS uses CLNS in pure OSI environment.
IS-IS uses hierarchical architecture with two level (L1,L2) hierarchy, where as OSPF uses Area 0 with ABRs. Contiguous L2 or L1/L2 routers (ISes) form the backbone in IS-IS environment.

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