isis interview questions

Face to face with ISIS – The cells of the regime – Episode 4

Comparison Table: OSPF vs ISIS

Below table enumerates the differences between OSPF and ISIS protocols –




Administrative Distance 110 115
Standard RFC 2328 (OSPFv2) ISO 10589, RFC1195
Operating OSI Layer OSPF operates on top of IP Layer ISIS operates over L2
Virtual Links Supported Yes No
DR/BDR election OSPF elects a DR and BDR on broadcast networks ISIS elects a single DIS on broadcast networks
IP connectivity OSPF requires IP connectivity between the routers to share the routing information ISIS doesn’t require IP connectivity between the routers as updates are sent via CLNS instead of IP.
Security Prone to attack and hence requires more security overheads for protection. Since ISIS runs on Layer 2 , hence very unlikely possibility of attack
Area/Level Types ·Backbone Area

·Standard Area (Non Backbone Area)

Different Levels used in place of area

·Level 1

·Level 2

·Level 1/2 Areas

Identification OSPF uses router id to identify a router on network ISIS uses System ID to identify a router on the network.
Table Refresh OSPF refreshes the entire routing table after 30 minutes. ISIS doesn’t refresh the entire SPF table periodically like OSPF.
Related terms Area, non-Backbone Area, Backbone Area, ABR, ASBR, Host IS,Level-1,Level-2,L1/L2,Sub Domain, ES
Flexibility Less flexible than ISIS More flexible to use than OSPF especially in provider domain
Scalability Less scalable than ISIS More scalable than OSPF

Download the comparison table: OSPF vs ISIS

Back when OSPF and IS-IS were developed, IP wasn’t the dominant protocol that it is today. When people think of OSI they automatically think of the OSI-model but back then, ISO (International Organization for Standardization) also created something similar to IP and UDP called CLNP (Connectionless-mode Network Protocol) and CLNS (Connectionless-mode Network Service).

R2 now has a second database, the level 2 database. Besides its level 1 database and level 1 LSP, it now also has a level 2 database. It generates a level 2 LSP and all prefixes for interfaces that are directly connected and advertised in IS-IS. Each IS-IS router only creates a single LSP for each level. This LSP carries multiple prefixes.

Nowadays, we use IP everywhere so you might wonder why we care about this. When working with IS-IS, you will see some references to CLNP/CLNS here and there. For example, when configuring a router ID (called a Network Entity Title), it has to be configured with the NSAP (Network Service Access Point Address) format. NSAP is similar to an IP address, and it is not automatically configured so we have to understand its format.

Let’s say we want to connect area 12 to another area, this means we need a level 2 router. Let’s convert R2 into a level 1-2 router so I can show you what will happen. At this moment, we start with a clean slate so there is no neighbor adjacency between R1 and R2:

R1 and R2 are in the same area so they will establish a level 1 neighbor adjacency. These routers will flood their LSPs within the area so that everyone knows about all LSPs in the area. The two routers add each others LSP in their database. These routers can now run SPF on their level 1 database and figure out the shortest path to each destination. IS-IS uses something called the DIS / Pseudonode which is similar to OSPF’s DR/BDR to reduce unneeded flooding.

Continue Reading:

If preparing for your next interview, please go through the list of Top 100 OSPF Interview questions.


How does IS-IS protocol work?

Intermediate System to Intermediate System (IS-IS, also written ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this by determining the best route for data through a packet switching network.

What is attach bit in IS-IS?

The Attach Bit is set by L1L2 routers to notify L1 routers that they can reach the rest of the network i.e. default routing.

What is difference between IS-IS and OSPF?

OSPF designates a backbone area and standard or non-backbone area for inter-area advertisements whereas ISIS organizes the domain into different levels. To identify a router on the network, OSPF uses Router ID and ISIS uses System ID. OSPF is less flexible with more strict requirements for forming neighbor adjacencies.

Why IS-IS-is preferred over OSPF?

One of the main reason is that ISIS used an LSP per each level that contains different TLV for the routing update, whereas OSPF need to generate different LSAs. It depends on the architecture and design of your network. ISIS has better scalability than OSPF and many other things. Thanks for the info.

Related Posts

Leave a Reply

Your email address will not be published.