OSPF vs ISIS : Detailed Comparison

Similarities between OSPF and ISIS

Before going into the details of differences between OSPF and ISIS, let’s explore some similarities:

OSPF and ISIS both are Link State Routing Protocols using the Dijkstra SPF Algorithm.
Both are Interior Gateway Protocols (IGP) that distribute routing information between routers belonging to a single Autonomous System (AS).
Both use Hello packets to create and maintain adjacencies between the neighboring routers.
Both the protocols are classless protocols and support classless Inter-Domain Routing (CIDR) and Variable Subnet Length Masking (VLSM)
Both support Authentication Mechanism
Both support multipath.
Both support IP unnumbered links.

OSPF operates on the top of IP layer whereas ISIS operates over Layer 2.
OSPF can support virtual links but ISIS can not support (as it operates on Layer 2 directly).
OSPF elects a DR and BDR on broadcast networks which can not be pre-empted however, ISIS elects a single DIS which can be pre-empted.
IP connectivity between the routers to share the routing information is required in case of OSPF, while ISIS doesn’t require IP connectivity as the updates are sent via CLNS instead of IP.
OSPF is prone to attacks hence security overheads are required for protection. The possibility of attacks is very less in case of ISIS as it runs over Layer 2.
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. The hello and dead intervals, and the subnet mask must match (except on point-to-point links).

Below table enumerates the differences between OSPF and ISIS protocols –

Parameter	OSPF	ISIS
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