IS IS stands for Intermediate System to Intermediate System. ISIS is a routing protocol for Internet Protocol Network
How ISIS works
ISIS is 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.
The IS-IS protocol was developed by a team of people working at Digital Equipment Corporation as part of DECnet. It was standardized by the ISO in 1992 for communication between network devices that are termed Intermediate Systems (as opposed to end systems or hosts) by the ISO. The purpose of IS-IS was to make possible the routing of datagrams using the ISO-developed OSI protocol stack called CLN
CLNS – Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI network layer datagram service that does not require a circuit to be established before data is transmitted, and routes messages to their destinations independently of any other messages
Comparing ISIS to OSPF;
- 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 uses 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.
The Differences between OSPF and ISIS
- 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).