IS-IS ( I ntermediate S ystem to I ntermediate S ystem Protocol ) is a link state interior gateway protocol (similar to OSPF ), which is specified by the ISO standard 10,589th Originally ( 1992 ) IS-IS was designed for use with the connectionless protocol CLNP , support for use with IP was made possible through extensions.
IS-IS knows four message types:
- Hello (Intermediate System to Intermediate System Hello): Serves the function Capability Announcement and Neighbor Discovery
- LSP (Link State PDU (Protocol Data Unit)): Transports network topological information as well as IP , CLNP and IPv6 addresses
- PSNP ( Partial Sequence Number PDU): For sending acknowledgments (ACK) and requests to the neighboring router for complete information on a route. The router uses CSNP messages to determine whether it is missing information.
- CSNP ( Complete Sequence Number PDU): A CSNP contains a summary of all LSPs that are stored on the router / IS in the form of the Link State Database (LSDB). In broadcast networks, the CSNP is sent every ten seconds by the Designated IS (DIS). With P2P links, this is only sent once as soon as the link is active. In this way, all neighboring ISs receive information about changes that have taken place in addition to the LSPs. This also ensures that all IS have the same information. If a router notices that the information in a received CSNP differs from that in its LSDB, it requests the complete information on the relevant entry by sending a PSNP. In the broadcast network, the DIS then sends him the requested information via LSP. In the case of the P2P connection, the corresponding neighboring IS answers.
Since the mid-1990s, IS-IS has experienced a renaissance among American and European network operators. The reason for this is to be seen in the very minimalistic and therefore very easy to implement protocol architecture. In addition, the first IS-IS implementation from the manufacturer Cisco was much more robust than z. B. the OSPF implementation, which has moved many network operators to use the IS-IS protocol.
Disadvantages of IS-IS compared to other routing protocols
When using IS-IS in the IP environment, there is a difference compared to other routing protocols such as B. OSPF, the disadvantage that a neighborhood relationship is established between routers when communication on OSI layer 2 is successful, but at the same time problems or incompatibilities exist on OSI layer 3 (example: two routers were incorrectly connected to a LAN connection configured with IP addresses from different subnets).
Free software implementations
- Quagga (further development of Zebra)
- IS-IS Working Group of the IETF
- RFC 1142 OSI IS-IS Intra-domain Routing Protocol, February 1990
- RFC 1195 Use of OSI IS-IS for Routing in TCP / IP and Dual Environments, December 1990