ISIS Overview¶

ISIS overview¶

IS-IS is a dynamic link-state routing protocol, generally used inside ASs. IS-IS is based on the Dijkstra algorithm, and is able to convey IP, and IPv6 networking information.

IS-IS has been initially defined by the ISO for the OSI networks, then it has been extended by the IETF to support IP networks.

IS-IS is similar to OSPF, as it establishes adjacencies between directly connected routers. From a scalability perspective, IS-IS performances are also similar.

IS-IS has some differences with OSPF:

It supports both IP and IPv6 network information.
It is an OSI protocol and operates over an LLC header, which requires an Ethernet v1 Header. IS-IS is not configurable on non Ethernet interfaces like GRE interfaces. Also, instead of multicast packets sent, a specific mac address is used
The level concept replaces the area concept to separate networks and introduces 2 levels: Level-2 is generally used to connect to the backbone area; Level-1 is used for intra-area communications. An LSD is maintained for each area. IS-IS level can either be configured globally or per interface. By default, the system acts as both Level-1 and Level-2 router : Level-1-2 router. Each level has its own separate database. There is no route distribution between areas. It is up to the user to configure which level a device is running.
A specific Ethernet destination MAC address is used to send traffic over a given interface. The destination MAC is one of the below ones:
- for broadcast interfaces
  01:80:c2:00:00:14 : all level-1 intermediate systems
  
  01:80:c2:00:00:15 : all level-2 intermediate systems
  
  01:80:2b:00:00:05 : all intermediate systems
- for point to point interfaces
  09:00:2b:00:00:05 : all
IS-IS is an extensible protocol, as it uses TLVs and sub-TLVs to carry information. As a reminder , TLV stands for Type-Length-Value and is an encoding scheme used for storing various information elements in network protocols.
The metric calculation defaults to 10 in IS-IS whereas OSPF relies on the link bandwidth settings.

ISIS terminology¶

IS-IS has been defined for the OSI networks and has introduced some concepts with specific OSI wording which are reused in the IS-IS user guide. To facilitate the understanding of the document, the below table describes the analogous concepts between the OSI world and the TCP/IP world.

OSI world	TCP/IP world

system	Node
end-system	Host
intermediate system	Router
circuit	Interface / link
domain	Autonomous System

ISIS packets¶

IS-IS exchanges information through various kinds of messages. All IS-IS PDUs include a header that tells the kind of packet.

IS-IS PDU header¶

IIH packets are hello messages that help discover its neighbors and elect a DIS.
CSNP packets are usually emitted by DIS and contain a complete list of LSPs. CSNP packets are used to trigger database synchronization in LSD.
LSP packets advertise NLRI and topological information.

Standards¶

Virtual Service Router provides the following features:

ISO10589 :: Intermediate system to intermediate system definition

RFC 1195:: Use of OSI IS-IS for Routing in TCP/IP and Dual Environments
RFC 5120:: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)
RFC 5308:: Routing IPv6 with IS-IS
RFC 5303:: Three-Way Handshake for IS-IS Point-to-Point Adjacencies
RFC 5304:: IS-IS Cryptographic Authentication
RFC 5305:: IS-IS Extensions for Traffic Engineering
RFC 6232:: Purge Originator Identification TLV For IS-IS
RFC 7308:: Extended Administrative Groups in MPLS Traffic Engineering (MPLS-TE)
RFC 8405:: Shortest Path First (SPF) Back-Off Delay Algorithm for Link-State IGPs
RFC 8570:: IS-IS Traffic Engineering (TE) Metric Extensions
RFC 8667:: IS-IS Extensions for Segment Routing
RFC 8491:: Signaling Maximum SID Depth (MSD) Using IS-IS
RFC 8919:: IS-IS Application Specific Link Attributes
RFC 9352:: IS-IS extensions to support Segment Routing over the IPv6 Data Plane