What is IS-IS?
- it is a link state protocol which originates from ISO 10589 - Connectionless Network Protocol (CLNP)
- was originally designed to support CLNS, still uses CLNS for its transport
- supports both IPv4 and IPv6
- mostly popular in large ISP environments
- similarly to OSPF it runs Dijkstra SPF algorithm
- use Hello packets to manage adjacencies (IIHs)
- use areas and two-level hierarchy, only two types of the areas
- summarisation is only possible on ABRs between areas
- elects designated router (DIS) similar to OSPF DR except no backup DR (backup DIS)
Sample lab diagram:
NOTE. It is essential to understand that the IS-IS connectivity between areas relays on the level-1-2 adjacencies.
Concepts and Operations:
- Router is an Intermediate System and host is End System
- SNPA - Subnetwork Point of Attachment - concept that relates to data link/data switch
- Frame is Data Link PDU / Packet is Network PDU
- LSP - Link State PDU - LSP is a packet itself
- two types of areas: Level 2 - backbone area and Level 1 - a non backbone area
- Level 1/2 routers (similar to OSPFs ABRs) must have databases for Level 1 and 2 areas
- Virtual-Link not suported by many vendors, but it is possible to expand L1/2 adjacency
- in general much simpler that OSPF
- supports two metric types: narrow(only IPv4) & wide (both IPv4 and IPv6 recommended)
Hierarchy:
Level 1 routing - routing within area
Level 2 routing - routing between areas - backbone
Design concepts for IS-IS:
- good addressing schema to summerize on ABRs
- need to plan CLNP addressing (NET)
- two layer hierarchy to limit LSP flooding and point of subnet summarization
- need to use wide metrics
- all interfaces by default are cost of 10
- cost needs to be set manually (plan cost per link speed then assign)
- allows up to 1000 routers per area - great for scalability
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