IS-IS Design Principles

IS-IS History

https://en.wikipedia.org/wiki/IS-IS
https://en.wikipedia.org/wiki/OSI_protocols
https://en.wikipedia.org/wiki/Type-length-value

• Created for the OSI Protocol Suite
• Integrated IS-IS: the mutation.
• IS-IS dictionary:
  • IS = Intermediate System  (Routers)
  • ES = End System
  • TLV = Type Length Value
  • NSAP = Network Service Access Point
    • (OSI protocols equivilent of the TCP/IP’s IP Address)
• IS-IS features:
  • Link State Routing Protocol  (Same as OSPF)
  • NSAP address assigned per router
  • Dijkstra SPF powered (Same as OSPF)
    • PRC efficient
  • Hellos Establisch neighbors at Layer 2 ( source MAC, Multicast Mac )
  • Two routing levels ( Level 1 and Level 2 )
  • Area Based Design ( Routers know their area )
  • default link cost = 10

IS-IS High level Design

• Two routing databases
  • Level1 and Level2
    • Level 1 routers find closest Level2 Exit.
  • Area defined by
  • 49.0001  ( Area 1 )
    • 49 private adressing , 0001 is area 1
• Three router types

IS-IS Neighbors and Area Design

• Router types defined, neighbors constricted
• Router is in one area ( Level1 ), links can lead to other areas ( Level 2 )
• Level1 Neighbors only speak to Level1 Area
• Level2 Neighbors can be in the same or different Areas
• Neighbor authentication possible

• https://en.wikipedia.org/wiki/Connectionless-mode_Network_Service

NSAP Addressing

• Original implementation of OSI defined five fields the NSAP address would represent
• Cisco’s implementation of IS-IS only uses three of those fields
  • The Area Address
  • The System ID
  • The NSAP Selector ( NSEL )
• Example address:
  • 49.1234.aa15.b322.1841.00
    • Reading from right to left:
      • 00 defines an IS
      • System ID ( MAC address in this case )
      • AREA
• Addressing rules:
  • NSAP Addresses can be up to 20 bytes
  • Last byte of the address is always 00 and it identifies an IS ( a router )
  • System ID is always a fixed 6 bytes in length

R1(config)#router isis
R1(config-router)#net
R1(config-router)#net ?
XX.XXXX. ... .XXX.XX Network entity title (NET)

R1(config-router)#net 49.0001.1111.1111.1111.00

IS-IS Modes of Operation

• IS-IS supports two modes of operation:
  • Broadcast.
  • Point-to-Point.
• Modes generate different IS-IS Hello (IIH) Messages.
• Prevents NBMA cloud designs.
• Two accepted designs:

  • Flat Design

    • One area
    • L1 or L2 mode only ( Saves resources )
    • Cisco recommendation L2  ( creates a backbone )
      
      • 

  • Herarchical Design

    • Large network / scalable
    • Follow the Three Tier Hierarchy
    • 

Three Tier Hirarchy:

• Summerization supported L1 -> L2
• External (redistribution) L1  -> or L2