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
OSPFIS-IS
Area borders inside routerRouters in one Area
Link in one AreaLinks define Area Borders
Backbone extensions can be difficultBackbone is easy to extend
IP Based, IP PoweredCLNS* (NSAP) Addresses define Routing
IP Carried in a backpack ( TLV )
Metric = Cost ( bandwidth based )Metric is 1O/ You can create the system
Widley known and supportedLess known

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

 

 

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