Tag: IS-IS

Intermediate System to Intermediate System (IS-IS)

Intermediate System to Intermediate System (IS-IS) is a dynamic link-state routing protocol widely used in large-scale networks, especially in service provider environments. In this post I will look at the following IS-IS configurations;

• Redistribution of OSPF into IS-IS, and the differences between L1 and L2.
• IS-IS Path selection when L1 and L2 routes are available.
• How and where to apply IS-IS Route Summarization.

IS-IS Lab Setup

Using Cisco’s Modeling Labs (CML) I build the following IS-IS and OSPF topology:

• 2 x L2 Router ( Top )  (R1, R2) running XRv with IOS-XR.
• 2 x L1L2 Router ( left ) (R3, R4) running XRv with IOS-XR.
• 2 x L1 Router ( left ) (R7, R8) running IOS with IOSv.
• 2 x L1L2 Router ( Center ) (R5, R6) running XRv with IOS-XR.
• 2 x L1 Router ( Center ) (R9, R10) running IOS with IOSv.
• 1 x OSPF Router (R11) running IOS with IOSv.

Logical View:

Physical View:

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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

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