A brief flashback of IS-IS – Part I

IS-IS is categorised under Link State type of protocols & it is based on Dijkstra Algorithm same as OSPF. Admin Distance defaults to 115 & metric is 10 for all type of links (contrast to OSPF which has different cost for different type of media). It was originally standardized by ISO in 1987 (ISO10589) which was later adopted for TCP/IP type of networks & was called as Integrated IS-IS under RFC1195 (1990).

Integrated/Dual IS-IS supports IP Routing while original IS-IS only supported CLNP routing. It supports both ISO’s Connectionless-Mode Network Service (CLNS) and IP.

IS-IS runs on Layer2 in contrast to OSPF which runs on Layer3. IS-IS is directly encapsulated in L2 while OSPF uses IP Port89 as transport.

Below table explains the Place of IS-IS in Routing Protocols world:

  • It supports much larger inter-networks,
  • Less susceptible to routing loops.
  • Hierarchical routing
  • Classless behavior
  • Rapid flooding of new information
  • Fast Convergence
  • Very scalable
  • Flexible timer tuning
  • Implementation of multi-area routing
  • Route-leaking
  • Cisco IOS implementation of overload-bit

 

Similarities between OSPF & IS-IS

  • Link-state routing protocol and use a Dijkstra-based SPF algorithm to calculate shortest-path tree
  • Hellos to maintain adjacencies
  • Hierarchical
  • Provide address summarization
  • Classless
  • Elect Designated Router for broadcast networks
  • Authentication Capabilities

 

 

IS-IS versus OSPF – Terminology

IS-IS OSPF Comments
ES (End System) Host  
IS (Intermediate System) Router  
Circuit Link  
SNPA (Subnetwork Point of Attachment) Datalink Address  
PDU (Protocol Data Unit) Packet  
DIS (Designated Intermediate System) DR (Designated Router)  
N/A BDR  
IIH (IS-to-IS Hello Packet) Hello packet  
LSP (Link-State Packet) LSA (Link -State Advertisement) LSAs are actually comparable to TLVs used in LSPs.
CSNP (Complete Sequence Number PDU or Packet) DBD (Data Base Description Packet)  
PSNP (Partial Sequence Number PDU or Packet) LSAck or LSR (Link State Request)  
Routing Domain AS The term routing domain is also used with OSPF.
Level 1 Area Area (non-backbone)  
Level 2 Area Backbone area (Area 0) IS-IS uses a backbone path connected by contiguous L2 routers.  There is no backbone area in IS-IS
Level 1 IS (router) Internal Non-backbone Router Internal, non-backbone router in a Totally Stubby Area
Level 2 IS (router) Internal Backbone Router or ASBR Any Level 2 router can distribute externals into the domain.  No special name.  (Cisco IOS allows Level 1 routers to distribute externals.)
Level 1-2 IS (router) ABR  
System ID Router ID The System ID is the key for SPF calculations.  Sometimes the NET address is thought of as the Router ID.
AFI = 49 RFC 1918 Addresses AFI is part of the NSAP.

 

IS-IS versus OSPF – Timers

Interface IS-IS OSPF
Point-to-Point Hello – 10 sec

Holdtime – 30 sec

Hello – 10 sec

Dead – 40 sec

Broadcast Hello – 10 sec

Holdtime – 30 sec

Hello – 10 sec

Dead – 40 sec

NBMA N/A Hello – 30 sec

Dead – 120 sec

 

Other IS-IS OSPF
LS Aging 1,200 sec or 20 min

(counts down)

3,600 sec or 60 min

(counts up)

LS Refresh Every 15 min Every 30 min
NBMA N/A Hello – 30 sec

Dead – 120 sec

SPF Delay/Holdtime 5.5 sec / 10 sec 5 sec / 10 sec

Written by 

Waqas Karim is a seasoned Network Expert … Geek. He is the founder of ATech. ATech was started for learning & sharing. Over time the platform has grown to include other resources which continue to attract fellow networkers. Today it sees upward trend of a hundred thousand visitors per month, scattered all over the globe. His specialty is networking, but his interest & expertise spans from traditional IT to Network Security including Programming, Virtualization, Service Provider & so on... no matter the badge on the box. He is CCIE Certified (CCIE#56732) in addition to below badges: Telecom Engineer (BE) + CCIE-RS (CCIE#56732) + Huawei Certified (HW#706632) + MBA Microsoft Certified MCITP, MCSE#109*26, CCNP-Sec, CCNP-SP, CCNA-DC, CompTIA Security+, Nokia NRS-I, JNCIA, ITIL Certified (ITIL#*6373), CEH (Certified Ethical Hacker).

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Falcon22
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Falcon22

How can L1 routes be redistributed into some different protocol ?

Anton
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Anton

Just wondering why “ISISI is better”?

Mozamel
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Mozamel

Why does IS-IS scale better than OSPF as both have most of similarities?

Mark Obrien
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Mark Obrien

I read in a cisco white paper a comparison between IS-IS and OSPF.
IS-IS was preferred because the core area in the IS-IS is extensible. What is the reason for it?

Derpy Networking
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Derpy Networking

Please also shed some light on IS-IS configuration. Thanks

Olawale Anikulapo
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Olawale Anikulapo

Dear sir, can IS-IS routing protocol used in present days. Please tell us. I cannot see IS-IS in either CCNA, CCNP and MCS, MCSE syllabus

Feedbrain24271@gmail.com
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Feedbrain24271@gmail.com

thanks, IS-IS has been made very easy to me through this article.

Howard Turner
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Howard Turner

Studying for my CCDP – ISIS is mentioned quite alot thanks for this!

Aren Abkarian
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Aren Abkarian

Just what I was looking for. Been looking into such single article of late. Thanks so much.

Gajendra Singh
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Gajendra Singh

Very helpful Article. Please can you post an article about relationship among BGP, IS-IS & OSPF?

Anonymous
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Anonymous

Thank you for the beautiful explanation. really nice draft

Mrock4
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Mrock4

This stuff gets more and more interesting the deeper I go