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 |
How can L1 routes be redistributed into some different protocol ?
Yes, it can be redistributed from both L1 and L2 domains into another routing protocol.
But it is not a good design. You should carry the external routing information from another domain through BGP.
Just wondering why “ISISI is better”?
Because it supports much larger internetworks. It is Less susceptible to routing loops. It supports much better hierarchical routing & it is highly scalable. That’s why IS-IS is picking up again.
Why does IS-IS scale better than OSPF as both have most of similarities?
Because fewer LSPs are used in IS-IS which means less CPU/Memory overhead.
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?
Because in IS-IS, the area border lie “on links” not “inside router” as for OSPF. So, in IS-IS you can add as many L2 router as you want to expand the core area.
Excellent! Thanks for the clarification, I was scratching my head since last night.
Please also shed some light on IS-IS configuration. Thanks
Please browse through the Configuration & lab sections
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
Yes, IS-IS is picking up again in recent years due to the Data Center revolution. It is heavily included in DC & SP streams. Have a look at:
http://tools.cisco.com/GlobalLearningLocator/courseDetails.do?actionType=executeCourseDetail&courseID=8568
https://www.cisco.com/c/en/us/training-events/training-certifications/exams/current-list/ccie-sp.html
thanks, IS-IS has been made very easy to me through this article.
Studying for my CCDP – ISIS is mentioned quite alot thanks for this!
Thanks for app!
Just what I was looking for. Been looking into such single article of late. Thanks so much.
Thanks!
Very helpful Article. Please can you post an article about relationship among BGP, IS-IS & OSPF?
Please refer to the BGP section of website:
https://aurumme.com/atech/category/networking/layer3/BGP/
Thank you for the beautiful explanation. really nice draft
Thanks!!
This stuff gets more and more interesting the deeper I go