Table of Contents

Network Design Fundamentals Course

02:28:37 Hours
17 Lectures


IS-IS Training

05:14:41 Hours
12 Lectures


Routing Protocols Design and Deployment Course

47:00:55 Hours
51 Lectures


IS-IS Suboptimal Routing Design

IS-IS Suboptimal Routing - If you design multi level IS-IS network and if you have more than one exit (L1-L2 routers) from the Level 1 domain, you will likely create a suboptimal routing. Multi-level IS-IS design is for large-scale network. What’s more, most of the real life networks use only flat Level 2 IS-IS as their interior gateway protocol (IGP).

In the figure shown above, Router A is in the Level 1 IS-IS domain. While Router B is in the Level 1 IS-IS domain, it is in different area. Router A has two exit point/default gateway (L1-L2 Routers) to reach Router B; however, unlike Router B, Router A operates in different domain.   In IS-IS Level 1-Level 2, routers don’t send anything except default route (with ATT bit in the Level 1 LSP) towards the internal Level 1 Routers.

Thus, Router B only trust it’s Level 1-Level 2 router information. In addition, Router A doesn’t know the entire IS-IS topology would be in the flat Level 2 IS-IS topology. Furthermore, both L1-L2 routers advertise the same subnet – towards Router A – for the Router B network. In fact, only the metric of Router B is different.

Left L1-L2 gateway sends the route of Router B with metric 5; right L1-L2 sends the route of Router B with metric 10.   Because of that, Router A chooses the Left L1-L2 router as its exit point/default gateway to reach Router B. Obviously, this creates sub-optimal routing since the left L1-L2 router sends the packet to top routers so that more hops packet can travel – an effect that we don’t want in our design.  

Sub-optimal routing is appealing if you know the requirements of the application. Some applications can tolerate suboptimal routing since their timeout, delay, and jitter expectation may not be sensitive. In sum, putting the low-end devices into an L1 domain provides fault isolation, which in turn provides scalability.

Created by
Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

Today, with his carefully selected instructors, OrhanErgun.Net is providing IT courses to tens of thousands of IT engineers. 

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