JuniperConfig389 downloads

Juniper JunOS IS-IS & Segment Routing

IS-IS Level 1/2 routing with Segment Routing MPLS (SR-MPLS) and traffic-engineering extensions for Juniper MX/PTX routers. Includes SRGB configuration and prefix-SIDs.

↓ example.txt

What is it?

IS-IS (Intermediate System to Intermediate System) is a link-state IGP protocol similar to OSPF but widely preferred in service provider and large data center networks. Unlike OSPF which runs over IP, IS-IS runs directly over Layer 2 — making it more resilient and faster to converge. IS-IS uses 'levels' instead of 'areas': Level 1 is intra-area, Level 2 is inter-area (backbone), Level 1-2 is a border router. Juniper is a dominant IS-IS platform — JunOS has the most mature IS-IS implementation including Segment Routing (SR-MPLS and SRv6) extensions for traffic engineering.

When to use

·Service provider core networks — IS-IS is the standard IGP for most major ISPs
·Large data center fabrics where IS-IS + ECMP provides fast convergence
·MPLS networks requiring traffic engineering — IS-IS-TE with RSVP or SR-MPLS
·When Segment Routing is required — IS-IS SR extensions are more mature than OSPF SR

When NOT to use

·Enterprise campus networks — OSPF is more familiar and equally capable for campus
·Small networks — IS-IS adds complexity without benefit at small scale
·Networks requiring IPv4-only routing — OSPF is simpler for pure IPv4 enterprise deployments

How it compares

vs OSPFOSPF runs over IP (protocol 89). IS-IS runs over Layer 2 (CLNS) — immune to IP routing loops during convergence. IS-IS scales better in very large networks. OSPF is more common in enterprise; IS-IS dominates in service provider.

vs BGP (as IGP)Some large DCs use BGP as IGP (Facebook, Microsoft). IS-IS has simpler configuration and faster convergence for IGP use cases. BGP-as-IGP offers better policy control but more complexity.

Prerequisites

→NET (Network Entity Title) address must be unique per router — format: 49.XXXX.YYYY.YYYY.YYYY.00
→All IS-IS interfaces must have 'family iso' enabled in addition to 'family inet'
→IS-IS area design must be decided before deployment (L1-only, L2-only, or L1/L2)
→For Segment Routing: SRGB (Segment Routing Global Block) must be agreed across all routers

Config Generator

Hostname *

Router ID (Loopback) *

NET Address (NSAP) *

Format: 49.AREA.SYSID.00 — SYSID is the router-id in hex (e.g. 1.1.1.1 → 0100.0100.0100)

IS-IS Level

Metric Style

Enable Segment Routing?

SRGB Start Label

SRGB End Label

Prefix-SID (node SID index)

Absolute label = SRGB_start + index

IS-IS Interfaces

SR-TE Policies (optional)

Verification commands

show isis adjacency

Verify IS-IS neighbor adjacencies are Up

ExpectedShows neighbors with 'Up' state, interface, level, and hold time

show isis route

Show IS-IS learned routes

ExpectedLists all IS-IS destinations with metric and next-hop interface

show route protocol isis

Show IS-IS routes installed in the main routing table

ExpectedRoutes marked with 'IS-IS' source. Missing routes indicate export policy or redistribution issue.

Debug commands

show isis statistics

Show IS-IS PDU send/receive statistics and error counts

Common mistakes & fixes

SymptomIS-IS adjacency not forming

CauseInterface not configured for IS-IS, missing 'family iso', or level mismatch

FixVerify 'family iso' is configured on the interface (in addition to 'family inet'). Check both sides run the same IS-IS level (L1, L2, or L1-L2).

SymptomIS-IS adjacency flapping

CauseMTU mismatch or high CPU on routing engine

FixCheck interface MTU matches on both sides. IS-IS uses max-frame-size for Hello PDUs. Check 'show isis statistics' for PDU errors.

SymptomSR labels not being distributed

CauseSRGB range mismatch or SR not enabled on all routers

FixVerify SRGB start-label and index-range match across all routers. Check 'show spring-traffic-engineering lsp' for SR-TE path state.

Further learning

Recommended books to go deeper on this topic.

Routing TCP/IP, Volume 1

Jeff Doyle & Jennifer Carroll · Cisco Press · The definitive IS-IS reference — levels, LSP, NET addresses, area design

Kindle ↗Hardcover ↗

BGP Design and Implementation

Randy Zhang & Micah Bartell · Cisco Press · IS-IS as IGP under BGP — common SP design using Juniper

Kindle ↗Hardcover ↗Paperback ↗