SRv6 on Juniper Junos¶

Juniper implements SRv6 in Junos OS (MX Series) and Junos OS Evolved (PTX and ACX7000 Series), with both full-length (classic) SIDs and micro-SIDs (uSID, the RFC 9800 NEXT-CSID flavor). This guide walks through a complete uSID deployment — locator, IS-IS, L3VPN and Internet services, Flex-Algo and TI-LFA — following the structure of Juniper's validated design for SRv6 core and edge, and shows the commands to verify each step.

Platforms and Releases¶

Release numbers are the "introduced" values from Juniper Feature Explorer or, where noted, Juniper TechLibrary; check Feature Explorer for your exact platform and line card. Later additions include static and PCEP SRv6-TE policies (MX 21.3R1), EVPN E-LAN over SRv6 (MX 23.4R1), the End.B6.Encaps binding SID (ACX7000 25.2R1, MX 26.2R1) and TWAMP performance measurement over SRv6 paths (PTX 24.4R1). The SRv6 Support Matrix compares Junos with other platforms feature by feature. cRPD supports SRv6 in IS-IS and L3 services in BGP with limitations (no flavor or Flex-Algo options on End SIDs, no ARM64). There is no official SRv6 support statement for vJunos-router.

Building Blocks¶

The default uSID length is 16 bits. In each block the GIB defaults to 0x0000–0xDFFF (static) and the LIB to 0xE000–0xFFFF (dynamic). With no flavor configured, micro-sid SIDs use PSP and USD.

Step 1: Platform Prerequisites (MX)¶

set chassis network-services enhanced-ip
set routing-options resolution preserve-nexthop-hierarchy
set routing-options forwarding-table srv6-chain-merge

enhanced-ip is needed on MX only. The other two enable the chained, hierarchical next-hop structures that SRv6 services resolve through; the JVD sets them on every router.

Step 2: uSID Block and Locator¶

The example uses the IANA SRv6 SID block 5f00::/16 (RFC 9602), block 5f00::/32 and node 1 = 5f00:0:1::/48 (F3216: 32-bit block, 16-bit node uSID, 16-bit function).

set routing-options source-packet-routing srv6 block BLK0 5f00::/32
set routing-options source-packet-routing srv6 block BLK0 local-micro-sid maximum-static-sids 2000
set routing-options source-packet-routing srv6 locator LOC0 5f00:0:1::/48
set routing-options source-packet-routing srv6 locator LOC0 micro-sid block-name BLK0

maximum-static-sids reserves part of the LIB for statically configured service SIDs. By default the static LIB range is empty, so any static micro-dt46-sid value would fail to commit. Changing GIB/LIB sizes later is disruptive (it requires restarting routing, and dynamic SIDs can change), so size it up front. Use the same block on every router; uSID compression only works between nodes that share a block.

set routing-options source-packet-routing srv6 locator myloc 2001:db8:0:a0::/64
set protocols isis source-packet-routing srv6 locator myloc end-sid 2001:db8:0:a0:d01:: flavor usd
set protocols isis interface xe-0/0/0:0.0 level 2 srv6-adjacency-segment protected locator myloc end-x-sid 2001:db8:0:a0:1a01:: flavor psp

Step 3: IS-IS¶

set protocols isis interface et-0/0/0.0 point-to-point
set protocols isis interface et-0/0/0.0 level 2 srv6-adjacency-segment unprotected locator LOC0 micro-adjacency-sid
set protocols isis interface lo0.0 passive
set protocols isis source-packet-routing srv6 locator LOC0 micro-node-sid
set protocols isis level 1 disable
set protocols isis level 2 wide-metrics-only

• micro-node-sid advertises the locator and the uN SID (RFC 9352 SRv6 Locator TLV). The locator must be configured under IS-IS even if you only want SRv6 for L3VPN.
• micro-adjacency-sid with no value allocates a dynamic uA from the LIB. The JVD uses unprotected uA SIDs and relies on TI-LFA to protect uN SIDs and prefixes.
• IS-IS multi-topology (set protocols isis topologies ipv6-unicast) is not required. The JVD only uses it during migration, so IPv4 SR-MPLS and IPv6 SRv6 topologies can differ.

Step 4: L3VPN over SRv6¶

On each PE, assign a service SID per VRF and enable SRv6 service signaling on the BGP families.

# VRF with a dynamic uDT46 SID (End.DT46 with NEXT-CSID)
set routing-instances CUST-A instance-type vrf
set routing-instances CUST-A interface ge-0/0/2.0
set routing-instances CUST-A route-distinguisher 192.0.2.1:100
set routing-instances CUST-A vrf-target target:65000:100
set routing-instances CUST-A protocols bgp source-packet-routing srv6 locator LOC0 micro-dt46-sid
#   or a static value from the LIB static range:  ... micro-dt46-sid 0xfc30
#   or per address family:                         ... micro-dt4-sid / micro-dt6-sid
#   classic SIDs:                                  ... locator myloc end-dt4-sid / end-dt6-sid / end-dt46-sid <sid>

# IBGP to the route reflectors (IPv6 transport)
set protocols bgp group RR type internal
set protocols bgp group RR local-address 2001:db8:ff::11
set protocols bgp group RR family inet-vpn unicast extended-nexthop
set protocols bgp group RR family inet-vpn unicast advertise-srv6-service
set protocols bgp group RR family inet-vpn unicast accept-srv6-service
set protocols bgp group RR family inet6-vpn unicast advertise-srv6-service
set protocols bgp group RR family inet6-vpn unicast accept-srv6-service
set protocols bgp group RR family route-target nexthop-resolution no-resolution
set protocols bgp group RR neighbor 2001:db8:ff::1
set protocols bgp rfc8950-compliant
set protocols bgp multipath list-nexthop

How it resolves: with SRv6 services enabled, Junos ignores the BGP NEXT_HOP for resolution and resolves the route on the locator inside the SRv6 SID, which IS-IS has installed in inet6.3 as an SRV6-ISIS route. No resolution rib tweaks are needed. Two details matter in practice:

• extended-nexthop and rfc8950-compliant make VPN-IPv4 routes carry IPv6 next hops in the standard (RFC 8950) encoding, which matters for interoperability with other vendors.
• Route-target (RT-constraint) routes otherwise stay hidden, because the PE loopbacks exist only in inet6.0. nexthop-resolution no-resolution fixes this.

The per-VRF SID is advertised in the BGP Prefix-SID attribute (SRv6 L3 Service TLV) with the function bits transposed into the label field, as described in RFC 9252. Inter-AS Option C works over SRv6; Option B over SRv6 is not supported. For SIDs learned only via BGP (no IGP route to the remote locator), Junos 24.2R1+ on MX can use routing-options dynamic-tunnels <name> srv6.

Internet (Global Table) over SRv6¶

set protocols bgp source-packet-routing srv6 locator LOC0 micro-dt46-sid
set protocols bgp group RR family inet unicast extended-nexthop
set protocols bgp group RR family inet unicast advertise-srv6-service
set protocols bgp group RR family inet unicast accept-srv6-service
set protocols bgp group RR family inet6 unicast advertise-srv6-service
set protocols bgp group RR family inet6 unicast accept-srv6-service

Step 5: Flex-Algo¶

A Flex-Algo needs a definition, IS-IS participation and its own locator. Services pick the algorithm through the locator their SID comes from.

# Algorithm 128 = low-latency
set routing-options flex-algorithm 128 definition metric-type delay-metric
set routing-options flex-algorithm 128 definition spf
set protocols isis source-packet-routing flex-algorithm 128

# Locator for algorithm 128, in its own /32 block
set routing-options source-packet-routing srv6 block BLK128 5f00:a1::/32
set routing-options source-packet-routing srv6 locator LOC128 5f00:a1:1::/48
set routing-options source-packet-routing srv6 locator LOC128 algorithm 128
set routing-options source-packet-routing srv6 locator LOC128 micro-sid block-name BLK128
set protocols isis source-packet-routing srv6 locator LOC128 micro-node-sid
set protocols isis interface et-0/0/0.0 level 2 srv6-adjacency-segment unprotected locator LOC128 micro-adjacency-sid

# Option A: the whole VRF uses the low-latency locator
set routing-instances CUST-LL protocols bgp source-packet-routing srv6 locator LOC128 micro-dt46-sid

# Option B: allocate a second SID in a VRF and select it per prefix by policy
set routing-instances CUST-A protocols bgp source-packet-routing srv6 locator LOC128 micro-dt46-sid non-default
set policy-options community VOICE members 65000:911
set policy-options policy-statement LL-PREFIXES term VOICE from community VOICE
set policy-options policy-statement LL-PREFIXES term VOICE then srv6 locator LOC128 micro-dt46-sid
set policy-options policy-statement LL-PREFIXES term VOICE then accept
set protocols bgp group RR export LL-PREFIXES

In option B the VRF advertises its default SID for most prefixes, and the export policy on the IBGP group swaps in the algorithm-128 SID for the matching ones, so remote PEs reach those prefixes over the low-latency topology.

Delay-based algorithms need link delay: enable delay-measurement on the IS-IS interfaces (TWAMP Light), and advertise it as an application-specific link attribute for Flex-Algo. IS-IS installs SIDs of non-zero algorithms in inet6color.0. On Junos, Flex-Algo participation applies to SR-MPLS and SRv6 together — you cannot define an algorithm for SRv6 only. See Flex-Algorithm.

Step 6: TI-LFA and Microloop Avoidance¶

set protocols isis backup-spf-options use-post-convergence-lfa
set protocols isis backup-spf-options use-source-packet-routing
set protocols isis interface et-0/0/0.0 level 2 post-convergence-lfa
set protocols isis interface et-0/0/0.0 level 2 post-convergence-lfa node-protection cost 16777214
set protocols isis spf-options microloop-avoidance post-convergence-path delay 5000

Link protection is the default once post-convergence-lfa is on an interface. The JVD adds node protection with a very high cost, so the node-protecting path is preferred but a link-protecting backup is still used when no node-protecting path exists. uSID-based TI-LFA, microloop avoidance and Flex-Algo protection arrived in Junos 23.4R1. Before Junos 26.2R1, SRLG is not considered for SRv6 TI-LFA; 26.2R1 adds IS-IS IPv6 SRLG (TLV 139) for SRLG-aware TI-LFA on MX, PTX and ACX7000. The JVD measured worst-case link-failure convergence around 5 ms. See TI-LFA for the algorithm.

Verification¶

show spring-traffic-engineering is for SR-TE policies, not for SRv6 locators and service SIDs.

Limits and Gotchas¶

• SID depth (from the Junos SRv6 docs): MX encapsulates up to 6 SIDs and inserts up to 5; ACX7000 encapsulates 12 and inserts 10; PTX10002-36QDD and PTX10008 (Express 5) support 7. uSID packs up to six 16-bit uSIDs per 128-bit container, which leaves plenty of headroom for TE and TI-LFA.
• Encap vs insert: ingress PEs always encapsulate, so egress SIDs must support USD; transit SRH insertion for TI-LFA needs a PSP-capable last SID.
• Static plus IS-IS SIDs: configuring the same static uA or End.X SID under both the SRv6 manager and IS-IS fails commit (a duplicate uN is allowed; IS-IS wins).
• IS-IS SRv6 limits: the IS-IS SRv6 concept page lists anycast locators, SRLG, End.X interface groups and Flex-Algo locator conflict resolution as unsupported. Feature Explorer has since added anycast locators and summarization (MX10004/MX10008 22.3R1, ACX7000 22.4R1) and SRLG (26.2R1), so check your platform and release.
• No next-hop-self knob: Junos IBGP needs an export policy with then next-hop self when you redistribute CE routes into plain IPv4/IPv6 BGP.

Hands-On¶

• netlab Labs - Junos SRv6 lab topologies, including the ipSpace.net SRv6 examples
• GNS3 Labs - Emulate Junos routers for SRv6 testing

Further Reading¶

• SRv6 Support Matrix - Junos feature by feature next to IOS-XR, EOS, SR OS, VRP and open-source platforms
• Implementations Overview - Compare Junos with IOS-XR, EOS, SR OS and Linux
• uSID Compression - How uN, uA and uDT SIDs pack into one address
• BGP Overlay Services - L3VPN and EVPN over SRv6 (RFC 9252)
• TI-LFA - P/Q-space repair paths and vendor configs
• Flex-Algorithm - Constraint-based topologies
• Juniper SRv6 Validated Design

References¶

1. Segment Routing User Guide - Juniper TechLibrary - Comprehensive Junos OS guide for configuring SR-MPLS and SRv6
2. JVD: Service Provider SRv6 Core and Edge (PDF) - Validated uSID design with full test-bed configurations, Flex-Algo, TI-LFA and convergence results
3. Understanding SRv6 Network Programming in IS-IS Networks - SRv6 in IS-IS: SID types, encapsulation modes, TI-LFA behavior and limits
4. Example: Configuring SRv6 Network Programming in IS-IS - Classic End and End.X SID configuration
5. SRv6 Network Programming and Layer 3 Services in BGP Networks - TechLibrary guide for SRv6 L3VPN and Internet services with BGP
6. srv6 block statement and locator micro-sid statement - uSID block, GIB/LIB and flavor defaults
7. Static SRv6 SIDs - Static uN/uA SIDs, interaction with IS-IS and Flex-Algo route tables
8. Understanding TI-LFA for IS-IS - Backup options, limits and IPv6 caveats
9. IS-IS Microloop Avoidance for SRv6 - Post-convergence-path microloop avoidance
10. Juniper Feature Explorer - Per-platform release information for each SRv6 feature
11. cRPD supported features - SRv6 support and limitations on containerized Junos routing
12. SRv6 Micro-SID (uSID) Basics (Juniper community) - Step-by-step uSID configuration and outputs on Junos 24.4R1
13. Day One: Introduction to SRv6 (PDF) - Juniper Day One book introducing SRv6 concepts and configuration
14. Configuring SRv6 on Junos (ipSpace.net) - Hands-on walkthrough of SRv6 locator and IGP configuration on Junos, tested in a netlab topology
15. Configuring SRv6 Layer-3 Services on Junos (ipSpace.net) - Follow-up covering SRv6 L3VPN service configuration on Junos
16. Junos OS 20.3 In Focus Guide (PDF) - First SRv6 release: IS-IS End/End.X, flavors, Flex-Algo and TI-LFA for the SRv6 data plane
17. Feature Explorer: SRLG-aware TI-LFA and anycast locators - Later additions to IS-IS SRv6