Configuring SRv6 on Junos (Locator and IGP)

I wanted to make my SRv6 lab examples usable with more platforms. Adding netlab Junos SRv6 support (included in the 26.10 release) was relatively easy; here’s what I learned while working on the pull request:

  • I decided to use micro-SIDs and used the excellent SRv6 Micro-SID (uSID) Basics article1 by Krzysztof Szarkowicz (of the EVPN-with-MPLS fame) as the starting point.
  • Krzysztof recommended using SRv6 block configuration. I was too lazy to calculate it from the SRv6 locator (which was already part of the data structure), so I used only the locator configuration. Everything worked fine without SRv6 block anyway (but maybe I’m missing something important).
SRv6 uSID locator configuration on Junos
routing-options {
  source-packet-routing {
    srv6 {
      locator dut {
        5f00:0:1::/48;
        micro-sid;
      }
    }
  }
}
  • Adding the SRv6 SID to IS-IS was trivial. All I had to do was tell the IS-IS protocol to get a node uSID from the already-configured locator:
SRv6 IS-IS configuration on Junos
protocols {
  isis {
    source-packet-routing {
      srv6 {
        locator dut micro-node-sid;
      }
    }
  }
}

Have I missed anything? Would you have done it differently? Please write a comment.

Why Do We Need SRv6 Locator in IS-IS?

Even if you plan to use SRv6 solely for layer-3/VPN services, you have to add the SRv6 locator to the IS-IS configuration to get the SRv6 TLVs and the locator prefix into the IS-IS LSP. The locator prefix is inserted into the IPv6 routing tables on other IS-IS routers and enables forwarding of SRv6-encapsulated packets.

SRv6 IS-IS LSP advertised by a Junos router configured with SRv6
pe2# show isis database detail dut.00-00Area Gandalf:IS-IS Level-2 link-state database:LSP ID                  PduLen  SeqNumber   Chksum  Holdtime  ATT/P/OLdut.00-00                 328   0x00000004  0x9652    1191    0/0/0  Protocols Supported: IPv4, IPv6  Area Address: 49.0001  MT Router Info: ipv4-unicast  MT Router Info: ipv6-unicast  Hostname: dut  TE Router ID: 10.0.0.1  IPv6 TE Router ID: 2001:db8:0:1::1  Router Capability: 10.0.0.1 , D:0, S:0    SR Algorithm:      0: SPF      1: Strict SPF    SRv6: O:0  MT Reachability: 0000.0000.0007.00 (Metric: 10) ipv6-unicast    Link Local  ID: 345    Link Remote ID: 1    Local Interface IPv6 Address(es): 2001:1::1    Remote Interface IPv6 Address(es): 2001:1::2  IPv4 Interface Address: 10.0.0.1  IPv6 Interface Address: 2001:db8:0:1::1  MT IPv6 Reachability: 5f00:0:1::/48 (Metric: 0) ipv6-unicast  MT IPv6 Reachability: 2001:1::/64 (Metric: 10) ipv6-unicast  MT IPv6 Reachability: 2001:db8:0:1::1/128 (Metric: 0) ipv6-unicast  MT IPv6 Reachability: 2001:db8:0:1::/64 (Metric: 0) ipv6-unicast  SRv6 Locator: 5f00:0:1::/48 (Metric: 0) ipv6-unicast    Sub-TLVs:      SRv6 End SID Endpoint Behavior: uN PSP/USD, SID value: 5f00:0:1::        Sub-Sub-TLVs:          SRv6 SID Structure Locator Block length: 32, Locator Node length: 16, Function length: 0, Argument length: 80,

  1. The article was published on community.juniper.net, and a few days after this blog post was published, someone in HPE marketing managed to bork that website; the last time I checked, the original link either ended on a generic HPE webpage or returned a DNS server error. I don’t care whether it’s a temporary glitch or not (it’s not the first time someone has had a DNS problem with that domain); if HPE doesn’t like inbound traffic, we can fix that. ↩︎

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3 comments:

  1. Shreeram 01 October 2026 01:09

    Hi Ivan, Block become useful for companies that have multiple networks. Eg - A big ISP may have multiple networks- internet/ transit network, core network, aggregation network, access network. They each can get an identity from the block. Generally, they ISP would then insert the AS number to make it a unique identity.

  2. Antti Ristimäki 04 October 2026 04:35

    About the SRv6 block configuration, I might be completely wrong but this is how I understand the thing. In micro-SID the header compression is achieved by not repeating the most significant bits of the SID, that is, the Locator Block. In order to make that work within an SR domain all the routers need to share the same block. How could that work if one router considers the first 32 bits to represent the block while some other router considers the first 48 bits to be the block?

    I guess configuring the block explicitly is not required if you are sure that all the routers use the industry standard F3216 format, but I don't know if any standard states that F3216 must be the default format. As can be seen in the example, your lab router advertises the F3216 locator structure.

    Finally, explicitly configuring the block becomes even more relevant if one uses Flex Algos.

    Replies
    1. Ivan Pepelnjak 05 October 2026 05:39

      Thanks a million! And yes, if you stick with F3216, things usually work out ;)

  3. Johannes Resch 06 October 2026 09:07

    Hi Ivan, the link in the article to the Juniper blog page (https://community.juniper.net/blogs/krzysztof-szarkowicz/2025/09/30/srv6-micro-sid-basics) appears to be broken, it just ends up on main HPE page.

    Replies
    1. Ivan Pepelnjak 06 October 2026 09:39

      Thank you. They managed to break the link within the last few days (it worked the last time I checked).

      I'll "fix" it by removing the link. If they don't want incoming traffic, I'm perfectly fine with that 🤦‍♂️

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