After the introduction to SR-MPLS demo I did during the Segment Routing workshop @ ITNOG10, we moved to dual-stack SR-MPLS – can we assign node segment identifiers (SIDs) to IPv4 and IPv6 prefixes? The demo used the same three-router network as the previous one, with IPv4 SIDs starting at one and IPv6 SIDs starting at 101:

Lab Topology

I had to make a single change to the previous lab topology – I had to define the IPv6 component of the address pools¹. I also used that opportunity to showcase² a few addressing options:

addressing:
  loopback:
    ipv4: 10.0.42.0/24
    ipv6: 2001:db8::/48
    prefix6: 128
  core:
    ipv4: 192.168.42.0/24
    prefix: 31
    ipv6: 2001:db8:cafe::/48

Notes:

• By default, netlab assigns /32 IPv4 prefixes to loopback interfaces.
• If you want to have IPv6 loopback prefixes, you have to specify the addressing.loopback.ipv6 parameter.
• I believed too much in the “/64 everywhere” gospel, so netlab assigns /64 IPv6 prefixes to loopbacks. You can change that with the addressing.loopback.prefix6 parameter, and you MUST change it to /128 if you want SR-MPLS to work on Arista EOS.
• Based on the above lab topology, the loopback interfaces in our lab will get IPv4 /32 prefixes from the 10.0.42.0/24³ address space (starting with subnet one, not zero) and IPv6 /128 prefixes from the 2001:db8::/48 address space
• The core links (we’ll get to what that pool is in a bit) will use /31 IPv4 prefixes from the 192.168.42.0/24 space, and /64 IPv4 prefixes from the 2001:db8:cafe::/48⁴ space.

On the topic of showcasing⁵, the topology uses a link group using a custom address pool:

links:
- group: core
  pool: core
  members: [ pe1-p, p-pe2 ]

The complete lab topology is in the SR-MPLS workshop GitHub repository, and here’s the addressing netlab generated based on our pool/link definitions:

Node/Interface	IPv4 Address	IPv6 Address	Description
pe1	10.0.42.1/32	2001:db8::1/128	Loopback
Ethernet1	192.168.42.1/31	2001:db8:cafe::2/64	pe1 -> p
p	10.0.42.2/32	2001:db8::2/128	Loopback
Ethernet1	192.168.42.0/31	2001:db8:cafe::1/64	p -> pe1
Ethernet2	192.168.42.2/31	2001:db8:cafe:1::1/64	p -> pe2
pe2	10.0.42.3/32	2001:db8::3/128	Loopback
Ethernet1	192.168.42.3/31	2001:db8:cafe:1::2/64	pe2 -> p

Exploring the SR-MPLS Setup

After setting up netlab, changing into the 1-intro/3-ds directory, and executing netlab up, you’ll have a working dual-stack SR-MPLS network and can start exploring.⁶

For example, do we get SR-MPLS TLVs advertised in IS-IS LSPs?

pe2#show isis database detail pe1.00-00Legend:H - hostname conflictU - node unreachableIS-IS Instance: Gandalf VRF: default  IS-IS Level 2 Link State Database    LSPID                   Seq Num  Cksum  Life Length IS  Received LSPID        Flags    pe1.00-00                     3  35182  1123    261 L2  0000.0000.0001.00-00  <>      LSP received time: 2026-05-17 08:50:21      Remaining lifetime received: 1199 s Modified to: 1200 s      NLPID: 0xCC(IPv4) 0x8E(IPv6)      Hostname: pe1      Area addresses: 49.0042      Topology: 2 (IPv6)      Topology: 0 (IPv4)      Interface address: 192.168.42.1      Interface address: 10.0.42.1      Interface address: 2001:db8:cafe::2      Interface address: 2001:db8::1      IS Neighbor          : p.00                Metric: 10        IPv4 Neighbor Address: 192.168.42.0        IPv4 Interface Address: 192.168.42.1        Adj-sid: 100000 flags: [L V] weight: 0x0      IS Neighbor (MT-IPv6): p.00                Metric: 10        Adj-sid: 100001 flags: [L V F] weight: 0x0      Reachability         : 192.168.42.0/31 Metric: 10 Type: 1 Up      Reachability         : 10.0.42.1/32 Metric: 10 Type: 1 Up        SR Prefix-SID: 1 Flags: [N] Algorithm: 0      Reachability (MT-IPv6): 2001:db8:cafe::/64 Metric: 10 Type: 1 Up      Reachability (MT-IPv6): 2001:db8::1/128 Metric: 10 Type: 1 Up        SR Prefix-SID: 101 Flags: [N] Algorithm: 0      Router Capabilities: Router Id: 10.0.42.1 Flags: []        SR Local Block:          SRLB Base: 965536 Range: 65536        Area leader priority: 250 algorithm: 0        Maximum SID depth:          Base MPLS imposition (MSD type 1):  6        SR Capability: Flags: [I V]          SRGB Base: 900000 Range: 65536

The IS-IS TLV is picture-perfect:

• It contains Segment Routing capabilities and the SR local block in Router Capabilities TLV
• It also contains Extended IP Reachability and multi-topology Extended IPv6 Reachability TLVs, each of them with a SR Prefix-SID sub-TLV.

The IPv6 SIDs are in the IS-IS segment routing prefix table:

pe2#show isis segment-routing prefix-segmentsSystem ID: pe2                  Instance: 'Gandalf'SR supported Data-plane: MPLS                   SR Router ID: 10.0.42.3Node: 6      Proxy-Node: 0      Prefix: 0       Total Segments: 6Flag Descriptions: R: Re-advertised, N: Node Segment, P: no-PHP                   E: Explicit-NULL, V: Value, L: Local, A: Proxy-Node attachedSegment status codes: * - Self originated Prefix, L1 - level 1, L2 - level 2, ! - SR-unreachable,                      # - Some IS-IS next-hops are SR-unreachable   Prefix                      SID   Label Type       Flags                        System ID       Level Protection           Algorithm   ------------------------- ----- ------- ---------- ---------------------------- --------------- ----- -------------------- -------------   10.0.42.1/32                  1  900001 Node       R:0 N:1 P:0 E:0 V:0 L:0      pe1             L2    unprotected          SPF   10.0.42.2/32                  2  900002 Node       R:0 N:1 P:0 E:0 V:0 L:0      p               L2    unprotected          SPF*  10.0.42.3/32                  3  900003 Node       R:0 N:1 P:0 E:0 V:0 L:0      pe2             L2    unprotected          SPF   2001:db8::1/128             101  900101 Node       R:0 N:1 P:0 E:0 V:0 L:0      pe1             L2    unprotected          SPF   2001:db8::2/128             102  900102 Node       R:0 N:1 P:0 E:0 V:0 L:0      p               L2    unprotected          SPF*  2001:db8::3/128             103  900103 Node       R:0 N:1 P:0 E:0 V:0 L:0      pe2             L2    unprotected          SPF

Not surprisingly, the MPLS forwarding table contains entries for IPv4 and IPv6 SIDs:

pe2#show mpls route... 900001  A[1]                via M, 192.168.42.2, swap 900001                    EgressACL: apply                    directly connected, Ethernet1                    ca:f0:00:02:00:02, vlan 1006 900002  A[1]                via M, 192.168.42.2, pop                    EgressACL: apply                    directly connected, Ethernet1                    ca:f0:00:02:00:02, vlan 1006 900101  A[1]                via M, fe80::c8f0:ff:fe02:2, swap 900101                    EgressACL: apply                    directly connected, Ethernet1                    ca:f0:00:02:00:02, vlan 1006 900102  A[1]                via M, fe80::c8f0:ff:fe02:2, pop                    EgressACL: apply                    directly connected, Ethernet1                    ca:f0:00:02:00:02, vlan 1006

The previous blog post goes into more detail, and explains why we see MPLS labels for SIDs advertised by PE1 and P, but not by PE2, in the PE2 MPLS table.

The Dirty Details

Most devices I could test with netlab implement dual-stack SR-MPLS⁷ except for Cisco IOS XE, which cannot assign SIDs to IPv6 prefixes⁸.

However, while FRRouting, Junos, and Nokia SR Linux assign IPv6 node SID to whatever IPv6 prefix you configure on the loopback interface, Arista EOS and Cisco IOS XR advertise the loopback prefix with the SR-MPLS SID sub-TLV only when the loopback IPv6 prefix is a /128. Nokia SR-OS rejects a loopback IPv6 prefix that is not a /128 anyway.

If you were duped by the “use /64 everywhere argument”, you’ll waste countless hours trying to troubleshoot the behavior (like I did). Making it worse, neither of these two generates any obvious error messages, and Cisco IOS XR makes it even more confusing because it assigns the SID (and MPLS label) to the /64 IPv6 loopback but does not advertise it in IS-IS:

RP/0/RP0/CPU0:pe1#show isis segment-routing label tableSat May 16 17:13:32.616 UTCIS-IS Gandalf IS Label TableLabel         Prefix                   Algorithm    Interface----------    ----------------         ---------    ---------16001         10.0.42.1/32             SPF          Loopback016002         10.0.42.2/32             SPF16003         10.0.42.3/32             SPF16101         2001:db8:0:1::1/64       SPF          Loopback016102         2001:db8:0:2::/64        SPF16103         2001:db8:0:3::/64        SPF

Try It Out

The workshop GitHub repository includes the installation guidelines; you might want to read them first. After that, you can:

• Start a GitHub Codespace
• Import an Arista cEOS container into it (alternate step-by-step instructions)
• Change directory to 1-intro/2-ds
• Execute netlab up
• Have fun