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SRv6 uN uSID — Leaf/Spine Config Reference (SONiC vs Arista EOS)

Reference for static SRv6 uN uSID programming on MRC leaf–spine fabrics. Covers the comparison between SONiC (FRR + ConfigDB + SAI) and Arista EOS 4.36 (7060XE7 / Tomahawk 6 — MRC rack reference design), with concrete configuration templates for a single MRC plane inside a 72-GPU rack.

See the Glossary for definitions of uN, uA, PSP/USD, F3216, ConfigDB, and related terms.

Scope

Templates use an illustrative addressing plan aligned with OCP MRC / OpenAI MRC semantics (F3216 uSID, static tables, no ECMP, no dynamic routing). Replace interface names, link-local next hops, and uSID values with your deployment spreadsheet before production use.

Documentation sources

System Document Role for uN/uSID
SONiC ConfigDB SRv6 schema, Static SRv6 HLD, SRv6 uSID HLD, FRR zebra SRv6, FRR static-sids Config + operational CLI
Arista EOS SRv6 uN Support TOI (local PDF) (4.34.2F+) · web TOI Authoritative CLI — router srv6, micro-segment domain, locator, static routes, ping srv6 / traceroute srv6
Arista EOS SRv6 uA TOI (4.35.0F+) Optional static uA adjacency configuration
Arista EOS 4.36 EOS 4.36.0FX-SRV6 Command API Guide (local PDF) eAPI JSON schema for show srv6 … output — observability/automation only; no config or ping srv6
Arista EOS 4.36 EOS 4.36.0FX-SRV6 Release Notes v0.1 (local PDF) 4.36 SRv6 build platform matrix and known caveats (e.g. zero CSID after active segment on 7060X6)
MRC OpenAI MRC paper Fabric semantics and path encoding

The public EOS 4.36 user manual does not include a dedicated SRv6 uSID configuration chapter. Use the TOI for syntax; use the Command API guide to parse show srv6 locator, show srv6 route, show srv6 capabilities, show srv6 domain, and show srv6 adjacency-segments via JSON-RPC. SONiC configuration is split between ConfigDB and FRR vtysh.

Arista EOS — CLI validation (SRv6 uN TOI 4.34.2F)

Validated against SRv6-uN-Support.pdf in this repo (Arista TOI, June 2026 export). Arista EOS SRv6 uN uses a different CLI tree from SONiC/FRR and from 3rd-party OS segment-routing srv6 examples.

Feature & platform (TOI)

Item TOI detail
Introduced EOS 4.34.2F — SRv6 uN on DCS-7060X6; F3216; static-route reachability to neighbor uNs; ping srv6
Later 4.35.0F traceroute srv6; 4.35.1F 7280R3/7500R3/7800R3 (TCAM profile srv6-usid-f3216)
uN dataplane Match local /48 locator → shift left 16 bits → LPM in IPv6 FIB (static routes to neighbor uNs)
Flavors show srv6 capabilities: PSP, USD, Next-CSID supported; End with NEXT-CSID in System SFIB

Platform note: This primer’s rack diagrams use 7060XE7 (Broadcom Tomahawk 6). Arista’s SRv6 uN TOI and the 4.36.0FX-SRV6 release notes first document uN on DCS-7060X6 (Broadcom Tomahawk 5) — a separate, concurrently shipping product line. F3216 uSID semantics and EOS CLI are the same; confirm SRv6 feature support and caveats against your exact SKU and EOS build before deployment.

Validated Arista CLI (from TOI)

Purpose Command / stanza TOI
Enable IPv6 routing ipv6 unicast-routing Troubleshooting §1
SRv6 config root router srv6 → vrf default Configuration
uSID block (F3216) micro-segment domain <NAME> → block fcbb:bbbb::/32 Configuration
Local uN locator locator <NAME> → prefix micro-segment domain <NAME> end usid <value> Configuration
Loopback for ping interface Loopback0 → ipv6 address <locator-prefix>/48 SRv6 Ping
Neighbor uN reachability ipv6 route <neighbor-uN>/48 <nh> or … Ethernet N fe80::… SRv6 Operation
Verify locator show srv6 locator Show Commands
Verify micro-segment domain show srv6 domain Command API 2.1944 (GIB range, block state)
Verify System SFIB show srv6 route Show Commands
Verify capabilities show srv6 capabilities Show Commands
Verify static uA (optional) show srv6 adjacency-segments Command API 2.1942 (4.36+); uA TOI 4.35.0F+
Path ping ping srv6 sid <target> via segment-list <sid1> <sid2> … SRv6 Ping (CLI only — not in Command API)
Path trace traceroute srv6 sid … via segment-list … (4.35.0F+) SRv6 Traceroute (CLI only — not in Command API)

Not Arista EOS syntax (do not use on EOS)

Syntax Belongs to
segment-routing → srv6 → locators 3rd-party OS / FRR-style examples
micro-segment behavior unode psp-usd 3rd-party OS
prefix … block-len … behavior usid SONiC / FRR
show segment-routing srv6 locator / sid 3rd-party OS
static-sids / behavior uA … SONiC / FRR (uA: separate Arista TOI)

Reference topology (one MRC plane)

A 72-GPU rack uses 16 × Arista 7060XE7 switches — one leaf and one spine per plane, with no cross-plane links. Each plane is an isolated 2-hop fabric:

  Node A (NIC n)          Node B (NIC n)
       │                        ▲
       │ 400G/800G              │
       ▼                        │
  ┌─────────┐   uplink   ┌─────────┐   downlink   (same leaf, different ports)
  │ Leaf Pn │───────────▶│ Spine Pn│──────────────┘
  └─────────┘            └─────────┘
       ▲                        │
       └──────── uplink ────────┘

Traffic path (same plane, node A → node B): A → Leaf → Spine → Leaf → B

SRv6 encodes that path in the outer IPv6 destination address as a uSID carrier. Each transit switch runs uN (shift-and-lookup). See MRC Packet Structure.

Addressing plan (F3216 — unified SONiC & Arista)

Uses F3216: 32-bit block + 16-bit uSID per locator (RFC 9800 / IETF uSID). Both vendors use the same /48 prefixes derived from one block and one 16-bit end usid per switch.

Derivation rule

Step SONiC / FRR Arista EOS (TOI) Result
1. Block prefix …/48 block-len 32 node-len 16 + behavior usid micro-segment domain … → block fcbb:bbbb::/32 F3216 block
2. Local uN static-sids → sid <prefix>/48 … behavior uN locator … → prefix micro-segment domain … end usid <value> /48 uN
3. Verify show segment-routing srv6 locator show srv6 locator → Prefix field Must match loopback / static routes

Formula: block fcbb:bbbb::/32 + end usid 0xWWWW → fcbb:bbbb:WWWW::/48

TOI example: fc00:42::/32 + 0xC901 → fc00:42:C901::/48. This template uses rack block fcbb:bbbb::/32.

uSID value allocation (rack R1)

Role end usid pattern Example (P1) Notes
Leaf plane n 0x01{n} 0x0101 n = 1…8 (hex digit in low byte)
Spine plane n 0x02{n} 0x0201 Spine tier for same plane
Leaf→spine uA (optional) 0xfe0{n} 0xfe01 Static adjacency on explicit uplink
Rack label (logical) high byte 0x01 / 0x02 — Encoded in uSID, not a separate address field

Shared block once per fabric: fcbb:bbbb::/32. Unique end usid per switch (GIB default 0x1–0xDFFF per TOI).

Side-by-side prefix mapping (8 planes)

Canonical /48 uN prefix is identical on SONiC and Arista. Columns show how each vendor names the same value.

Plane Tier Hostname end usid /48 uN prefix (both vendors) SONiC locator Arista locator Arista CLI
P1 Leaf leaf-p1-r1 0x0101 fcbb:bbbb:0101::/48 MRC-R1-P1 MRC-R1-P1 end usid 0x0101
P1 Spine spine-p1-r1 0x0201 fcbb:bbbb:0201::/48 MRC-R1-S1 MRC-R1-S1 end usid 0x0201
P2 Leaf leaf-p2-r1 0x0102 fcbb:bbbb:0102::/48 MRC-R1-P2 MRC-R1-P2 end usid 0x0102
P2 Spine spine-p2-r1 0x0202 fcbb:bbbb:0202::/48 MRC-R1-S2 MRC-R1-S2 end usid 0x0202
P3 Leaf leaf-p3-r1 0x0103 fcbb:bbbb:0103::/48 MRC-R1-P3 MRC-R1-P3 end usid 0x0103
P3 Spine spine-p3-r1 0x0203 fcbb:bbbb:0203::/48 MRC-R1-S3 MRC-R1-S3 end usid 0x0203
P4 Leaf leaf-p4-r1 0x0104 fcbb:bbbb:0104::/48 MRC-R1-P4 MRC-R1-P4 end usid 0x0104
P4 Spine spine-p4-r1 0x0204 fcbb:bbbb:0204::/48 MRC-R1-S4 MRC-R1-S4 end usid 0x0204
P5 Leaf leaf-p5-r1 0x0105 fcbb:bbbb:0105::/48 MRC-R1-P5 MRC-R1-P5 end usid 0x0105
P5 Spine spine-p5-r1 0x0205 fcbb:bbbb:0205::/48 MRC-R1-S5 MRC-R1-S5 end usid 0x0205
P6 Leaf leaf-p6-r1 0x0106 fcbb:bbbb:0106::/48 MRC-R1-P6 MRC-R1-P6 end usid 0x0106
P6 Spine spine-p6-r1 0x0206 fcbb:bbbb:0206::/48 MRC-R1-S6 MRC-R1-S6 end usid 0x0206
P7 Leaf leaf-p7-r1 0x0107 fcbb:bbbb:0107::/48 MRC-R1-P7 MRC-R1-P7 end usid 0x0107
P7 Spine spine-p7-r1 0x0207 fcbb:bbbb:0207::/48 MRC-R1-S7 MRC-R1-S7 end usid 0x0207
P8 Leaf leaf-p8-r1 0x0108 fcbb:bbbb:0108::/48 MRC-R1-P8 MRC-R1-P8 end usid 0x0108
P8 Spine spine-p8-r1 0x0208 fcbb:bbbb:0208::/48 MRC-R1-S8 MRC-R1-S8 end usid 0x0208

Optional uA (leaf uplink) — same prefix on both vendors:

Plane end usid /64 uA prefix SONiC SRV6_MY_SIDS key suffix Arista
P1 0xfe01 fcbb:bbbb:fe01::/64 MRC-R1-P1\|FCBB:BBBB:FE01::/64 SRv6 uA TOI (4.35.0F+)
P2 0xfe02 fcbb:bbbb:fe02::/64 MRC-R1-P2\|FCBB:BBBB:FE02::/64 …
… 0xfe0{n} fcbb:bbbb:fe0{n}::/64 … …

When uA is configured, validate with show srv6 adjacency-segments (keyed by domain/locator and usid; reports l3Intf, IPv6 next-hop, flavors, and active/inactive reasons). See Command API 2.1942 and the SRv6 uA TOI.

SONiC ConfigDB: set SRV6_MY_LOCATORS.<name>.prefix to the /48 without CIDR (e.g. "FCBB:BBBB:0101::"). Arista: set interface Loopback0 to the same /48 shown in show srv6 locator.

Example uSID carrier for Leaf P1 → Spine P1 → Leaf P1 (return hop on same leaf, different port):

fcbb:bbbb:0101:0201:0101:0000:0000
│           │    │    │    └── EoC padding (must not be the next CSID after active uSID in transit — see MRC constraints)
│           │    │    └── dst leaf uSID (0x0101) — final hop before NIC decap
│           │    └── spine uSID (0x0201) — active after 1st shift at leaf
│           └── leaf uSID (0x0101) — active at leaf ingress
└── 32-bit SRv6 block (fcbb:bbbb)

At each hop the switch compares the first 48 bits to its configured locator + uSID, left-shifts 16 bits, then does a static FIB lookup. Tables are installed at build time and not changed during training.

Side-by-side: implementation comparison

Topic SONiC Arista EOS 4.36
uSID format F3216 (block-len 32, node-len 16) F3216 — micro-segment domain + block /32 + end usid (TOI)
uN behavior End + NEXT-CSID, PSP/USD End with NEXT-CSID (System SFIB); PSP/USD per show srv6 capabilities
uA behavior End.X + NEXT-CSID; needs interface + optional nexthop Static micro-SID adjacency (SRv6 uA TOI 4.35.0F+)
MRC fabric control plane None — static only None — static only
Config path ConfigDB → bgpcfgd → FRR → srv6orch → SAI router srv6 → SRv6 agent → ASIC System SFIB
Persistence /etc/sonic/config_db.json + config save write memory / startup-config
Operational CLI Mostly vtysh (FRR) show srv6 …, ping srv6 …
Verify locators vtysh -c "show segment-routing srv6 locator" show srv6 locator
Verify uN in SFIB vtysh -c "show segment-routing srv6 sid" show srv6 route
Verify uA (optional) vtysh -c "show segment-routing srv6 sid" show srv6 adjacency-segments

uN forwarding (identical semantics)

Both platforms implement the same MRC dataplane steps described in the OpenAI MRC paper:

  1. Match DA[:48] against local locator + uSID.
  2. Left-shift the uSID portion by 16 bits (NEXT-CSID).
  3. Longest-prefix / static lookup on the shifted address.
  4. Forward out the configured egress port — no ECMP, no control-plane convergence.

Template A — Leaf P1 (plane 1)

Role: Accept node uplinks; uN-shift toward spine; static route to spine uSID prefix.

Arista EOS 4.36 (7060XE7 — Tomahawk 6)

! ── Global ─────────────────────────────────────────────────────────
hostname leaf-p1-r1
ipv6 unicast-routing

! Management / underlay (link-local or GUA — illustrative)
interface Ethernet1
   description node-r1-nic1
   ipv6 enable
   ipv6 address 2001:db8:1:1::1/64

interface Ethernet49
   description uplink-to-spine-p1
   ipv6 enable
   ipv6 address 2001:db8:1:f001::1/64

! ── SRv6 uSID locator + local uN (TOI 4.34.2F syntax) ───────────────
! Shared F3216 block once per fabric; unique end usid per switch.
! Confirm Prefix from "show srv6 locator" — must match Loopback0 below.
! end usid 0x0101 → fcbb:bbbb:0101::/48 — see prefix mapping table
router srv6
   vrf default
      micro-segment domain MRC-R1
         block fcbb:bbbb::/32
      !
      locator MRC-R1-P1
         prefix micro-segment domain MRC-R1 end usid 0x0101
      !
   !
!

interface Loopback0
   ipv6 address fcbb:bbbb:0101::/48

! Optional static uA: see SRv6 uA Support TOI (4.35.0F+) — not FRR static-sids syntax

! ── Static transit FIB (post-shift lookups) ─────────────────────────
! After uN shift at this leaf, active uSID is spine 0x0201 → fcbb:bbbb:0201::/48
! TOI allows GUA or link-local next hop (see SRv6-uN-Support.pdf)
ipv6 route fcbb:bbbb:0201::/48 Ethernet49 2001:db8:1:f001::2

! Return path: after shift, DA carries next uSID slots → hairpin to node ports
! (Program one static route per downlink uSID / node port from your path matrix)
ipv6 route fcbb:bbbb:0101:0002::/80 Ethernet2 fe80::node-r2-nic1

! ── Explicitly disable dynamic routing in MRC fabric ────────────────
! no router bgp …
! no router isis …

Verify (Arista):

show srv6 locator
show srv6 domain
show srv6 route
show srv6 capabilities
show srv6 adjacency-segments    ! optional — when static uA is configured
show ipv6 route fcbb:bbbb:0201::/48
! Path reachability (requires segment-list; target needs Loopback0 /48 on peer)
! CLI only — not exposed in the 4.36 Command API guide
ping srv6 sid fcbb:bbbb:0201:: via segment-list fcbb:bbbb:0101::

SONiC (ConfigDB + compiled FRR)

Step 1 — /etc/sonic/config_db.json (persistent):

{
  "SRV6_MY_LOCATORS": {
    "MRC-R1-P1": {
      "prefix": "FCBB:BBBB:0101::",
      "block_len": 32,
      "node_len": 16,
      "func_len": 16,
      "arg_len": 0
    }
  },
  "SRV6_MY_SIDS": {
    "MRC-R1-P1|FCBB:BBBB:0101::/48": {
      "action": "uN",
      "decap_dscp_mode": "pipe"
    },
    "MRC-R1-P1|FCBB:BBBB:FE01::/64": {
      "action": "uA",
      "decap_dscp_mode": "pipe",
      "interface": "Ethernet49",
      "adj": "2001:DB8:1:F001::2"
    }
  },
  "INTERFACE": {
    "Ethernet1|2001:DB8:1:1::1/64": {},
    "Ethernet49|2001:DB8:1:F001::1/64": {}
  }
}

Apply: sudo config reload -y (or let bgpcfgd push to FRR on table update).

Step 2 — Equivalent FRR (vtysh) after bgpcfgd compilation:

configure terminal
 segment-routing
  srv6
   locators
    locator MRC-R1-P1
     prefix fcbb:bbbb:0101::/48 block-len 32 node-len 16 func-bits 16
     behavior usid
    exit
   !
   static-sids
    sid fcbb:bbbb:0101::/48 locator MRC-R1-P1 behavior uN
    sid fcbb:bbbb:fe01::/64 locator MRC-R1-P1 behavior uA interface Ethernet49 nexthop 2001:db8:1:f001::2
   exit
  !
 !
 exit
!
ipv6 route fcbb:bbbb:0201::/48 Ethernet49 2001:db8:1:f001::2
ipv6 route fcbb:bbbb:0101:0002::/80 Ethernet2 fe80::node-r2-nic1
end
write memory

Verify (SONiC):

vtysh -c "show segment-routing srv6 locator"
vtysh -c "show segment-routing srv6 sid"
vtysh -c "show ipv6 route fcbb:bbbb:0201::/48"
sudo ip -6 route show table all | grep fcbb:bbbb

FRR persistence on SONiC

Changes made only in vtysh are lost on reboot unless saved via ConfigDB or integrated-vtysh-config in /etc/sonic/frr/vtysh.conf. For MRC fabrics, prefer ConfigDB as the source of truth.

Template B — Spine P1 (plane 1)

Role: Receive from leaf uplink; uN-shift; static forward back to leaf downlink uSIDs.

Arista EOS 4.36

hostname spine-p1-r1
ipv6 unicast-routing

interface Ethernet1
   description downlink-from-leaf-p1
   ipv6 enable
   ipv6 address 2001:db8:1:f001::2/64

router srv6
   vrf default
      micro-segment domain MRC-R1
         block fcbb:bbbb::/32
      !
      locator MRC-R1-S1
         prefix micro-segment domain MRC-R1 end usid 0x0201
      !
   !
!

interface Loopback0
   ipv6 address fcbb:bbbb:0201::/48

! After uN shift, forward to leaf downlink / node uSID prefixes
ipv6 route fcbb:bbbb:0101::/48 Ethernet1 2001:db8:1:f001::1
ipv6 route fcbb:bbbb:0101:0001::/80 Ethernet1 2001:db8:1:f001::1
! … repeat for each node downlink uSID in the path matrix

SONiC

ConfigDB excerpt:

{
  "SRV6_MY_LOCATORS": {
    "MRC-R1-S1": {
      "prefix": "FCBB:BBBB:0201::",
      "block_len": 32,
      "node_len": 16,
      "func_len": 16
    }
  },
  "SRV6_MY_SIDS": {
    "MRC-R1-S1|FCBB:BBBB:0201::/48": {
      "action": "uN",
      "decap_dscp_mode": "pipe"
    }
  }
}

FRR static routes (same as Arista logic):

ipv6 route fcbb:bbbb:0101::/48 Ethernet1 2001:db8:1:f001::1
ipv6 route fcbb:bbbb:0101:0001::/80 Ethernet1 2001:db8:1:f001::1

Template C — Full plane matrix (8 planes × 2 tiers)

Use a path matrix spreadsheet (one row per reachable shifted DA prefix) generated at fabric design time. Each row maps:

Shifted DA prefix Egress port Next-hop IPv6 Plane
fcbb:bbbb:0201::/48 Ethernet49 2001:db8:1:f001::2 P1 leaf → spine
fcbb:bbbb:0101::/48 Ethernet1 2001:db8:1:f001::1 P1 spine → leaf
fcbb:bbbb:0202::/48 … … P2 leaf → spine
… … … …

Scaling rule: For N nodes per plane and P planes, each leaf/spine pair needs static routes for every uSID prefix that can appear after a uN shift on that device — not merely its own locator.

Prefixes match the side-by-side prefix mapping table above.

Planes are fully independent — P1 switches never parse P2 uSIDs in the data path.

uN vs uA — when to use which

Behavior SONiC action Arista Use on leaf/spine
uN uN locator + end usid → System SFIB Every switch gets a /48 uN for shift-and-lookup transit
uA uA + interface + optional adj static uA (SRv6 uA TOI — not FRR static-sids) Pin traffic to a specific uplink/downlink when multiple parallel links exist or paths must survive reload without IGP

In a minimal 1 leaf : 1 spine plane, uN + static ipv6 route entries are usually sufficient. Add uA when the path matrix names explicit adjacencies (OpenAI MRC uses algorithmic EV→path mapping that may reference uplink indices).

SONiC vs Arista — CLI syntax cheat sheet

Locator (uSID F3216)

SONiC / FRR Arista EOS (TOI)
Enter SRv6 mode segment-routing → srv6 → locators router srv6 → vrf default
uSID block (F3216) prefix fcbb:bbbb:0101::/48 block-len 32 node-len 16 + behavior usid micro-segment domain NAME → block fcbb:bbbb::/32
Local uN /48 sid fcbb:bbbb:0101::/48 … behavior uN end usid 0x0101 → fcbb:bbbb:0101::/48
Loopback (for ping) (varies) interface Loopback0 → ipv6 address <locator>/48

Static local SID

SONiC / FRR Arista EOS
uN static-sids → sid …/48 locator … behavior uN Automatic from locator (show srv6 route)
uA sid fcbb:bbbb:fe01::/64 … behavior uA interface Et49 nexthop 2001:…::2 Per SRv6 uA TOI (not FRR static-sids)

Supported behaviors in static ConfigDB (SONiC)

Alias SRv6 behavior
uN End with NEXT-CSID
uA End.X with NEXT-CSID
uDT46 End.DT46 with CSID

Source: SONiC Static SRv6 HLD.

MRC-specific operational constraints

Both platforms in an MRC training fabric should observe the same discipline documented in OCP MRC Rev 1.0:

Requirement Leaf/spine action
No ECMP Single next-hop per shifted DA prefix; disable ECMP / multi-path
No dynamic routing Do not enable BGP, IS-IS, or OSPFv3 on MRC backend planes
No PFC Lossy Ethernet; MRC NIC handles retry
Static FIB Install full path matrix at provisioning; avoid runtime route changes
Line-rate uN 7060XE7 (Tomahawk 6) in this primer’s rack design; 7060X6 (Tomahawk 5) is a separate Arista line with the same F3216 EOS CLI — confirm SRv6 on your SKU/build. Spectrum-4/5 + SONiC also support line-rate uN in production MRC deployments
EoC / zero CSID on transit On DCS-7060X6, EOS drops packets when the CSID immediately after the active uN or uA is 0000 (EoC marker) — see RN 1294522. NIC path encoders must place EoC padding only in trailing slots; every post-shift active CSID during transit must be a non-zero uSID until the consuming node

The 32-bit EV in the outer header is not used for forwarding — it exists for SACK/NACK feedback on the NIC. See MRC Packet Structure.

SRv6 agent restarts

The SRv6 uN TOI notes that SRv6 agent, SandTunnel, or StrataL3Unicast agent restarts are hitful — expect brief SRv6 forwarding impact during process restart.

flowchart LR
  A[Path matrix spreadsheet] --> B[Assign uN /48 per switch]
  B --> C[Derive post-shift DA prefixes]
  C --> D[Static ipv6 routes per prefix]
  D --> E1[Arista: startup-config]
  D --> E2[SONiC: config_db.json + FRR]
  E1 --> F[Lab validation: show srv6 + ping srv6]
  E2 --> F
  F --> G[Freeze config for production job]
  1. Design — Build the uSID path matrix from rack topology (9 nodes, 8 planes, 2 hops).
  2. Assign — Allocate /48 uN locators per switch from the prefix mapping table (end usid → fcbb:bbbb:WWWW::/48).
  3. Program — Install locators + static routes on every leaf and spine.
  4. Validate — show srv6 locator / route / capabilities (and adjacency-segments if uA); inject test packets with known uSID carriers; confirm shift-and-forward on each hop; ping srv6 for path reachability.
  5. Freeze — Treat running config as immutable for the training job (MRC handles path health at the NIC).

External references