Arista EOS SRv6 uN + uA Config — Plane 1

Complete Arista EOS configuration template for MRC Plane 1, covering both uN (node) and uA (adjacency) micro-segments on a T0 leaf and T1 spine. Syntax is validated against the Arista SRv6 uN Support TOI (EOS 4.34.2F) and SRv6 uA Support TOI (EOS 4.35.2 GA).

For the conceptual walkthrough of how packets traverse this plane, see SRv6 Plane 1 — Packet Traversal. For a SONiC vs Arista comparison and rack-wide addressing templates, see SRv6 uN uSID — Leaf/Spine Config (SONiC vs Arista).

Scope

This is a lab-ready Plane 1 slice — one leaf (leaf1-p1) and one spine (spine1-p1) with illustrative GPU downlinks. Extend Ethernet port patterns and static routes for additional leaves in the plane. Replace management addresses and link-local next hops for production.

Addressing plan

Resource	Prefix / uSID	Range	Notes
uSID block	`fc00:1::/32`	—	Domain `PLANE1-DOMAIN`, F3216 format
leaf1-p1 uN	`fc00:1:1::/48`	GIB `0x0001`	Node SID — shift-and-lookup transit
leaf2-p1 uN	`fc00:1:2::/48`	GIB `0x0002`	Referenced in static routes / verification
spine1-p1 uN	`fc00:1:6501::/48`	GIB `0x6501`	Spine tier node SID
leaf1 → spine uA	`fc00:1:e001::/48`	LIB `0xe001`	On leaf1 `Ethernet64` uplink
spine → leaf1 uA	`fc00:1:e101::/48`	LIB `0xe101`	On spine `Ethernet1` downlink
spine → leaf2 uA	`fc00:1:e102::/48`	LIB `0xe102`	On spine `Ethernet2` downlink
leaf2 → server uA	`fc00:1:f001::/48`	LIB `0xf001`	Server-facing adjacency (leaf2)

uSID allocation discipline:

GIB (0x0001–0xDFFF) — node segments (uN)
LIB (0xE000–0xFFFF) — adjacency segments (uA)

What the config covers

Section	Device	Contents
Part A	`leaf1-p1`	Loopback uN, GPU downlinks, uplink uA on `Ethernet64`, `router srv6` domain/locator, static route to spine uN, ECN on transit ports, MGMT VRF
Part B	`spine1-p1`	Loopback uN, per-leaf downlinks with uA, `router srv6` locator, static routes to leaf uNs, ECN on all downlinks, MGMT VRF
Verification	Both	`show srv6` commands, `ping srv6` / `traceroute srv6` examples, uA neighbor-discovery checks
Architecture notes	—	When to use uN vs uA, mixing segments in one uSID container, ECMP caveat

uN vs uA — quick reference

Behavior	CLI	Forwarding
uN	`locator … end usid <value>`	Match local locator → shift 16b → static FIB lookup
uA	`adjacency-segment srv6 … behavior end.x p2p link-local`	Match adjacency uSID → forward out that interface, bypass FIB

Both coexist in the same SFIB. Standard MRC spraying uses uN throughout; uA is for explicit link steering (traffic engineering, failure avoidance).

Full configuration

Source file: Arista_EOS_SRv6_uN_uA_Config.txt

! ============================================================
! Arista EOS — SRv6 uN + uA Configuration
! Plane 1 MRC fabric — T0 Leaf and T1 Spine
!
! Platform:  DCS-7060X6 / DCS-7060XE7
! EOS:       4.35.2+ (uN from 4.34.2F, uA GA from 4.35.2)
! uSID fmt:  F3216 — 32-bit block + 16-bit uSID
! Domain:    PLANE1-DOMAIN  block fc00:1::/32
!
! uN GIB range:  0x0001–0xDFFF (1–57343)   — node SIDs
! uA LIB range:  0xE000–0xFFFF (57344–65535) — adjacency SIDs
!
! Address plan — Plane 1:
!   Block prefix:      fc00:1::/32
!   leaf1-p1 locator:  fc00:1:0001::/48  uSID 0x0001 = 1
!   leaf2-p1 locator:  fc00:1:0002::/48  uSID 0x0002 = 2
!   spine1-p1 locator: fc00:1:6501::/48  uSID 0x6501 = 25857
!
!   leaf1 uA uplink to spine:  fc00:1:e001::/48  uSID 0xe001 = 57345
!   spine uA downlink leaf1:   fc00:1:e101::/48  uSID 0xe101 = 57601
!   spine uA downlink leaf2:   fc00:1:e102::/48  uSID 0xe102 = 57602
!   leaf2 uA downlink to srv:  fc00:1:f001::/48  uSID 0xf001 = 61441
!
! Verified CLI syntax — source documents:
!   Arista EOS SRv6 uN Support TOI (EOS 4.34.2F)
!   Arista EOS SRv6 uA Support TOI (EOS 4.35.2 GA)
! ============================================================


! ############################################################
!  PART A — T0 LEAF SWITCH:  leaf1-p1
!  uN:  fc00:1:0001::/48  (node SID, GIB range)
!  uA:  fc00:1:e001::/48  (adj SID on uplink Ethernet64, LIB range)
! ############################################################

hostname leaf1-p1

! ── Prerequisite: IPv6 unicast routing in default VRF ────────
! Required for SRv6 to be operational (see uN TOI troubleshooting)
ipv6 unicast-routing

! ── Loopback — carries this node's uN prefix ─────────────────
! Must be configured as /48 (block 32b + uSID 16b = 48b)
! Linux kernel responds to SRv6 ping on this address
interface Loopback0
   description SRv6 uN source — leaf1-p1
   ipv6 address fc00:1:1::/48
!

! ── Server-facing downlinks (plane 1 NIC connections) ────────
! ECN marking NOT applied here — last-hop rule (MRC paper)
! No adjacency segments on server-facing ports (not needed for uA)
interface Ethernet1
   description to gpu-server-01 NIC-plane1 — 800G
   no switchport
   ipv6 address 2001:db8:1:1::1/127
   mtu 9214
!
interface Ethernet2
   description to gpu-server-02 NIC-plane1 — 800G
   no switchport
   ipv6 address 2001:db8:1:2::1/127
   mtu 9214
!
! ... Ethernet3–9 follow same pattern for servers 3–9
!

! ── Uplink to spine — uA adjacency segment configured here ───
! uA is configured ON the egress interface (uA TOI page 1)
! uSID 0xe001 is in LIB range (57344–65535) — distinct from uN GIB range
! p2p link-local: nexthop auto-discovered via IPv6 Router Advertisement
! Note: allow up to 150-200s for link-local discovery on first boot
interface Ethernet64
   description to spine1-p1 uplink — 800G — plane 1
   no switchport
   ipv6 address 2001:db8:1:fe::1/127
   mtu 9214
   !
   ! uA adjacency segment — explicit link steering to spine
   ! SID prefix: fc00:1:e001::/48  (block fc00:1:: + uSID 0xe001)
   ! Behavior:   End.X with NEXT-CSID  (confirmed in uA TOI show output)
   ! Use case:   Source encodes 0xe001 to force traffic out this exact link,
   !             bypassing FIB lookup — useful for traffic engineering
   adjacency-segment srv6 domain PLANE1-DOMAIN usid 57345 behavior end.x p2p link-local
!

! ── SRv6 uN configuration ─────────────────────────────────────
! Configures the node SID (locator) for this leaf switch
! Both uN and uA share the same micro-segment domain and block prefix
router srv6
   vrf default
      !
      ! Source address for SRv6 ping and traceroute
      local address 2001:db8:1:1::1
      !
      ! Micro-segment domain — shared by all plane 1 nodes
      ! Block prefix defines the uSID domain (F3216: /32 block + 16b uSID)
      ! All locators and adjacency segments in plane 1 use this domain
      ! CRITICAL: block prefix must NOT overlap with other domains
      micro-segment domain PLANE1-DOMAIN
         block fc00:1::/32
         ! GIB range default: 1–57343 (0x0001–0xDFFF) — uN values
         ! LIB range default: 57344–65535 (0xE000–0xFFFF) — uA values
         ! No explicit gib range command needed — defaults are correct
      !
      ! Locator — this node's uN (node SID)
      ! uSID value 1 (0x0001) — in GIB range
      ! Resulting locator prefix: fc00:1:1::/48
      ! SFIB entry: fc00:1:1::/48, End with NEXT-CSID
      locator LEAF1-P1-LOC
         prefix micro-segment domain PLANE1-DOMAIN end usid 1
      !
   !
!

! ── Static routes to neighboring node uNs ────────────────────
! Required for uN forwarding — one static route per neighbor
! No BGP, no OSPF, no IS-IS — static only (MRC architecture)
! Nexthop = interface address on the far end of the point-to-point link
!
! Route to spine1-p1 uN — via uplink Ethernet64
ipv6 route fc00:1:6501::/48 2001:db8:1:fe::0
!
! Alternative using link-local nexthop (also valid per uN TOI):
! ipv6 route fc00:1:6501::/48 Ethernet64 fe80::<spine-link-local>
!

! ── ECN marking — transit ports only ─────────────────────────
! MRC: ECN enabled on all transit hops, DISABLED on last hop (server-facing)
! leaf1 uplink (Ethernet64) is a transit hop — ECN enabled
! leaf1 downlinks (Ethernet1-9) are last-hop — ECN disabled
policy-map type qos ECN-MARK-TRANSIT
   class class-default
      set ecn
!
interface Ethernet64
   service-policy type qos input ECN-MARK-TRANSIT
!
! Note: Ethernet1-9 (server-facing) do NOT get ECN policy

! ── Management plane — MGMT VRF ───────────────────────────────
! Completely separate from SRv6 data plane address space
! Used by Clustermapper agents (gNMI telemetry, 1ms polling)
vrf instance MGMT
!
interface Management1
   description OOB management — Clustermapper
   vrf MGMT
   ipv6 address 2001:db8:fe:1::1/64
!
ipv6 route vrf MGMT ::/0 2001:db8:fe:1::fe
!


! ############################################################
!  PART B — T1 SPINE SWITCH:  spine1-p1
!  uN:  fc00:1:6501::/48  (node SID, GIB range 0x6501=25857)
!  uA:  fc00:1:e101::/48  (adj SID on downlink to leaf1, 0xe101=57601)
!  uA:  fc00:1:e102::/48  (adj SID on downlink to leaf2, 0xe102=57602)
! ############################################################

hostname spine1-p1

ipv6 unicast-routing

! ── Loopback — spine uN prefix ───────────────────────────────
interface Loopback0
   description SRv6 uN source — spine1-p1
   ipv6 address fc00:1:6501::/48
!

! ── Downlinks to T0 leaves ────────────────────────────────────
! Each leaf connects via one 800G port
! uA adjacency segments configured here for explicit link steering

interface Ethernet1
   description to leaf1-p1 downlink — 800G — plane 1
   no switchport
   ipv6 address 2001:db8:1:fe::0/127
   mtu 9214
   !
   ! uA to leaf1 — forces traffic out this exact downlink to leaf1
   ! SID prefix: fc00:1:e101::/48  (block fc00:1:: + uSID 0xe101)
   ! Behavior:   End.X with NEXT-CSID
   adjacency-segment srv6 domain PLANE1-DOMAIN usid 57601 behavior end.x p2p link-local
!
interface Ethernet2
   description to leaf2-p1 downlink — 800G — plane 1
   no switchport
   ipv6 address 2001:db8:1:fd::0/127
   mtu 9214
   !
   ! uA to leaf2 — forces traffic out this exact downlink to leaf2
   ! SID prefix: fc00:1:e102::/48  (block fc00:1:: + uSID 0xe102)
   adjacency-segment srv6 domain PLANE1-DOMAIN usid 57602 behavior end.x p2p link-local
!
! ... Ethernet3–64: one per T0 leaf in plane 1, same pattern
! uA uSID values: 0xe103 (57603), 0xe104 (57604) ... 0xe140 (57664)
!

! ── SRv6 uN configuration ─────────────────────────────────────
router srv6
   vrf default
      !
      local address 2001:db8:1:6501::1
      !
      ! Same domain and block as all plane 1 nodes
      micro-segment domain PLANE1-DOMAIN
         block fc00:1::/32
      !
      ! Spine locator — uSID 0x6501 = 25857 (in GIB range)
      ! Locator prefix: fc00:1:6501::/48
      ! SFIB: fc00:1:6501::/48, End with NEXT-CSID
      locator SPINE1-P1-LOC
         prefix micro-segment domain PLANE1-DOMAIN end usid 25857
      !
   !
!

! ── Static routes to neighboring leaf uNs ────────────────────
! One static route per T0 leaf — nexthop is far-end interface address
ipv6 route fc00:1:1::/48 2001:db8:1:fe::1
ipv6 route fc00:1:2::/48 2001:db8:1:fd::1
! ... ipv6 route fc00:1:<n>::/48 <nexthop>  for each leaf in plane 1
!
! Note: spine does NOT need routes to its own uA prefixes —
! those are programmed directly into SFIB by the adjacency-segment config

! ── ECN marking — spine is always a transit hop ───────────────
! All spine ports are transit — ECN marking on all downlinks
policy-map type qos ECN-MARK-TRANSIT
   class class-default
      set ecn
!
interface Ethernet1-64
   service-policy type qos input ECN-MARK-TRANSIT
!

! ── Management plane ──────────────────────────────────────────
vrf instance MGMT
!
interface Management1
   description OOB management — Clustermapper
   vrf MGMT
   ipv6 address 2001:db8:fe:2::1/64
!
ipv6 route vrf MGMT ::/0 2001:db8:fe:2::fe
!


! ############################################################
!  VERIFICATION COMMANDS — run on both leaf1 and spine
! ############################################################

! ── Domain and locator status ─────────────────────────────────
!
! show srv6 domain
!   Confirms PLANE1-DOMAIN is active, block fc00:1::/32
!   GIB Range: 1-57343  uSID Length: 16
!
! show srv6 locator
!   Confirms locator is active, type micro-SID, uSID value correct
!   State must be "active" — if inactive, check:
!     1. ipv6 unicast-routing is enabled in default VRF
!     2. micro-segment domain block is configured
!     3. uSID value is within GIB range (1–57343)
!     4. No conflicting locator prefix from another domain
!
! ── SFIB table — both uN and uA entries ──────────────────────
!
! show srv6 route
!   Expected entries on leaf1-p1:
!     S fc00:1:1::/48,    End with NEXT-CSID,   locator LEAF1-P1-LOC
!   (uN node SID — matches incoming packet whose active SID = this leaf)
!
! show srv6 route end behavior end-x
!   Expected entries on leaf1-p1:
!     S fc00:1:e001::/48,  End.X with NEXT-CSID, domain PLANE1-DOMAIN
!       via fe80::<spine-link-local>, Ethernet64
!   (uA adj SID — matches packet that explicitly steers via this link)
!
! show srv6 adjacency-segments
!   Lists all uA entries grouped by domain:
!   Domain PLANE1-DOMAIN
!     uSID  Hex     Prefix              Interface  Via                    State
!     57345 0xe001  fc00:1:e001::/48    Ethernet64 fe80::xxxx:xxxx:xxxx   active
!   State must be "active" — if inactive, check:
!     1. Domain state is active (show srv6 domain)
!     2. uSID is in LIB range (57344–65535)
!     3. Link-local of remote interface has been discovered
!        (may take 150-200s after interface comes up)
!     4. Interface is a routed port in default VRF with IPv6 enabled
!
! show srv6 adjacency-segments detail
!   Shows: Behaviour: End.X with NEXT-CSID  (confirmed uA behavior)
!   Shows: Source: static
!   Shows: Via: fe80::<addr>, Ethernet64
!
! ── Reachability tests ────────────────────────────────────────
!
! Test uN path: leaf1 → spine (via uN node SIDs only)
!   ping srv6 sid fc00:1:6501:: via segment-list fc00:1:1::
!   (pings spine uN via leaf1 uN — verifies uN forwarding)
!
! Test uN full path: leaf1 → spine → leaf2
!   ping srv6 sid fc00:1:2:: via segment-list fc00:1:1:: fc00:1:6501::
!   (verifies full 2-hop path with two uN hops)
!
! Test uA path: explicit link steering leaf1 → spine via Ethernet64
!   ping srv6 sid fc00:1:6501:: via segment-list fc00:1:e001::
!   Note: source node must have IPv6 route to first SID in segment list
!   Note: cannot use locally configured uA as FIRST SID (uA TOI page 7)
!   Correct: use uN of source node first, then uA of next hop
!   ping srv6 sid fc00:1:6501:: via segment-list fc00:1:1:: fc00:1:e001::
!
! Traceroute to see DA transformation at each hop:
!   traceroute srv6 sid fc00:1:6501:: via segmentlist fc00:1:1::
!   Output shows DA changing at each hop (uN shift-and-lookup)
!
! ── Neighbor discovery check (required for uA p2p link-local) ─
!
! show ipv6 nd ra neighbors
!   Verifies spine link-local has been learned on Ethernet64
!   uA state is "inactive" until this entry appears
!
! show ipv6 neighbors
!   Verifies IPv6 neighbor entries exist for uA via addresses
!   Required for traffic forwarding (uA TOI page 9)
!
! show srv6 capabilities
!   Confirms: End supported, End.X supported (4.35.2+), Next-CSID supported
!   uSID Formats: F3216
!   Note: End.X was unsupported in 4.34.x — requires 4.35.2+ for uA


! ############################################################
!  ARCHITECTURAL NOTES
! ############################################################
!
! uN vs uA — when to use each:
!
!   uN (End with NEXT-CSID) — node segment:
!     - Standard MRC forwarding — source encodes destination path
!       as a stack of node uSIDs (leaf → spine → leaf)
!     - Switch matches its own locator prefix, shifts 16b, does
!       LPM lookup in FIB to determine egress (standard uN behavior)
!     - No explicit link specified — FIB chooses the outgoing port
!     - Use for: all standard MRC packet spraying
!
!   uA (End.X with NEXT-CSID) — adjacency segment:
!     - Explicit link steering — source encodes a specific interface
!       as a uSID; switch forwards out THAT interface, bypasses FIB
!     - Configured on the egress interface itself
!     - uSID value must be in LIB range (0xE000–0xFFFF)
!     - Use for: traffic engineering, pinning flows to a specific
!       physical link, bypassing a specific link (failure avoidance)
!
!   Both run in the same SFIB on the same switch simultaneously.
!   A source NIC can mix uN and uA uSIDs in the same uSID container:
!     fc00:1 : 0001 : e101 : 0002 : ...
!              leaf1  spine-uA  leaf2
!   This forces leaf→spine via the specific physical link identified
!   by uA 0xe101 while leaving leaf ingress/egress as standard uN.
!
!   ECMP caveat (uA TOI page 9):
!   "While the same adjacency segment can be configured on multiple
!    interfaces forming ECMP, it is not supported in the hardware."
!   Only single-interface uA is hardware-enforced.
!
! ============================================================
! End of configuration
! ============================================================

Applying in the lab

Enable ipv6 unicast-routing before SRv6 stanzas.
Allow 150–200 seconds after link-up for uA link-local discovery (show ipv6 nd ra neighbors).
Confirm locators are active with show srv6 locator and adjacency segments with show srv6 adjacency-segments.
Test uN reachability: ping srv6 sid fc00:1:6501:: via segment-list fc00:1:1::

Arista EOS SRv6 uN + uA Config — Plane 1

Addressing plan

What the config covers

uN vs uA — quick reference

Full configuration

Related pages