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BGP Overlay Services over SRv6

SRv6 provides a clean, unified underlay for delivering L3VPN and L2VPN (EVPN) services using BGP — without MPLS, without LDP, without RSVP-TE. Just IPv6 + BGP.

Architecture Overview

graph TB
    subgraph Overlay - BGP Control Plane
        BGP1[PE1 BGP] <-->|MP-BGP with SRv6 SIDs| BGP2[PE2 BGP]
    end

    subgraph Underlay - SRv6 Data Plane
        PE1[PE1] -->|SRv6 Transport| P1[P Node]
        P1 -->|SRv6 Transport| PE2[PE2]
    end

    CE1[CE1 - Customer] --- PE1
    CE2[CE2 - Customer] --- PE2

Key principle: BGP carries service information (VPN prefixes, EVPN routes) with SRv6 SIDs instead of MPLS labels. The SRv6 SID tells the remote PE exactly what to do with the packet (decap to which VRF, cross-connect to which interface, etc.).

L3VPN over SRv6

How It Works

  1. PE1 learns customer prefix 10.1.0.0/24 from CE1 in VRF CUSTOMER-A
  2. PE1 advertises via MP-BGP VPNv4/VPNv6 with an SRv6 SID instead of a VPN label:
    • Prefix: 10.1.0.0/24
    • Next-Hop: PE1 IPv6 address
    • SRv6 Service SID: fcbb:bb01:0001::DT4 (End.DT4 behavior)
  3. PE2 receives the BGP update, installs the route
  4. When PE2 receives traffic for 10.1.0.0/24, it encapsulates with:
    • IPv6 DA = fcbb:bb01:0001::DT4
    • Inner packet = original IPv4
  5. PE1 receives, matches End.DT4 SID, decapsulates, looks up in VRF CUSTOMER-A

SRv6 SID Allocation Modes

Mode Description SID per...
Per-VRF One SID shared by all prefixes in a VRF VRF
Per-CE One SID per CE neighbor CE
Per-Prefix Unique SID for each prefix Prefix

Per-VRF is most common

Per-VRF allocation (End.DT4 / End.DT6) is the default and most scalable option. The SID says "decap and lookup in this VRF table."

Configuration

!! VRF definition
vrf CUSTOMER-A
 address-family ipv4 unicast
  import route-target 100:1
  export route-target 100:1
 !
!

!! BGP with SRv6
router bgp 65000
 address-family vpnv4 unicast
  segment-routing srv6
   locator MAIN
  !
 !
 vrf CUSTOMER-A
  rd 100:1
  address-family ipv4 unicast
   segment-routing srv6
    locator MAIN
    alloc mode per-vrf
   !
  !
 !
!

set routing-instances CUSTOMER-A instance-type vrf
set routing-instances CUSTOMER-A route-distinguisher 100:1
set routing-instances CUSTOMER-A vrf-target target:100:1
set routing-instances CUSTOMER-A protocols bgp source-packet-routing srv6 locator myloc

EVPN over SRv6

EVPN (Ethernet VPN) provides L2 services (E-Line, E-LAN, E-Tree) and integrated L2/L3 services over SRv6 transport.

EVPN Route Types with SRv6

Route Type Name SRv6 Behavior
Type 2 MAC/IP Advertisement End.DX2 or End.DT2
Type 3 Inclusive Multicast BUM traffic handling
Type 5 IP Prefix Route End.DT4 / End.DT6 (IP-VRF)

EVPN-VPWS (E-Line) with SRv6

Point-to-point L2 service using End.DX2 behavior:

CE1 ──[L2 Frame]──► PE1 ──[IPv6(DA=PE2::DX2)][SRH][L2 Frame]──► PE2 ──[L2 Frame]──► CE2

The End.DX2 SID tells PE2: "decapsulate and cross-connect this L2 frame to the specified attachment circuit."

EVPN-VXLAN vs EVPN-SRv6

Aspect EVPN-VXLAN EVPN-SRv6
Encapsulation VXLAN (UDP) SRv6 (IPv6 + SRH)
Underlay IPv4 or IPv6 IPv6 native
TE capability Limited Full SR Policy support
Network programming None Rich (service chaining, slicing)
MTU overhead 50 bytes 40 bytes (no SRH) to 56+ bytes (with SRH)

BGP SRv6 Service TLV (RFC 9252)

RFC 9252 defines how SRv6 SID information is carried in BGP updates using the SRv6 Services TLVs within the BGP Prefix-SID attribute:

BGP Update Message:
  ├── NLRI: 10.1.0.0/24 (VPNv4)
  ├── Next-Hop: 2001:db8::1
  ├── Extended Community: RT 100:1
  └── BGP Prefix-SID Attribute:
      └── SRv6 L3 Service TLV:
          ├── SRv6 SID: fcbb:bb01:0001::DT4
          ├── SRv6 SID Behavior: End.DT4 (19)
          └── SRv6 SID Structure:
              ├── Locator Block Length: 32
              ├── Locator Node Length: 16
              ├── Function Length: 16
              └── Argument Length: 0

SID Transposition

To save space in BGP updates, the function part of the SID can be transposed into (encoded within) the VPN prefix itself, rather than carrying the full 128-bit SID. This is an optimization defined in RFC 9252.

Verification

!! Show VPNv4 routes with SRv6 SIDs
show bgp vpnv4 unicast vrf CUSTOMER-A

!! Show SRv6 SID details
show segment-routing srv6 sid

!! Show EVPN routes
show bgp l2vpn evpn

vtysh -c "show bgp ipv4 vpn"
vtysh -c "show bgp l2vpn evpn"
vtysh -c "show segment-routing srv6 locator"

Further Reading

References

  1. RFC 9252 - BGP Overlay Services Based on SRv6 - Defines SRv6 Service TLVs in BGP for L3VPN, EVPN, and Internet services over SRv6
  2. RFC 8986 - SRv6 Network Programming - Defines End.DT4, End.DT6, End.DX2, and other service endpoint behaviors
  3. RFC 7432 - BGP MPLS-Based Ethernet VPN - Foundational EVPN specification defining route types and MAC learning in the control plane
  4. Cisco XRdocs: Implementing EVPN ELAN over SRv6 Transport - Step-by-step tutorial for configuring EVPN multipoint L2 service over SRv6 uSID
  5. Cisco XRdocs: SRv6 Transport on NCS5500 - Part 1 - Tutorial covering SRv6 transport fundamentals and L3VPN configuration on NCS 5500