LDP to SR-MPLS Migration in OcNOS: A Phased Approach

Migrating from LDP to Segment Routing is one of the most common infrastructure projects facing service provider network teams. The good news: OcNOS supports a phased migration model where LDP and SR-MPLS coexist on the same network simultaneously — allowing operators to migrate one node at a time without any service disruption or maintenance windows.

Why Migrate from LDP to SR?

Migration Strategy: Three Phases

Phase 1: Enable SR with LDP Coexistence

In Phase 1, Segment Routing is enabled on the IS-IS domain while LDP continues to operate. Both protocols distribute labels simultaneously. Traffic continues to use LDP labels — this phase is purely additive and carries zero risk.

! OcNOS Phase 1 -- Enable IS-IS SR alongside existing LDP
!
! Step 1: Enable SR globally under IS-IS (non-disruptive)
router isis CORE
  segment-routing mpls
  !
  address-family ipv4 unicast
    segment-routing mpls
  exit-address-family
!
! Step 2: Assign Node SID to loopback (new config, no traffic impact)
interface lo
  ip address 10.0.0.1/32
  ip router isis CORE
  isis segment-routing prefix-sid index 1
!
! Step 3: LDP remains fully operational -- no changes needed
! Both LDP and SR labels are now distributed
!
! Verify SR is active alongside LDP:
show isis segment-routing prefix-sids
show mpls ldp neighbor                  ! LDP still active
show mpls forwarding-table              ! Both LDP and SR labels present

Phase 2: Prefer SR Labels (Traffic Cutover)

Phase 2 switches traffic from LDP labels to SR labels using the sr-prefer knob under IS-IS. LDP sessions remain active as a fallback during validation. This is the actual traffic migration step — it can be applied per-node and rolled back immediately if needed.

! OcNOS Phase 2 -- Prefer SR labels, keep LDP as fallback
!
router isis CORE
  segment-routing mpls
  segment-routing sr-prefer            ! Prefer SR labels over LDP labels
  !
  address-family ipv4 unicast
    segment-routing mpls
    fast-reroute per-prefix ti-lfa     ! Optionally enable TI-LFA now
  exit-address-family
!
! Verify traffic is using SR labels (LDP labels still installed as backup):
show mpls forwarding-table
! Look for: primary label = SR label, backup = LDP label
!
! Verify MPLS path for a specific destination:
show ip route 10.0.0.5
show mpls forwarding-table 10.0.0.5
!
! Check LDP is still operational (fallback):
show mpls ldp neighbor
show mpls ldp bindings

Phase 3: Decommission LDP

Once all nodes are running SR-prefer and traffic has been validated on SR labels, LDP can be removed. This is the final cleanup step.

! OcNOS Phase 3 -- Remove LDP, SR-only operation
!
! Step 1: Remove LDP configuration
no mpls ldp
!
! Step 2: Remove LDP-IGP sync if configured
router isis CORE
  no mpls ldp sync
!
! Step 3: Optionally enable SR-TE policies now that SR is sole protocol
segment-routing
  traffic-eng
    ! Configure SR-TE policies here
  !
!
! Final verification:
show isis segment-routing prefix-sids   ! All nodes have SIDs
show mpls forwarding-table              ! Only SR labels remain
show isis fast-reroute summary          ! TI-LFA coverage (should be 100%)
!
! Confirm no LDP sessions remain:
show mpls ldp neighbor
! Expected: empty (no LDP sessions)

Rollback Procedure

! OcNOS -- Phase 2 rollback (revert to LDP preference)
!
router isis CORE
  segment-routing mpls
  no segment-routing sr-prefer         ! Remove sr-prefer, LDP takes over
  !
  address-family ipv4 unicast
    segment-routing mpls
    no fast-reroute per-prefix ti-lfa
  exit-address-family
!
! Traffic immediately reverts to LDP labels
! Verify:
show mpls forwarding-table             ! LDP labels active again

• Segment Routing Explained: SR-MPLS Fundamentals
• ISIS-SR with TI-LFA: Fast Reroute Guide
• Flex-Algo in OcNOS
• OcNOS-SP Product Page
• Contacter IP Infusion

IP Infusion Engineering Team