LDP-zu-SR-MPLS-Migration in OcNOS: Ein stufenweiser Ansatz

Part of IP Infusion's guide to Segment Routing.

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
• IP Infusion kontaktieren

IP Infusion Engineering Team