Segment Routing MPLS Interworking with LDP
Extracted elements (25)
SR and LDP Ships-in-the-Night (SIN) coexistence is possible because label uniqueness is guaranteed by the label manager's static allocation capability, which has existed across vendors for years and is not new—the same mechanism used by MPLS-TP. Each MCC's incoming labels are distinct, so multiple MPLS forwarding entries for the same prefix can coexist.
SR can eliminate the need for targeted LDP sessions used in RLFA-based FRR by providing SR-based repair tunnels expressed as node/adjacency segment stacks, while keeping steady-state traffic over LDP LSPs. The SR deployment can be confined to only the network region where these benefits are desired.
SR provides guaranteed FRR coverage in topologies where no RLFA solution exists (no intersection between extended P-space and Q-space) by using SR node and adjacency segments to construct repair tunnels that bypass the failed link without any signaling or targeted LDP sessions.
SRMS advertisements are propagated through standard inter-area advertisement procedures in link-state protocols (IS-IS, OSPF), and a preference mechanism supports primary/secondary SRMS schemes allowing controlled SID migration. Multiple SRMSes can be provisioned for redundancy, with the expectation that operator policy keeps their mappings consistent.
In Inter-AS Option C, SR may be independently deployed in each AS. If AS1 operates SR, the tunnel from an ingress PE to the AS border router (ASBR) is provided by the SR node segment, with a three-label stack: node segment to ASBR, labeled BGP label for the egress PE, and the service label.
SR-mapping advertisement for a given prefix provides no information about Penultimate Hop Popping (PHP). Other mechanisms, such as IGP-specific mechanisms defined in RFC 8665, RFC 8666, and RFC 8667, MAY be used to determine PHP in such cases.
The SR-to-LDP behavior does not propagate the status of the LDP FEC signaled by LDP when configured in ordered mode; the label binding is equivalent to independent LDP Label Distribution Control Mode (RFC 5036), where a label is bound to a FEC independently from the received binding for the same FEC.
When LSPs are defined by stitching LDP LSPs with SR LSPs, verification of LSP connectivity and path validation MUST use mechanisms described in RFC 8287 (LSP Ping/Traceroute for SR IGP-Prefix and IGP-Adjacency SIDs with MPLS Data Planes).
A local policy on a router MUST allow preferring the SR-provided IP2MPLS entry over the LDP-provided one, and this policy MAY be locally defined with no requirement that all routers use the same policy.
An MS MAY advertise SR mappings for any prefix whether or not it advertises reachability for the prefix and irrespective of whether that prefix is advertised by or even reachable through any router in the network.
An SR node having LDP neighbors MUST create LDP bindings for each Prefix-SID learned in the SR domain by treating SR-learned labels as if they were learned through an LDP neighbor, and must stitch the incoming LDP label to the outgoing SR label for each FEC.
An SR node having LDP-only neighbors MUST stitch the incoming SR label (whose SID is advertised by the SRMS) to the outgoing LDP label.
At least one SRMS server MUST exist in a routing domain to advertise Prefix-SIDs on behalf of non-SR nodes, thereby allowing non-LDP routers to send and receive labeled traffic from LDP-only routers.
For any prefix for which no Prefix-SID Advertisement was received, an MCC applies SRMS mapping advertisements with the highest preference value. If an incoming label collision occurs as specified in RFC 8660, the steps in RFC 8660 MUST be applied to resolve the collision.
When a Mapping Client (MC) receives both an SR-mapping advertisement from a Mapping Server and a Prefix-SID Advertisement in a prefix reachability advertisement for the same prefix, the MC MUST prefer the SID from the prefix reachability advertisement; the Mapping Server Advertisement MUST be ignored for that prefix.
When both SR and LDP coexist and both propose an IP2MPLS entry for the same IP prefix, the LDP route SHOULD be selected by default, because SR is typically introduced into networks containing non-SR routers and LDP preference avoids traffic disruption.
A phased migration from LDP to SR is described: at T0 all routers run LDP; at T1 all are upgraded to SR but LDP remains preferred for IP2MPLS; at T2–T3 the operator enables SR IP2MPLS preference progressively at edge routers; at T4 LDP is unconfigured. At each phase the network remains operational and changes are reversible.
LDP-to-SR interworking stitches an LDP LSP from the LDP domain to the SR/LDP boundary node, and the SR node segment from the boundary to the SR destination. No additional signaling or state is required for the LDP-to-SR direction; the SR node with LDP neighbors creates LDP bindings for each Prefix-SID learned.
SR Protection of LDP-Based Traffic allows SR-capable nodes to provide FRR repair tunnels (using SR node segments and adjacency segments) for LDP-based primary paths, eliminating the need for targeted LDP sessions and providing guaranteed FRR coverage even where no RLFA solution exists.
SR-to-LDP interworking stitches an SR node segment from the SR ingress to the SR/LDP boundary node, then an LDP LSP from the boundary to the LDP destination. The boundary node swaps the incoming SR label (advertised by SRMS) to the outgoing LDP label.
The Segment Routing Global Block (SRGB) is the range of labels allocated to SR, allowing SR to coexist with other MPLS Control-Plane Clients (MCCs) such as LDP, RSVP-TE, and BGP by ensuring unique incoming label allocation per MCC.
The Segment Routing Mapping Server (SRMS) is an IGP node that advertises mappings between Prefix-SIDs and prefixes owned by non-SR-capable routers, enabling SR-to-LDP interworking. It consists of a Mapping Server (MS) that advertises SID-to-prefix assignments and a Mapping Client (MC) that receives and applies them.
The SRMS preference value is an unsigned 8-bit integer: 0 is reserved and indicates advertisements from that node MUST NOT be used; values 1–255 represent preference with 255 being most preferred. Nodes that do not advertise a preference value are assigned a default preference of 128.
The SRMS introduces a centralized form of label binding advertisement on behalf of nodes advertising IP reachability, presenting a different and potentially higher risk profile than node-originated advertisements. Errors in SRMS configuration or programming may produce false or conflicting label binding advertisements compromising traffic forwarding; operators should implement strict configuration control, monitor advertised SIDs, prefer IP-reachability SID Advertisements over centralized ones, and log errors.
This document introduces no change to the MPLS data plane (RFC 3031) and therefore requires no additional MPLS data-plane security. Security of SRMS advertisements relies on the authentication mechanisms of underlying routing protocols IS-IS (RFC 5304) and OSPF (RFC 5709), both of which optionally support cryptographic authentication.