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+# SR-MPLS: Segment Routing for MPLS    {#srmpls_doc}
+
+This is a memo intended to contain documentation of the VPP SR-MPLS implementation.
+Everything that is not directly obvious should come here.
+For any feedback on content that should be explained please mailto:pcamaril@cisco.com
+
+## Segment Routing
+
+Segment routing is a network technology focused on addressing the limitations of existing IP and Multiprotocol Label Switching (MPLS) networks in terms of simplicity, scale, and ease of operation. It is a foundation for application engineered routing as it prepares the networks for new business models where applications can control the network behavior.
+
+Segment routing seeks the right balance between distributed intelligence and centralized optimization and programming. It was built for the software-defined networking (SDN) era.
+
+Segment routing enhances packet forwarding behavior by enabling a network to transport unicast packets through a specific forwarding path, different from the normal path that a packet usually takes (IGP shortest path or BGP best path). This capability benefits many use cases, and one can build those specific paths based on application requirements.
+
+Segment routing uses the source routing paradigm. A node, usually a router but also a switch, a trusted server, or a virtual forwarder running on a hypervisor, steers a packet through an ordered list of instructions, called segments. A segment can represent any instruction, topological or service-based. A segment can have a local semantic to a segment-routing node or global within a segment-routing network. Segment routing allows an operator to enforce a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the segment-routing network. Segment routing also supports equal-cost multipath (ECMP) by design.
+
+Segment routing can operate with either an MPLS or an IPv6 data plane. All the currently available MPLS services, such as Layer 3 VPN (L3VPN), L2VPN (Virtual Private Wire Service [VPWS], Virtual Private LAN Services [VPLS], Ethernet VPN [E-VPN], and Provider Backbone Bridging Ethernet VPN [PBB-EVPN]), can run on top of a segment-routing transport network.
+
+**The implementation of Segment Routing in VPP covers both the IPv6 data plane (SRv6) as well as the MPLS data plane (SR-MPLS). This page contains the SR-MPLS documentation.**
+
+## Segment Routing terminology
+
+* SegmentID (SID): is an MPLS label.
+* Segment List (SL) (SID List): is the sequence of SIDs that the packet will traverse.
+* SR Policy: is a set of candidate paths (SID list+weight). An SR policy is uniquely identified by its Binding SID and associated with a weighted set of Segment Lists. In case several SID lists are defined, traffic steered into the policy is unevenly load-balanced among them according to their respective weights.
+* BindingSID: a BindingSID is a SID (only one) associated one-one with an SR Policy. If a packet arrives with MPLS label corresponding to a BindingSID, then the SR policy will be applied to such packet. (BindingSID is popped first.)
+
+## SR-MPLS features in VPP
+
+The SR-MPLS implementation is focused on the SR policies, as well on its steering. Others SR-MPLS features, such as for example AdjSIDs, can be achieved using the regular VPP MPLS implementation.
+
+The <a href="https://datatracker.ietf.org/doc/draft-filsfils-spring-segment-routing-policy/">Segment Routing Policy (*draft-filsfils-spring-segment-routing-policy*)</a> defines SR Policies.
+
+## Creating a SR Policy
+
+An SR Policy is defined by a Binding SID and a weighted set of Segment Lists.
+
+A new SR policy is created with a first SID list using:
+
+    sr mpls policy add bsid 40001 next 16001 next 16002 next 16003 (weight 5)
+
+* The weight parameter is only used if more than one SID list is associated with the policy.
+
+An SR policy is deleted with:
+
+    sr mpls policy del bsid 40001
+
+The existing SR policies are listed with:
+
+    show sr mpls policies
+
+### Adding/Removing SID Lists from an SR policy
+
+An additional SID list is associated with an existing SR policy with:
+
+    sr mpls policy mod bsid 40001 add sl next 16001 next 16002 next 16003 (weight 3)
+
+Conversely, a SID list can be removed from an SR policy with:
+
+    sr mpls policy mod bsid 4001 del sl index 1
+
+Note that this CLI cannot be used to remove the last SID list of a policy. Instead the SR policy delete CLI must be used.
+
+The weight of a SID list can also be modified with:
+
+    sr mpls policy mod bsid 40001 mod sl index 1 weight 4
+    sr mpls policy mod index 1    mod sl index 1 weight 4
+
+### SR Policies: Spray policies
+
+Spray policies are a specific type of SR policies where the packet is replicated on all the SID lists, rather than load-balanced among them.
+
+SID list weights are ignored with this type of policies.
+
+A Spray policy is instantiated by appending the keyword **spray** to a regular SR-MPLS policy command, as in:
+
+    sr mpls policy add bsid 40002 next 16001 next 16002 next 16003 spray
+
+Spray policies are used for removing multicast state from a network core domain, and instead send a linear unicast copy to every access node. The last SID in each list accesses the multicast tree within the access node.  
+
+## Steering packets into a SR Policy
+
+To steer packets in Transit into an SR policy, the user needs to create an 'sr steering policy'.
+
+    sr mpls steer l3 2001::/64 via sr policy bsid 40001
+    sr mpls steer l3 2001::/64 via sr policy bsid 40001 fib-table 3
+    sr mpls steer l3 10.0.0.0/16 via sr policy bsid 40001