3 SR-MPLS: Segment Routing for MPLS
4 =================================
6 This is a memo intended to contain documentation of the VPP SR-MPLS
7 implementation. Everything that is not directly obvious should come
8 here. For any feedback on content that should be explained please
9 mailto:pcamaril@cisco.com
14 Segment routing is a network technology focused on addressing the
15 limitations of existing IP and Multiprotocol Label Switching (MPLS)
16 networks in terms of simplicity, scale, and ease of operation. It is a
17 foundation for application engineered routing as it prepares the
18 networks for new business models where applications can control the
21 Segment routing seeks the right balance between distributed intelligence
22 and centralized optimization and programming. It was built for the
23 software-defined networking (SDN) era.
25 Segment routing enhances packet forwarding behavior by enabling a
26 network to transport unicast packets through a specific forwarding path,
27 different from the normal path that a packet usually takes (IGP shortest
28 path or BGP best path). This capability benefits many use cases, and one
29 can build those specific paths based on application requirements.
31 Segment routing uses the source routing paradigm. A node, usually a
32 router but also a switch, a trusted server, or a virtual forwarder
33 running on a hypervisor, steers a packet through an ordered list of
34 instructions, called segments. A segment can represent any instruction,
35 topological or service-based. A segment can have a local semantic to a
36 segment-routing node or global within a segment-routing network. Segment
37 routing allows an operator to enforce a flow through any topological
38 path and service chain while maintaining per-flow state only at the
39 ingress node to the segment-routing network. Segment routing also
40 supports equal-cost multipath (ECMP) by design.
42 Segment routing can operate with either an MPLS or an IPv6 data plane.
43 All the currently available MPLS services, such as Layer 3 VPN (L3VPN),
44 L2VPN (Virtual Private Wire Service [VPWS], Virtual Private LAN Services
45 [VPLS], Ethernet VPN [E-VPN], and Provider Backbone Bridging Ethernet
46 VPN [PBB-EVPN]), can run on top of a segment-routing transport network.
48 **The implementation of Segment Routing in VPP covers both the IPv6 data
49 plane (SRv6) as well as the MPLS data plane (SR-MPLS). This page
50 contains the SR-MPLS documentation.**
52 Segment Routing terminology
53 ---------------------------
55 - SegmentID (SID): is an MPLS label.
56 - Segment List (SL) (SID List): is the sequence of SIDs that the packet
58 - SR Policy: is a set of candidate paths (SID list+weight). An SR
59 policy is uniquely identified by its Binding SID and associated with
60 a weighted set of Segment Lists. In case several SID lists are
61 defined, traffic steered into the policy is unevenly load-balanced
62 among them according to their respective weights.
63 - BindingSID: a BindingSID is a SID (only one) associated one-one with
64 an SR Policy. If a packet arrives with MPLS label corresponding to a
65 BindingSID, then the SR policy will be applied to such packet.
66 (BindingSID is popped first.)
68 SR-MPLS features in VPP
69 -----------------------
71 The SR-MPLS implementation is focused on the SR policies, as well on its
72 steering. Others SR-MPLS features, such as for example AdjSIDs, can be
73 achieved using the regular VPP MPLS implementation.
75 The Segment Routing Policy
76 (*draft-filsfils-spring-segment-routing-policy*) defines SR Policies.
81 An SR Policy is defined by a Binding SID and a weighted set of Segment
84 A new SR policy is created with a first SID list using:
88 sr mpls policy add bsid 40001 next 16001 next 16002 next 16003 (weight 5)
90 - The weight parameter is only used if more than one SID list is
91 associated with the policy.
93 An SR policy is deleted with:
97 sr mpls policy del bsid 40001
99 The existing SR policies are listed with:
103 show sr mpls policies
105 Adding/Removing SID Lists from an SR policy
106 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
108 An additional SID list is associated with an existing SR policy with:
112 sr mpls policy mod bsid 40001 add sl next 16001 next 16002 next 16003 (weight 3)
114 Conversely, a SID list can be removed from an SR policy with:
118 sr mpls policy mod bsid 4001 del sl index 1
120 Note that this CLI cannot be used to remove the last SID list of a
121 policy. Instead the SR policy delete CLI must be used.
123 The weight of a SID list can also be modified with:
127 sr mpls policy mod bsid 40001 mod sl index 1 weight 4
129 SR Policies: Spray policies
130 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
132 Spray policies are a specific type of SR policies where the packet is
133 replicated on all the SID lists, rather than load-balanced among them.
135 SID list weights are ignored with this type of policies.
137 A Spray policy is instantiated by appending the keyword **spray** to a
138 regular SR-MPLS policy command, as in:
142 sr mpls policy add bsid 40002 next 16001 next 16002 next 16003 spray
144 Spray policies are used for removing multicast state from a network core
145 domain, and instead send a linear unicast copy to every access node. The
146 last SID in each list accesses the multicast tree within the access
149 Steering packets into a SR Policy
150 ---------------------------------
152 Segment Routing supports three methods of steering traffic into an SR
158 In this variant incoming packets match a routing policy which directs
159 them on a local SR policy.
161 In order to achieve this behavior the user needs to create an ‘sr
162 steering policy via sr policy bsid’.
166 sr mpls steer l3 2001::/64 via sr policy bsid 40001
167 sr mpls steer l3 2001::/64 via sr policy bsid 40001 fib-table 3
168 sr mpls steer l3 10.0.0.0/16 via sr policy bsid 40001
169 sr mpls steer l3 10.0.0.0/16 via sr policy bsid 40001 vpn-label 500
174 In this variant incoming packets have an active SID matching a local
175 BSID at the head-end.
177 In order to achieve this behavior the packets should simply arrive with
178 an active SID equal to the Binding SID of a locally instantiated SR
184 In this variant incoming packets match a BGP/Service route which
185 recurses on the BSID of a local policy.
187 In order to achieve this behavior the user first needs to color the SR
188 policies. He can do so by using the CLI:
192 sr mpls policy te bsid xxxxx endpoint x.x.x.x color 12341234
194 Notice that an SR policy can have a single endpoint and a single color.
195 Notice that the *endpoint* value is an IP46 address and the color a u32.
197 Then, for any BGP/Service route the user has to use the API to steer
202 sr steer l3 2001::/64 via next-hop 2001::1 color 1234 co 2
203 sr steer l3 2001::/64 via next-hop 2001::1 color 1234 co 2 vpn-label 500
205 Notice that *co* refers to the CO-bits (values [0|1|2|3]).
207 Notice also that a given prefix might be steered over several colors
208 (same next-hop and same co-bit value). In order to add new colors just
209 execute the API several times (or with the del parameter to delete the
212 This variant is meant to be used in conjunction with a control plane
213 agent that uses the underlying binary API bindings of
214 *sr_mpls_steering_policy_add*/*sr_mpls_steering_policy_del* for any BGP
215 service route received.