Introduction to Segment Routing Santiago Álvarez, Distinguished Technical Marketing Engineer BRKRST-2124
Agenda Technology Overview Use Cases A Closer Look to Control and Data Plane Traffic Protection Traffic Engineering Conclusion
Technology Overview
Segment Routing Source Routing the source chooses a path and encodes it in the packet header as an ordered list of segments the rest of the network executes the encoded instructions without any further per-flow state Segment: an identifier for any type of instruction forwarding or service
IGP Prefix Segment Shortest-path to the IGP prefix Global 16000 + Index Signaled by ISIS/OSPF 12 13 10 1 2 4 16005 7 3 6 5 14 11 DC (BGP-SR) WAN (IGP-SR) PEER
IGP Adjacency Segment Forward on the IGP adjacency Local 1XY X is the from 12 10 124 2 4 Y is the to Signaled by ISIS/OSPF 13 1 7 3 6 5 11 14 DC (BGP-SR) WAN (IGP-SR) PEER
BGP Prefix Segment Shortest-path to the BGP prefix Global 16000 + Index Signaled by BGP 12 13 16001 10 1 2 4 7 3 6 5 14 11 DC (BGP-SR) WAN (IGP-SR) PEER
BGP Peering Segment Forward to the BGP peer Local 1XY X is the from Y is the to Signaled by BGP-LS (topology information) to the controller 12 13 10 11 1 3 2 6 4 5 147 Low Lat, Low BW High Lat, High BW 7 14 DC (BGP-SR) WAN (IGP-SR) PEER
WAN Controller WAE collects via BGP-LS IGP segments BGP segments Topology 12 BGP-LS 10 BGP-LS BGP-LS 2 4 Low Lat, Low BW 13 1 7 3 6 5 11 14 DC (BGP-SR) WAN (IGP-SR) PEER
An end-to-end path as a list of segments WAE computes that the green path can be encoded as 16001 16002 124 147 WAE programs a single per-flow state to create an applicationengineered end-toend policy 12 {16001, 16002, 124, 147} 13 14 PCEP, Netconf, BGP 10 11 DC (BGP-SR) 1 3 2 4 50 6 5 Default ISIS cost metric: 10 WAN (IGP-SR) Low Lat, Low BW 7 PEER
Segment Routing Standardization Sample IETF Documents IETF standardization in SPRING working group Protocol extensions progressing in multiple groups IS-IS OSPF PCE IDR 6MAN Broad vendor and customer support Segment Routing Architecture (draft-ietf-spring-segment-routing) Problem Statement and Requirements (draft-ietf-spring-problem-statement) IPv6 SPRING Use Cases (draft-ietf-spring-ipv6-use-cases) Segment Routing Use Cases (draft-filsfils-spring-segment-routing-use-cases) Topology Independent Fast Reroute using Segment Routing (draft-francois-spring-segment-routing-ti-lfa) IS-IS Extensions for Segment Routing (draft-ietf-isis-segment-routing-extensions) OSPF Extensions for Segment Routing (draft-ietf-ospf-segment-routing-extensions) PCEP Extensions for Segment Routing (draft-ietf-pce-segment-routing) Close to 30 IETF drafts in progress
Segment Routing Product Support Platforms: ASR9000, CRS-1/CRS-3 (shipping) IS-IS IPv4 / IPv6 (shipping) Node/Adjacency SID advertisement LDP interworking (mapping server/client) Traffic protection (topology independent LFA link protection) OSPFv2 (shipping) Node SID advertisement Traffic protection (LFA) Upcoming SR Traffic Engineering (manual provisioning and PCEP) OAM (Ping/Trace) IOS XE Support (ASR1000, ASR902, ASR903, ISR4400)
Use Cases
IPv4/6 VPN/Service transport 16007 vpn IGP only No LDP, no RSVP-TE ECMP pkt 1 pkt 2 3 4 vpn pkt 7 pkt 6 5 Site1 16007 Site2 vpn pkt
Seamless interworking with LDP 16007 Seamless deployment vpn pkt 2 3 LDP(7) vpn pkt vpn pkt pkt 1 4 7 pkt 6 5 Site1 16007 Site2 vpn pkt
Topology-Independent LFA (TI-LFA FRR) 50msec FRR in any topology IGP Automated No LDP, no RSVP-TE 16007 2 3 7 Optimum Post-convergence path pkt 1 4 No midpoint backup state Detailed operator report 6 5 16007 pkt S. Litkowski, B. Decraene, Orange Mate Design How many backup segments Capacity analysis 16005 16007 pkt
Automated Traffic Matrix Collection 1 2 3 4 Traffic Matrix is fundamental for capacity planning centralized traffic engineering IP/Optical optimization Most operators do not have an accurate traffic matrix 1 2 3 4 2 With SR, the traffic matrix collection is automated 1 3 4
Optimized Content Delivery On a per-content, per-user basis, the content delivery application can engineer 16003 AS7 5 7 6 the path within the AS the selected border router 16002 126 AS5 AS6 the selected peer pkt Also applicable for engineering egress traffic from DC to peer 1 2 BGP Prefix and Peering Segments 4 3 AS1
Application Engineered Routing Per-application flow engineering End-to-End DC, WAN, AGG, PEER Millions of flows No signaling No midpoint state No reclassification at boundaries Low-Latency to 7 for application A12 12 Push {16001, 200, 147} 13 10 Low-Lat to 4 1 3 200: pop and push {16002, 16004} BSID: 200 ISIS: 35 2 4 6 5 PeerSID: 147, Low Lat, Low BW PeerSID: 147, High Lat, High BW Low Lat, Low BW 7 14 11 DC (or AGG) Default ISIS cost metric: 10 Default Latency metric: 10 WAN PEER
Application Engineered Routing Per-application flow engineering End-to-End DC, WAN, AGG, PEER Millions of flows No signaling No midpoint state No reclassification at boundaries Low-Latency to 7, DC Plane 0 only, for application A12 12 Push {16010, 16001, 200, 147} 13 10 Low-Lat to 4 1 3 200: pop and push {16002, 16004} BSID: 200 ISIS: 35 2 4 6 5 PeerSID: 147, Low Lat, Low BW PeerSID: 147, High Lat, High BW Low Lat, Low BW 7 14 11 DC (or AGG) Default ISIS cost metric: 10 Default Latency metric: 10 WAN PEER
A Closer Look to Control and Data Plane
MPLS Control and Forwarding Operation with Segment Routing Services PE1 BGP / LDP PE2 IPv4 IPv6 IPv4 VPN IPv6 VPN VPWS VPLS No changes to control or forwarding plane Packet Transport PE1 IGP PE2 LDP RSVP BGP Static IS-IS OSPF MPLS Forwarding IGP label distribution for IPv4 and IPv6, same forwarding plane
SID Encoding Prefix SID SID encoded as an index Index represents an offset from SRGB base Index globally unique SRGB may vary across LSRs SRGB (base and range) advertised with router capabilities Adjacency SID SID encoded as absolute (i.e. not indexed) value Locally significant Automatically allocated for each adjacency SR-enabled Node SRGB = [ 16000-23999 ]. Advertised as base = 16,000, range = 7,999 Prefix SID = 16041. Advertised as Prefix SID Index = 41 Adjacency SID = 24000. Advertised as Adjacency SID = 24000
SR IS-IS Control Plane Overview Level 1, level 2 and multi-level routing Prefix Segment ID (Prefix-SID) for host prefixes on loopback interfaces Adjacency SIDs for adjacencies Prefix-to-SID mapping advertisements (mapping server) MPLS penultimate hop popping (PHP) signaling MPLS explicit-null label signaling
IS-IS Configuration Required Wide metrics SR enabled under unicast address family Optional Prefix-SID configured under loopback(s) AF IPv4 MPLS forwarding enabled automatically on all (non-passive) IS-IS interfaces Adjacency-SIDs are automatically allocated for each adjacency
SR OSPF Control Plane Overview IPv4 Prefix Segment ID (Prefix-SID) for host prefixes on loopback interfaces MPLS penultimate hop popping (PHP) signaling MPLS explicit-null label signaling
OSPF Configuration OSPFv2 control plane Required Enable segment-routing under instance or area(s) Command has area scope, usual inheritance applies Enable segment-routing forwarding under instance, area(s) or interface(s) Command has interface scope, usual inheritance applies Optional Prefix-SID configured under loopback(s) MPLS forwarding enabled on all OSPF interfaces with segment-routing forwarding configured
MPLS Data Plane Operation Prefix SID SRGB [16,000 23,999 ] Adjacency SID SRGB [16,000 23,999 ] Swap Pop Adjacency SID = X X Y X Y Y Payload Payload Payload Payload Packet forwarded along IGP shortest path (ECMP) Swap operation performed on input label Same top label if same/similar SRGB PHP if signaled by egress LSR Packet forwarded along IGP adjacency Pop operation performed on input label Top labels will likely differ Penultimate hop always pops last adjacency SID
MPLS Data Plane Operation (Prefix SID) SRGB [16,000 23,999 ] SRGB [16,000 23,999 ] SRGB [26,000 23,999 ] SRGB [16,000 23,999 ] A B C D Loopback X.X.X.X Prefix SID Index = 41 Push Push Swap Pop Pop 16041 26041 VPN Label VPN Label VPN Label Payload Payload Payload Payload Payload
MPLS Data Plane Operation (Adjacency SIDs) MPLS Label Range MPLS Label Range MPLS Label Range MPLS Label Range [ 24000 265535 ] [ 24000 265535 ] [ 24000 265535 ] [ 24000 265535 ] A B C D Push Push Push Adjacency SID = 24010 24000 24000 VPN Label Pop Adjacency SID = 24000 24000 VPN Label Pop Adjacency SID = 24000 VPN Label Pop Payload Payload Payload Payload Payload
MPLS LFIB with Segment Routing LFIB populated by IGP (ISIS / OSPF) Forwarding table remains constant (Nodes + Adjacencies) regardless of number of paths Other protocols (LDP, RSVP, BGP) can still program LFIB Network Node Segment Ids Node Adjacency Segment Ids In Label PE PE PE PE Out Label P Out Interface L1 L1 Intf1 L2 L2 Intf1 L8 L8 Intf4 L9 L9 Intf2 L10 Pop Intf2 Ln Pop Intf5 PE PE PE PE Forwarding table remains constant
Traffic Protection
Topology Independent LFA (TI-LFA) Benefits 100%-coverage 50-msec link and node protection Simple to operate and understand automatically computed by the IGP Prevents transient congestion and suboptimal routing leverages the post-convergence path, planned to carry the traffic Incremental deployment also protects LDP traffic
Topology Independent LFA Implementation Leverages existing and proven LFA technology P space: set of nodes reachable from node S (PLR) without using protected link L Q space: set of nodes that can reach destination D without using protected link L Enforcing loop-freeness on post-convergence path Where can I release the packet? At the intersection between the post-convergence shortest path and the Q space How do I reach the release point? By chaining intermediate segments that are assessed to be loop-free
TI-LFA Zero-Segment Example TI-LFA for link R1R2 on R1 Calculate LFA(s) Packet to Z A R1 Z R2 Packet to Z Calculate post-convergence SPT 1000 Find LFA on post-convergence SPT R1 will steer the traffic towards LFA R5 prefix-sid(z) Packet to Z R5 R4 R3 Default metric:10
TI-LFA Single-Segment Example TI-LFA for link R1R2 on R1 Calculate P and Q spaces Packet to Z A R1 Z R2 Packet to Z They overlap in this case P-space Calculate post-convergence SPT Find PQ node on postconvergence SPT prefix-sid(r4) prefix-sid(z) Packet to Z R5 R4 R3 prefix-sid(z) Packet to Z R1 will push the prefix-sid of R4 on the backup path Default metric:10 Q-space
TI-LFA Double-Segment Example TI-LFA for link R1R2 on R1 Calculate P and Q spaces Packet to Z A R1 Z R2 Packet to Z Calculate post-convergence SPT Find Q and adjacent P node on post-convergence SPT R1 will push the prefix-sid of R4 and the adj-sid of R4-R3 link on the backup path prefix-sid(r4) adj-sid(r4-r3) prefix-sid(z) Packet to Z P-space adj-sid(r4-r3) prefix-sid(z) R5 R4 1000 Default metric:10 R3 Q-space prefix-sid(z) Packet to Z Packet to Z
Traffic Engineering
Traffic Engineering with Segment Routing Provides explicit routing Supports constraint-based routing Supports centralized admission control No RSVP-TE to establish LSPs Uses existing ISIS / OSPF extensions to advertise link attributes Supports ECMP Segment Routing TE LSP
How Traffic Engineering Works Head end Mid-point TE LSP IP/MPLS Tail end Link information Distribution ISIS-TE OSPF-TE Path Calculation Path Setup Forwarding Traffic down path Auto-route (announce / destinations) Static route PBR PBTS / CBTS Forwarding Adjacency Pseudowire Tunnel select
Stateful PCE PCE maintains topology and path database (established paths) More optimal centralized path computation Enables centralized path initiation and update control Well suited for SDN deployments PCC PCEP Stateful PCE LSP DB TED
Topology Acquisition An external PCE requires some form of topology acquisition A PCE may learn topology using BGP-LS, IGP, SNMP, etc. BGP-LS characteristics aggregates topology across one or more domains provides familiar operational model New BGP-LS attribute TLVs for SR IGP: links, nodes, prefixes BGP: peer node, peer adjacency, peer set PCE BGP-LS Domain 0 RR BGP-LS BGP-LS Domain 1 Domain 2 TED
Active Stateful PCE PCC or PCE may initiate path setup PCC may delegate update control to PCE PCC may revoke delegation PCE may return delegation PCE has update control over delegated paths Stateful PCC PCEP Active Stateful PCE LSP DB TED
Active Stateful PCE PCE-Initiated and PCC-Initiated LSPs PCE-Initiated (Active Stateful PCE) PCC-Initiated (Active Stateful PCE) PCE initiates LSP and maintains update control PCEP LSP DB TED PCC initiates LSP and delegates update control PCEP LSP DB TED Stateful PCC Stateful PCC PCE part of controller architecture managing full path life cycle Tighter integration with application demands PCC may initiate path setup based on distributed network state Can be used in conjunction with PCEinitiated paths
PCE Extensions for Segment Routing (SR) Segment routing enables source routing based on segment ids distributed by IGP PCE specifies path as list of segment ids PCC forwards traffic by pushing segment id list on packets No path signaling required Minimal forwarding state Maximum network forwarding virtualization Application PCEP Segment List:: 10,20,30,40 Stateful PCC Stateful PCE Path Request LSP DB TED The state is no longer in the network but in the packet Paths may be PCE- or PCC-initiated Node SID Adjacency SID In Out Int L1 L1 Intf1 L7 L7 Int3 L8 Pop Intf3 L9 Pop Intf5 Forwarding table remains constant
Conclusion
Conclusion Simple routing extensions to implement source routing Packet path determined by prepended segment identifiers (one or more) Data plane agnostic (MPLS, IPv6) Network scalability and agility by reducing network state and simplifying control plane Traffic protection with 100% coverage with more optimal routing
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Backup Slides
Configuring Segment Routing for IPv4 Using IS-IS (Cisco IOS XR) router isis DEFAULT net 49.0001.1720.1625.5001.00 address-family ipv4 unicast metric-style wide segment-routing mpls interface Loopback0 passive address-family ipv4 unicast prefix-sid absolute 16041 interface GigabitEthernet0/0/0/0 point-to-point address-family ipv4 unicast Enable Segment Routing for IPv4 with MPLS data plane Advertise prefix SID 16041 (index 41) for Loopback0
Configuring Segment Routing for IPv6 Using IS-IS (Cisco IOS XR) router isis DEFAULT net 49.0001.1720.1625.5001.00 address-family ipv6 unicast metric-style wide segment-routing mpls interface Loopback0 passive address-family ipv6 unicast prefix-sid absolute 16061 interface GigabitEthernet0/0/0/0 point-to-point address-family ipv6 unicast Enable Segment Routing for IPv6 with MPLS data plane Advertise prefix SID 16061 (index 61) for Loopback0
Configuring Segment Routing for IPv4 Using OSPF (Cisco IOS XR) router ospf DEFAULT router-id 172.16.255.1 segment-routing mpls segment-routing forwarding mpls area 0 interface Loopback0 passive prefix-sid absolute 16041 interface GigabitEthernet0/0/0/0 network point-to-point Enable Segment Routing with MPLS data plane Advertise prefix SID 16041 (index 41) for Loopback0
Configuring Topology Independent Fast Reroute for IPv4 using Segment Routing and IS-IS (Cisco IOS XR) router isis DEFAULT net 49.0001.1720.1625.5001.00 address-family ipv4 unicast metric-style wide segment-routing mpls interface Loopback0 passive address-family ipv4 unicast prefix-sid absolute 16041 interface GigabitEthernet0/0/0/0 address-family ipv4 unicast fast-reroute per-prefix fast-reroute per-prefix ti-lfa Enable TI-LFA for IPv4 prefixes on interface GigabitEthernet0/0/0/0
Configuring Topology Independent Fast Reroute for IPv6 using Segment Routing and IS-IS (Cisco IOS XR) router isis DEFAULT net 49.0001.1720.1625.5001.00 address-family ipv6 unicast metric-style wide segment-routing mpls interface Loopback0 passive address-family ipv6 unicast prefix-sid absolute 16061 interface GigabitEthernet0/0/0/0 address-family ipv6 unicast fast-reroute per-prefix fast-reroute per-prefix ti-lfa Enable TI-LFA for IPv6 prefixes on interface GigabitEthernet0/0/0/0
Configuring a Mapping Server for SR and LDP Interworking for IPv4 Using IS-IS (Cisco IOS XR) router isis DEFAULT net 49.0001.1720.1625.5001.00 address-family ipv4 unicast metric-style wide segment-routing mpls segment-routing prefix-sid-map receive segment-routing prefix-sid-map advertise-local... segment-routing address-family ipv4 prefix-sid-map 172.16.255.1/32 4041 range 8 Construct active mapping policy using remotely learned and locally configured mappings (mapping client) Advertise local mapping policy (mapping server) Local prefix-to-sid mapping policy 172.16.255.1/32 4041 : 172.16.255.8/32-4048
Configuring a Mapping Client for SR and LDP Interworking for IPv4 Using IS-IS (Cisco IOS XR) router isis DEFAULT net 49.0001.1720.1625.5001.00 address-family ipv4 unicast metric-style wide segment-routing mpls segment-routing prefix-sid-map receive interface Loopback0 passive address-family ipv4 unicast prefix-sid absolute 16041 interface GigabitEthernet0/0/0/0 point-to-point address-family ipv4 unicast Construct active mapping policy using remotely learned and locally configured mappings (mapping client)
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