This document discusses Segment Routing, which uses source routing to allow a packet's source to specify the path through the network. Segment Routing represents path instructions as segments, which can be forwarding segments or service segments. Forwarding segments in MPLS are represented as labels. There are two types of segments distributed by IGPs: prefix segments that represent the shortest path to a prefix, and adjacency segments that represent forwarding on a specific adjacency. Combining different segments allows traffic to be steered along any path through the network.

1. Clarence Filsfils Kris Michielsen
Cisco Fellow @ cisco Systems Technical Leader Engineering @ cisco Systems
November 2015
Segment Routing

2. Clarence Filsfils
Kris Michielsen
Segment Routing
Introduction

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© 2015 Cisco and/or its affiliates. All rights reserved.
Illustration and Examples Conventions
• The examples in this presentation follow these conventions:
– Router-id of NodeX: 1.1.1.X
– Link address XY:
IPv4 99.X.Y.X/24 with X<Y
IPv6 99::X:Y:X/112 with X<Y and X, Y in decimal representation
– Prefix-SID index of NodeX: X
– Prefix-SIDs are labels in the range [16000 – 23999]
> This is the default Segment Routing Global Block (SRGB)
– Adjacency-SIDs are labels with the format 24NXY for the Nth adjacency XàY
– LDP/RSVP/BGP3107/… labels are in the range [90000 – 99999]

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© 2015 Cisco and/or its affiliates. All rights reserved.
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
• Segment: an identifier for any type of instruction
– forwarding or service
• In this presentation: IGP-based forwarding construct

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© 2015 Cisco and/or its affiliates. All rights reserved.
Segment Routing – Forwarding Plane
• MPLS: an ordered list of segments is represented as a stack
of labels
– Segment Routing re-uses MPLS data plane without any change
– Segment represented as MPLS label
– Applicable to IPv4 and IPv6 address families
• IPv6: an ordered list of segments is encoded in a routing
extension header
• This presentation: MPLS data plane

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© 2015 Cisco and/or its affiliates. All rights reserved.
Global and Local Segments
• Global Segment
– Any node in SR domain understands associated instruction
– Each node in SR domain installs the associated instruction in its
forwarding table
– MPLS: global label value in Segment Routing Global Block (SRGB)
• Local Segment
– Only originating node understands associated instruction
– MPLS: locally allocated label

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Global Segments – Global Label Indexes
• Global Segments always distributed as a label range
(SRGB) + Index
– Index must be unique in Segment Routing Domain
• Best practice: same SRGB on all nodes
– “Global model”, requested by all operators
– Global Segments are global label values, simplifying network
operations
– Default SRGB: 16,000 – 23,999
> Other vendors also use this label range

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© 2015 Cisco and/or its affiliates. All rights reserved.
IGP segments
• Two basic building blocks distributed by IGP
– Prefix Segments
– Adjacency Segments

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© 2015 Cisco and/or its affiliates. All rights reserved.
IGP Prefix Segment
• Shortest-path to the IGP prefix
– Equal Cost MultiPath (ECMP)-aware
• Global Segment
• Label = 16000 + Index
– Advertised as index
• Distributed by ISIS/OSPF
1 2
3 4
5
16005
16005
16005
16005
16005
16005
16005
1.1.1.5/32
All nodes use default SRGB
16,000 – 23,999

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© 2015 Cisco and/or its affiliates. All rights reserved.
IGP Prefix Segment
• Shortest-path to the IGP prefix
– Equal Cost MultiPath (ECMP)-aware
• Global Segment
• Label = 16000 + Index
– Advertised as index
• Distributed by ISIS/OSPF
1 2
3 4
5
16004
16004
16004
16004
16004
16004
16004
1.1.1.4/32
All nodes use default SRGB
16,000 – 23,999

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IGP Adjacency Segment
• Forward on the IGP adjacency
• Local Segment
• Advertised as label value
• Distributed by ISIS/OSPF
1 2
3 4
5
24024
24025
Adj to 5
Adj to 4
All nodes use default SRGB
16,000 – 23,999

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© 2015 Cisco and/or its affiliates. All rights reserved.
Combining IGP Segments
• Steer traffic on any path through
the network
• Path is specified by list of
segments in packet header, a
stack of labels
• No path is signaled
• No per-flow state is created
• Single protocol: IS-IS or OSPF
1 2
3 4
5
16004 24045
16004
24045
Packet to 5
24045
Packet to 5
All nodes use default SRGB
16,000 – 23,999

13. Visit us:
cisco.com
segment-routing.net

14. Acknowledgements:
Ahmed Bashandy
Robert Hanzl
Steven Luong
Stefano Previdi
Peter Psenak

15. Thank you.