This session provides an overview of the Next Generation Network Architecture with Segment Routing technology that helps Service Providers to simplify the network. You will get an understanding of the basic concepts behind the technology and its wide applicability ranging from simple transport for MPLS services, disjoint routing, traffic engineering and its benefits in the context of software defined networking. Previous knowledge of IP routing and MPLS is beneficial to understand Segment Routing.

1. Segment Routing
Leonir Hoxha
Network Consulting Engineer
Cisco @ccie49534

2. Agenda
• Drivers
• Segment Routing model
• Use cases
• Summary

3. Why Segment Routing?
Helping operators by tackling issues such as
- Fast Reroute for any type of topology - Introduction of TI LFA
- Operation simplicity, and scalable explicit routing (no midpoint state, RSVP-TE)
- Smooth incremental deployment
SDN ready
- Offers a better and scalable interaction with orchestration,
applications and the network.
- Centralized or Distributed control plane

4. 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

5. 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
• IPv6: an ordered list of segments is encoded in a routing
extension header
• SR Operations: PUSH, CONTINUE, NEXT.
• This presentation: MPLS data plane

6. 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

7. 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
• All vendors also use this label range

8. IGP Segment Identifiers
• Two basic building blocks distributed by IGP
- Prefix Segments (also known as Node Segment)
- Adjacency Segment (Adj-SID)

9. Node Segment ID
• Shortest-path to the IGP prefix
• Global Segment
• Label = 16000 + Index
• Advertised as index

10. Node Segment ID
• Shortest-path to the IGP prefix
- Equal Cost MultiPath (ECMP)-aware
• Distributed by ISIS/OSPF
- A node segment to 16078 distributes traffic across all ECMP paths to node O.

11. Adjacency Segment
• C advertises the Adjacency Segment via ISIS
• C is the only node to install the adjacency segment in FIB
• Advertised as label value

12. 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. Use Cases

14. Topology Independent LFA (TI-LFA) – Benefits
• 100%-coverage across any topology
• 50-msec Protection upon link, node or SRLG failure
• Prevents transient congestion and suboptimal routing
• leverages the post-convergence path, planned to carry the traffic
• Avoid any intermediate flap via alternate path
• Simple to operate and understand
• automatically computed by the IGP
• Incremental deployment
• also protects LDP and IP traffic

15. Disjoint Routing
• A to Z any plane
• IGP shortest-path
• PrefixSID of Z (16065)
• A to Z via blue plane
• SRTE policy pushes one additional
Anycast segment “Blue” (16111)
• Benefits
• ECMP
• No midpoint state

16. Centralized Traffic Engineering
• Highly programmable and responsive to rapid changes.
• Path Computation Element Protocol (PCEP) based controller
• BGP-LS used for feeding information to the controller

17. • Future ready architecture considering Automation & Orchestration
• Node/Link failure coverage, 100% with TI-LFA.
• MicroLoop (uLoop) avoidance
• Low Opex/Capex
Summary
SR hands-on demo?... contact me 

18. Questions?

19. Thanks…
@ccie49534