sEGMENT ROUTING

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Segment Routing - Traffic
Engineering

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Upon completing this module, you will be able
to:
Explore the components of SR-TE
Examine Anycast and Binding SIDs
Enable and Verify SR-TE
Instantiate SR-TE policies from a configured tunnel
Instantiate SR-TE policies using BGP Dynamic

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This module contains the following lessons:
Exploring SR-TE
Introducing Anycast
Introducing Binding SIDs
Enabling and Verifying SR-TE

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This module contains the following lessons:
Instantiating SR-TE Policies from a Configured Tunnel - Explicit
Instantiating SR-TE Policies from a Configured Tunnel - Dynamic
Introducing BGP Dynamic SR-TE policy instantiation
Configure and Verify BGP Dynamic SR-TE policy instantiation

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Exploring SR-TE

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Upon completion of this lesson, you should
be able to:
Articulate the difference between circuit and SR
optimization
Describe the various TE optimizations and
constraints
Define multi-domain and multi-layer TE
Explain disjointed TE services

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2
4
1
5 3
6
7
8 9
Pre-SR-TE is circuit-based
CSPF => non-ECMP path
RE-using this for SR-TE is not good
SID List: {4, 5, 7, 3}
Poor ECMP, big SR list, ATM optimized
SR-native TE is needed
!No more circuit!
SID List: {7, 3}
ECMP, Small SR list, IP-optimized
2
4
1
5 3
6
7
8 9 Default IGP metric: 10
100
Find a path (1) – (3) that
avoids RED link (2) – (3)
Default IGP metric: 10
100
Find a path (1) – (3) that
avoids RED link (2) – (3)

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SR-TE offers a comprehensive support for all useful
optimizations and constraints
Latency
Bandwidth
Disjointness
Resource avoidance
SR-TE Policy path can be computed locally (distributed) or
centrally

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SR-TE path can be computed local (distributed) or centralized
Distributed Centralized
Latency ✔ ✔
Avoid a topological resource ✔ ✔
Disjoint from another service ✔
(same headend)
✔
Bandwidth ✖ ✔
Multi Domain ✖ ✔
Multi Layer (IP/Optical) ✖ ✔

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SR-TE uses a “Policy” to steer traffic through the network
Since many SR-TE Policies don’t require a tunnel-te interface, the term
“tunnel” is avoided in the contest of SR-TE
An SR-TE Policy path is expressed as a “SID list”
The list of segments that specifies the path
If a packet is steered into an SR-TE policy, the SID list is pushed
on the packet by the head-end
The rest of the network executes the instructions embedded in the SID
list (source routing)

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Binding Segment is a fundamental building block of SR-TE
The Binding Segment is a local segment identifying an SR-TE Policy
Each SR-TE Policy is associated 1-for-1 with a Binding-SID
The Binding-SID can be used to steer traffic into the SR-TE Policy
The instruction associated with a Binding Segment is: “Pop and steer
into SR-TE Policy”
The Binding-SID is a local label, automatically allocated for each
SR-TE Policy
A Binding-SID can also be allocated for RSVP-TE tunnels (configurable)

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Bandwidth optimization models:
Distributed: Head-ends independently calculate BW placement
Centralized: Central controller globally optimizes BW placement

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RSVP-TE in full mesh: distributed signaling and BW bookkeeping
 requires a full-mesh of non-zero bandwidth RSVP-TE tunnel
 requires that all traffic rides RSVP-TE tunnel
 requires auto bw
 suffers from k*n^2 scale problem
 suffers from longer and unpredictable convergence due to bw contention

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Central controller monitors traffic load and optimizes bandwidth
with the creation of the minimum number of tunnels to balance
the traffic
Segment Routing uses the centralized model
More optimized and predictable
Faster (fewer states to program)
Simpler (less protocols, 100 to 1000 times less tunnels)

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Multi-Domain and Multi-Layer must be centralized
Head-end has no visibility on other domain or layer
Multi-domain:
Central controller calculates end-to-end path
Encodes path as list of segments
Leverages the Binding Segment

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Binding Segments isolate SR-TE Policy path control in different
domains
 Maintain a seamless end-to-end LSP
Each domain controls local SR-TE Policies
No reclassification on border nodes
Isolates head-end from remote domains’ topology changes
 SR-TE Policy not updated when remote domain’s topology changes

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Primary traffic steering mechanisms for SR-TE use the Binding
SID
Locally programmed: BGP SR-TE Dynamic – (IOS XR 6.0)
 Destination based
 Flow based
Remotely programmed: “nesting” and "stitching” SR-TE Policies
“Classic” mechanisms: static route, autoroute, PBTS, ... can also
be used but are not the primary mechanisms for SR-TE

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A to Z any plane
 IGP shortest-path
 Prefix SID of Z (65)
A to Z via blue plane
 SR-TE policy pushes one
additional
segment “Blue Anycast” (111)
Benefits
 ECMP
 No hop-by-hop signaling load and
delay
 No midpoint state
16065
pkt
16065
pkt
16111

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Data from Tokyo to Brussels
 IGP shortest-path via US, higher and
cheaper capacity
 PrefixSID of Brussels
Voice from Tokyo to Brussels
 SR-TE policy pushes one additional
segment “Russia Anycast”
 Low-latency path
Benefits
 ECMP
 Availability of the anycast segment
against node failure
 No hop-by-hop signaling load and delay
 No midpoint state
Node segment to Brussels
Node segment to Russia
Brussels
pkt
Data
Brussels
pkt
Russia
Voice

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In this lesson, you examined the following topics:
Pre-SR Traffic Engineering techniques are circuit-based. SR-TE
optimization is IP optimized and can utilize ECMP.
SR-TE optimizations and constraints options can utilize latency,
bandwidth, disjointness and resource avoidance when defining
SR-TE policies.
Multi-Domain and Multi-Layer TE must centralized because the
head-end has no visibility on other domain or layer. A Central
controller calculates the end-to-end path and encodes it as a list
of segments, through the use of the binding segment.
Lesson

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In this lesson, you examined the following topics:
Disjointed TE services allow different traffic types or applications
to traverse different paths. It is a simple way to implement disjoint
traffic-engineering paths which would be very complex using
traditional MPLS TE techniques.
Lesson