Access Chandan-L2 L3 Multicast

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2006 Cisco Systems, Inc. All rights reserved.CiscoConfidentialPresentation_ID 1

Comparing Layer 2 andLayer 3 Metro Ethernet

Access Rings for MulticastTraffic

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Abstract Currently the Service Providers are implementing Metro Ethernet across the cities,

and are looking at mainly two options

1. Having Layer 2 based rings or

2. Layer 3 / MPLS control plane based.

The primary services that Service Providers will implement are

1. Residential : HSI, IPTV, VoIP

2. Business: Layer 2, Layer 3 point to point and also multipoint to multipoint.

The session will discuss advantages and disadvantages that Layer 3 / IP basedmulticast provides as compared to a Layer 2 based approach for a ringconfiguration.

The session will describe how basic multicast streams / flows behave when therings are based on layer2 or layer3. The session would also look at how the twoimplementations differ in responding to fibre cuts / ring failures / node failure.

Finally we conclude with the importance that services have in developing a flexiblemetro ethernet architecture.

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Agenda

Network and Service Rollout scenarios

Layer 2 and Layer 3 solution

Failure Scenarios Comparison

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Ethernet DSL Services Considerations

Service Transport TopologyService

GovernanceApplication

Elements IP Edge Element

Internet Access P2MP, Unicast Subscriber Policy Server,

Portal BRAS

VoIP Telepony P2P, Unicast Application Call Control Server BRAS, Aggregation

Node

VoD P2MP, Unicast Application Video Middleware Aggregation Node

TV Broadcast P2MP, Multicast Application Video Middleware Aggregation Node

MPLS VPN partial MP, Unicast Subscriber None MPLS PE

Ethernet VirtualLines P2P Subscriber None Aggregation Node

Ethernet VirtualLANs MP Subscriber None

Aggregation,Distribution Node,MPLS PE

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Current Architecture Options Driven by the Services and SLAs

Internet

IP/MPLS

U-PE

SONET/SDH

Ring

10/100/

1000 Mbps

10/100/

1

000Mbps

Metro D

N-PE

N-PE

N-PE

P P

PP

Hub &

Spoke

Metro C

10/100/

1000 Mbps

U-PE

DWDM/

CDWM

Metro B

U-PE

GE Ring

Metro A U-PE

PE-AGG

GE

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Access / Aggregation Network Control Plane Options

Layer 3 - IP, MPLS

Distributed L3 Service Edge

Layer 2 MPLSEoMPLS/ H-VPLS

Centralised L3 Service EdgeTransparent Ethernet Services

EoMPLS

DSL

Access Node

DistributionNode

BRAS

MPLS PE

SCE

Business

Corporate

Residential

STB

AggregationNode

AggregationNode

Core NetworkIP / MPLS

VoD

Content Network

TV SIP

Business

Corporate

Business

Corporate

AggregationNode

EthernetAccess Node

Aggregation NetworkMPLS, Ethernet, IP

DistributionNode

Access L2/3 Edge

VoD

Content Network

TV SIP

Layer 2 Bridged EthernetIEEE 802.1q / 802.1ad

802.1q

Centralised L3 Service EdgeTransparent Ethernet Services

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Layer 2 based Ring

architectures

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TV/VoD Distribution MechanismsEthernet Layer 2 Aggregation Network

IGMP snooping

DR

Querier Router

PIM

L2L2

L2L2

L2 L2

IGMP/PIM snooping

IEEE 802.1q

RSTP

Multicast

Unicast and multicast IEEE 802.1qbridging

Forwarding with IEEE bridging relies on

flooding Consistent with the logical STP derived topology

Flooding for unicast is constrained by MAC

learning

Flooding for multicast is constrained by PIM orIGMP snooping

Subscriber STBs isolation needs to be secured

across the entire multicast VLAN

Redundant topologies solved by RSTP Convergence is in range of seconds

L3 multicast might be delivered inefficiently, as

STP is multicast unaware

Multicast convergence dependent on L3 Only one Multicast Router can inject traffic

Redundant Multicast Routers; Failover between

Multicast Routers could potentially take 120s

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TV/VoD Distribution MechanismsMPLS Layer 2 Aggregation Network

IGMP snooping

Designated Router

Querier Router

PIM

PW PW

PWPW

PW PW

IGMP/PIM snooping

MPLS

Overlay Topology:

VPLS, EoMPLS PW

VSI

VSI

VSI

VSI

VSI

VSI

Multicast

Overlay bridged H-VPLS topology Each VPLS node runs a bridged VSI (Virtual

Forwarding Instance

All physical links and bridges are replaced by

EoMPLS tunnels terminating into VFIs

Including the PW between the two switches that

connect to the multicast routers

Topology protection may be addressed at

different levels: MPLS TE FRR:

Protects the VPLS pseudo-wires

Does not protect the VPLS VFIs (the VFIs)

Can lead to 50 ms convergence but ONLY

in case of link failure

STP must be used to avoid the discontinuous

subnet

Still results into several seconds of STPconvergence in case of node failure

The H-VPLS overlay topology implies the

same considerations related to bridged

multicast distribution

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Layer 3 based Ring

architectures

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IP Multicast and Unicast TV and VoD

L3 control plane is now leveraged to run IPmulticasting (potentially together with MPLS)

Forwarding topology created at network layer

Both P2MP and MP2MP services are available toupper layers

No discontiguous subnet problems

Supports multiple injection points

Multiple active trees can co-exist

Dynamic topology driven by PIM

Enables anycasting

Node and service injection point failures solved atthis layer

Enables multicast (receiver driven) distributionmodel

Optimal forwarding

P2MP trees (TV) with PIM SSM

Predictable sub-second convergence (Ciscospecific)

Layer 3 BUS

Multicast

PIM

PIM

PIM

PIM

PIM

PIM

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TV / VoD distribution mechanisms with Layer3 / IP control plane

1PIM-SM

L3

L3L3

L3

L3L3

2 3

4

LocalAd Server or Multilingual Server

IP Multicast Implementation

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Failure Scenarios

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TV Multicast Distribution Failure CasesEthernet/MPLS La er 2 A re ation Network

Topology failures Link

Addressed by MPLS FRR with 50ms convergenceor by STP with1-3s convergence

Node

Addressed only by STP

Otherwise recovered by the DR failover Inconsistency: the layer 2 topology depends onlayer 3 topology convergence

Functional element failures

Querier Router failure

Requires Querier re-election, around 120 s

Designated Router failure Relies on DR failover, that (ex. Cisco) can be veryfast (sub second)

Source and other Headend failures

Anycast-Source possible if they are routed to theDR/QR

Conclusion:

Video Multicast redundancy has functional

DR

Querier Router

PIM

Multicast

Failure

Layer 2 BUS

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The Discontiguous Subnet ProblemFailure scenarios: Box Failure

Assume: R1 IGMP querier: IfR2 does not see IGMP-queries from R1 any more itusually starts being aquerier after 2 x query-

interval (default 120s) Requires significant tuning/

enhancements on PIMrouters to achieve fastfailover (> 50 ms!)

L2 Access Problem ispushed to the L3 Edge

IGMP

R1

R2

MPLS-FRR, backup LSP

PW

PW

PW

PW PW PIM

Logical view

R1

R2

IGMP

DN2

DN1

AN1

AN2

AN3

AN4

IGMP query

IGMP query

VSI

VSI

VSI

VSI

VSI

VSI

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The Discontiguous Subnet ProblemFailure scenarios: Box Failure (II)

Layer-2 subnet is partitionedinto two pieces

Violation of the fundamentalrule that an L2 segmentmust be contiguous

Usually there is unicastcontrol traffic in the multicastVLAN (e.g. RTCP, HTTP,MiddleWare traffic)

Unicast control traffic couldbe blackholed

R1

R2PW

PW

PW

PW PW

Logical view

R1

R2

DN2

DN1AN1

AN2

AN3

AN4

?

VSI

VSI

VSI

VSI

VSI

VSI

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Packet Duplication

N-PE 1 N-PE 2

U-PE A U-PE C

U-PE B

N-PE 1 N-PE 2

U-PE A U-PE C

U-PE B

Traffic-Flow from N-PE1 to U-PE B:Traffic traverses link twice

Ring-VPLSImplementation

1 PIM-SSM

L3

L3L3

L3

L3L3

2 3

4

IP MulticastImplementation

PW/FRR

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Conclusion and

Comparison

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Final Words

Is it Layer 2 or Layer 3 or may be Hybrid ?

There are quite a lot of advantages of Layer 2, cheaper, easier tomaintain

Layer 2 may be the best solution for various other services likeERS, EWS etc..

However as we clearly see there are some clear advantages ofLayer 3 specially for Video traffic

We see these two continue in hybrid way whereas the TV trafficwill be L3 primarily and the other traffic may continue on Layer 2pseudo wires or plane Layer 2 itself.

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Aggregation Network Models

Ethernet/IP Requires a STP operational domain for the

Layer 2 BUS transport

Provides optimal layer 2 multipoint transportthat is topology independent

Supports virtualised Layer 2 services thru

native 802.1q and 802.1ad bridges The scalability of the L2 Service BUS is

dependent on the aggregation network

Service recovery, in average of seconds, isimplemented with RSTP and Fast IGP

convergence

Possibility to aggregate other accessservices as: Mobile RAN, Legacy ATM/FR/

TDM with L2TPV3

Allows different or common administrativedomains for the aggregation and core network

Supports virtualized Layer 2 and 3 services thruMPLS based VPNs

Supports Traffic Engineering thru MPLS TE

mechanisms Service recovery, as low as 50ms, is

implemented with MPLS FRR and Fast IGPconvergence

The scalability for Layer 2 Service BUS isdependent on the network element

Flexibility for aggregating other access servicesas: Mobile RAN, Legacy ATM/FR/TDM with

MPLS AToM

MPLS/IP

S

imilarities

Charac

teristics

Similar Layer 2 and Layer 3 BUS mechanisms Support point to point and multipoint layer 2 and layer 3 transport Support the same residential, business and wholesale broadband services

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