Adapting MPLS Fast Reroute to Create
Resilient Rings
Ahmed Abdelhalim
Foundry Networks

www.mpls.jp

Ring Topology:
Multiple Scenarios, Multiple Technologies

Backbone

BackboneAccessCost effective design– To be used when reducing fiber cost is paramount– Access or backbone applications

Multiple technologies evolved over the years––––FDDISONET Ring APS / SDH Ring MSP
Vendor proprietary ring resiliency for Ethernet rings
IEEE 802.17 RPR

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Goals of a Good Ring Resiliency Mechanism
Highly survivable– Distributed control with no single point of failure

Sub-second resiliency– Ideally sub-50 ms resiliency

Provides traffic path optimization after link failure
ScalableCan be easily applied/extended to multiple interconnected rings
Provides protection to upper layer services:– Routed services, VPN services, multicast-based services, etc.

Efficient bandwidth use (spatial reuse)
Simple to manage

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Fast Reroute & Ring Topologies
What Makes Rings Different?

Fast Reroute mainly envisioned for rich meshed / partially meshedtopologies– Potential for several backup paths

A ring topology does not offer rich connectivity. Only:– Primary path in one direction– Backup path has to be the other way around the ring

An FRR implementation with reasonable adaptation can efficiently
offer resiliency for ring-based topologies

42Why MPLS Fast Reroute for Rings?
Standards-based mechanism
Well established in the industry– Especially, for meshed topologies

Inherent rapid traffic protection
MPLS experiencing increasing popularity– Increasing demand for MPLS-capable hardware

Avoid running additional ring-specific resiliency
mechanisms– Simplify whenever/wherever possible

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Which Flavor of FRR?

Detour LSPs can easily allow for an implementation thatoffers– automatic protection– optimal forwarding paths under local repair and after path
optimization

By extension, automatic bypass tunnels can also beused for higher scalability
Question: can FRR be used to efficiently provide ringprotection?
YES

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Optimal Calculation of Backup Paths Aroundthe RingiLER
eLER

iLER

eLER??

General Direction

LegendiLER = ingress LER
eLER = egress LER= Protected LSP= Backup Path (detour/bypass)

CSPF should be used

What may be a good heuristic for meshed topologies may notbe good for rings– For a mesh topology, trying to find a backup path that flows towards downstream nodes of theprotected path may work well
– Yet for a ring, a backup path will have to flow in a direction opposite to the protected path
– Excluding upstream nodes/links may make sense for loop avoidance in a mesh topology– Yet, upstream nodes/links must not be excluded for CSPF on a ring

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Optimal Calculation of Backup Paths Aroundthe Ring – cont.
iLER

CSPF should optimize a detour (case B) toavoid awkward forwarding patterns (case A)
in case of failure
XCase AeLERiLER

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Case BeLER– CSPF primary goal should be reaching eLER,
not merely merging at NHop/NNHop– CSPF should be recursive in finding the optimal
merge point (MP): NHop, NNHop, …
– eLER becomes the optimal merge point (MP) for
the detour

Bypass tunnels with automatic calculation
can offer the scalability of many-to-one
protection– Algorithm must create an optimized bypass
tunnel for each group of LSPs targeting the sameeLER*!

* To avoid the undesirable case A!
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Reducing Overhead and Increasing
ScalabilityiLER

In theory, an LSP spanning N nodes canresult in N-1 detours/bypass tunnels (case A)
For detours, a ring offers many opportunities
for straight forward detour merging
Case A– E.g., at each MP, merge detours having same
SESSION, SENDER_TEMPLATE, outgoinginterface, and next hop LSR*eLERiLER

MPMerging can dramatically simplify the overall
picture (case B)

MP– Reduce signaling, state, and cross connects

MP

Case B

Reduce to practically one large detour
spanning all nodes involvedeLER
LegendMP = Merge Point* An example of path-specific merging
9Reducing Overhead and Increasing
Scalability – cont.

iLEReLER

For bypass tunnels, many LSPs can beprotected via the same bypass tunnel– On a link, protected LSPs targeting thesame eLER can be mapped to one
bypass tunnel

All LSPs targeting eLER on thislink are protected by one
common bypass tunnel

105Ensuring Bandwidth Availability in Case ofFailures
Straight Forward Approach
Rely on spare capacity engineered into the ring
Do not request bandwidth protection for the detours/bypass
tunnels– It can mislead CSPF

A Variant of Above: A Differentiated Services Approach
Business traffic has higher priority than residential traffic
In case of failure, when capacity is reduced, business traffic isguaranteed* a percentage of the capacity at the expense ofresidential traffic* Through traffic management / scheduling
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Putting It All Together
What is the End Result Here?
iLER

iLER

MPPathErr

iLER

MPMP
Fault
Normal ConditioneLER

WrapeLER

Fault
SteereLER

A plug-n-play resiliency mechanism– All backup paths set up automaticallyRapid, sub-50 ms traffic protection via local repair Î ring wrap
Provides traffic path optimization after link failure– “Tunnel locally repaired” notification causes iLER to switch to the more optimal path Î ringsteer

Provides protection to all MPLS-based services:– IP routing, pseudo-wire, VPLS, BGP/MPLS VPN, …

Combine with emerging MPLS OAM for improved visibility and manageability

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Pros & Cons
Pros
Reduce network complexity– One technology for enabling services, and offering resiliency– Avoid excessive and confusing resiliency at different layersNo lower layer technology operating in a ships in the night modeNo need to worry about interaction between those layers– Simplify training and operations
No need to learn another L-2 mechanism – like RPR, or RSTP, …

Operates on any MPLS capable hardware– No need for additional costly hardware provisions

Reduce TCO– TCO ≅ k . Complexity

More scalable and viable and than many would think
ConsRequires set up of the LSPs – not the backup paths– For large number of nodes (N), N2 in a worst case*: each node shares service
instances with every other node– An external tool, or mesh membership discovery can help rapidly set up thoseLSPs

Each LSP requires maintaining state in the protected path direction, andits opposite direction* Classical MPLS problem. Not specific to the resiliency mechanism used
* Using advertisement of membership in IGP (draft-ietf-ccamp-automesh)13

Scoping the Viability of this Approach
The Metro Network

MTU
Metro Ethernet
NetworkOverlay• Pseudowire• H-VPLS• BGP/MPLS VPN

MTUUse• VLAN
• H-VPLS Spoke (Q-in-Q / PW)• .1Q-based Multi-VRF

Attractive solution for Metro Ethernet Networks– Can be easily extended to span multiple rings

Contrary to common perceptions, an optimized
implementation can offer– Well groomed signaling, paths, cross connects– Optimized paths and backup paths– Backup paths contained within their own rings Î simplifies
manageability
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Scoping the Viability of this Approach
Beyond the Metro Network

Backbone
Metro Ethernet
Network

Metro Ethernet
Network

It is recommended to break up the LSP and resiliency domains– Better manageability– Allow for autonomous control within the different markets/regions

Choice of backbone resiliency mechanism is independent of themetro– May use same approach in case of rings

Utilize emerging services that inherently support inter-domain
connectivity– MS-PW, BGP/MPLS VPN, inter-AS VPLS

15Summary
MPLS Fast Reroute is a powerful and viable approach for providing ringresiliency– Provides rapid protection and optimization equivalent to the well known “Wrapthen Steer” behavior– Can be set up with minimum effort – using today’s automatic detour/bypass
tunnels– Far scalable than many would think

Simplify overall design and operation– No need for an additional resiliency layer for handling ring resiliency– No need for an additional OAM layer – rely on MPLS OAM

Reduce cost
– Many service providers invest in MPLS capable hardware to offer next
generation services– No need for special hardware designed specifically for addressing ring resiliency

168Thank You!

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