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Automatic Protection Switching. Yaakov (J) Stein CTO RAD Data Communications. Mar 2012. Course Outline General protection switching principles Examples of protection mechanisms SONET/SDH Ethernet linear protection Ethernet ring protection MPLS fast reroute

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automatic protection switching

AutomaticProtection Switching

Yaakov (J) Stein


RAD Data Communications

Mar 2012


Course Outline

  • General protection switching principles
  • Examples of protection mechanisms
    • Ethernet linear protection
    • Ethernet ring protection
    • MPLS fast reroute
general principles
General principles



Traffic types

Network topologies


Protection classes


Protection types



Automatic Protection Switching (APS)

is a functionality of carrier-grade transport networks

is often called resilience

since it enables service to quickly recover from failures

is required to ensure high reliability and availability

APS includes :

  • detection of failures (signal fail or signal degrade) on a working channel
  • switching traffic transmission to a protection channel
  • selecting traffic reception from the protection channel
  • (optionally) reverting back to the working channel once failure is repaired

Automatic means uses (at most) control plane protocols

– no management layer or manual operations needed

some useful references
Some useful references

G.808.1 – generic linear protection

G.808.2 – generic ring protection (not yet written)

G.841 and G.842 – SDH

G.774.3/4/9/10 – SDH protection management

G.870 and G.873.1 – OTN

G.8031 – Ethernet linear protection

G.8032 – Ethernet ring protection

G.8131 – T-MPLS APS

Y.1720 – MPLS

I.630 – ATM

M.495 – analog signal protection

G.781 – clock selection (can be used to protect synchronization)

RFC 4090 – MPLS Fast ReRoute

RFC 6372 – MPLS-TP Survivability Framework

RFC 6378 – MPLS-TP Linear Protection

traffic types
Traffic types

In a network with APS capabilities, there are three types of traffic :

  • protected traffic
    • traffic that may be rapidly switched to protection channel
    • at any time it may be on the working channel or protection channel
  • NonpreemptibleUnprotected Traffic (NUT)
    • noncritical traffic that does not require protection mechanism
    • not affected by protection mechanism
    • somewhat less expensive to customer
  • extra (preemptible) traffic
    • best effort background traffic that runs on protection channel
    • preempted (blocked) when protection channel is needed
    • very inexpensive to customer
network topologies
Network topologies

APS can be defined for any topology with redundant links

e.g., for tree topologies no protection is possible

We will often discuss protection of individual links

However, there are two topologies that are of particular interest :

  • rings
    • protection is natural for rings
      • although there are other reasons for using rings as well
    • rings are so important that protection for other topologies
      • is often called linear protection
  • dense meshes
    • for this topology multiple local bypasses can be preconfigured
    • protection switching is similar to routing change, but faster
      • often called “Fast ReRoute” (FRR)

Protection switching is usually triggered by a failure

although the operator may manually force a protection switch

A failure is declared when a fault condition

persists long enough

for the ability to perform the required function

to be considered terminated

Failures are Signal Fail (SF) or Signal Degrade (SD) (of various types)

and may be :

  • detected by physical layer
  • indicated by signaling (e.g. AIS)
  • detected by OAM mechanisms

When there is no SF or SD, the state is called No Request (NR)

switching time 1
Switching time (1)

SONET/SDH protection switching takes place in under 50 ms

Regarding multiplex section shared protection rings, G.841 states :

The following network objectives apply:

1) Switch time – In a ring with no extra traffic, all nodes in the idle state (no detected failures,

no active automatic or external commands, and receiving only Idle K-bytes), and with less

than 1200 km of fibre, the switch (ring and span) completion time for a failure on a single

span shall be less than 50 ms. On rings under all other conditions, the switch completion

time can exceed 50 ms (the specific interval is under study) to allow time to remove extra

traffic, or to negotiate and accommodate coexisting APS requests.

while for linear VC trail protection, it says :

The following network objectives apply:

1) Switch time – The APS algorithm for LO/HO VC trail protection shall operate as fast as

possible. A value of 50 ms has been proposed as a target time. Concerns have been

expressed over this proposed target time when many VCs are involved. This is for further

study. Protection switch completion time excludes the detection time necessary to initiate the

protection switch, and the hold-off time.

There are similar statements in other clauses as well

switching time 2
Switching time (2)

This 50 ms time has become the golden standard

and new protection schemes are expected to meet this objective

However, studying the literature that lead up to SONET/SDH standards

shows that the objective was to attain the minimum possible time

for the sum of

    • persistent (i.e. non-transient) failure detection
    • speed of light propagation
    • signaling protocol time
    • regaining sync alignment

and 50 ms was the minimum that was considered practical !

Many modern standards have “built in” 50 ms

and much marketing literature boasts “faster than 50 ms”

But there is really nothing special about 50 ms

  • 50 ms gaps in voiced speech are noticeable,
  • but not fatal if infrequent
  • 50 ms of data at high rates can not be stored and later forwarded
  • timing circuits can withstand much more than 50 ms without clock
protection classes
Protection classes

It is useful to distinguish two different protection classes

  • path protection (AKA trail protection, end-to-end protection)
    • when a failure is detected on the end-to-end path

we switch to an alternative end-to-end path

    • the failure is usually detected by end-to-end OAM
  • local protection (AKA local restoration, SNC protection, bypass, detour)
    • we protect individual network elements, links, or groups of same
    • when such an entity fails

only that local entity is bypassed

    • the failure may be detected by link OAM or physical layer means
aps entities 1
APS entities (1)

The following entities are important in APS

  • working channel – channel used when no failure exists
  • protection channel – channel used when a failure exists
  • head-end – entity transmitting data to working/protection channel
  • tail-end – entity receiving data from the working/protection channel

Note: we will usually consider traffic to be bidirectional

so that the head-end for one direction

is the tail-end for the opposite direction

working channel

protection channel



aps entities 2
APS entities (2)
  • Bridge – function at head-end that connects traffic (including extra traffic) to the working and protection channels
  • Selector –function at tail-end that extracts traffic (perhaps extra traffic) from the working or protection channel
  • APS signaling channel – channel used to communicate between head-end and tail-end for APS purposes
  • Trail termination –function responsible for failure detection

including injection and extraction of OAM

working channel





protection channel

signaling channel

revertive operation
Revertive operation

Reversion means returning to use the working channel

after the failure has been rectified

Protection mechanisms can be revertive or nonrevertive

Revertive mechanisms may be preferable

  • when the working channel has better performance (free BW, BER, delay)
  • when there are frequent switches (easier to manage)
  • when there is extra traffic

but nonrevertive also has advantages

  • only one service disruption due to protection switching
  • may be simpler to implement
uni bi directional

We will usually consider bidirectional traffic

but even then the failures can be uni- or bi- directional

and for unidirectional failures there can be uni- or bi- directional switching









working channel

working channel

protection channel in use

protection channel in use

working channel

working channel

protection channel in use

protection channel

uni bi directional switching
Uni- / bi- directional switching

Unidirectional switching may be advantageous

  • for 1+1 - faster and no signaling channel is needed
  • no unnecessary service disruption for direction without failure
  • higher chance of protection under multiple failures
  • easier to implement for local protection
  • maintains extra traffic in direction without failure

But bidirectional may be preferable

  • easier management since directions traverse same network elements
  • does not disrupt delay balance between direction
  • may simplify repair since failed spans are unused
protection types
Protection types

We distinguish several different protection types

  • 1+1
  • 1:1
  • 1:n
  • m:n
  • (1:1)n

Each type has its applicability, advantages, and disadvantages

and there are trade-offs between

  • simplicity
  • BW consumption
  • protection switch time
  • signaling requirements
1 1 protection
1+1 protection

Simplest and fastest form of protection

but wasteful - only 50% of actual physical capacity is used

Head-end bridge always sends data on both channels

Tail-end selector chooses channel to use (based on BER, dLOS, etc.)

For unidirectional1+1 switching there is no need for APS signaling

If non-revertive

there is no distinction between working and protection channels

channel A

channel B

1 1 protection19
1:1 protection

Head-end bridge usually sends data on working channel

When failure detected it starts sending data over protection channel

and tail-end needs to select the protection channel

When not in use, protection channel can be used for extra traffic

However, since failure is detected by tail-end, APS signaling is needed

Protection channel should have OAM running to ensure its functionality

working channel

extra traffic

protection channel

APS signaling

1 n protection
1:n protection

One protection channel is allocated for n working channels

Only can protect one working channel at a time

but improbable that more than 1 working channel will simultaneously fail

Only 1/(n+1) of total capacity is reserved for protection

working channels

protection channel

m n protection
m:n protection

To enable protection of more than 1 channel

m protection channels are allocated for n working channels (m < n)

m simultaneous failures can be protected

Less protection capacity dedicated than for n times 1:1

When failure detected,

1 of the m protection channels need to be assigned and signaled

High complexity but conserves resources

working channels

protection channels

1 1 n protection
(1:1)n protection

This is like n times 1:1 but the n protection channels share bandwidth

Only 1 failed working channel can be protected

This is different from 1:n since

  • n protection channels are preconfigured
  • n working channels need not be of the same type

Protection bandwidth must be at least that of the largest working channel

aps algorithm
APS algorithm

We have seen that protection switching is a tricky business

So it is not surprising that network elements that support APS

run an APS algorithm

This algorithm inputs :

  • configuration (protection type, revertive?, available channels, …)
  • failure indications (NR, SF, SD)
  • operator commands
  • APS signaling (more on that soon)

and makes switching decisions

The algorithm maintains state information for head-end and tail-end

APS algorithms are detailed in standards documents


Not every failure event / operator command results in a protection switch

For example

in 1:n protection the protection channel may already be in use !

Conflicts are resolved by assigning priorities to events/commands

When an event is detected or a command received

the APS algorithm will not act

if an event/command or equal or higher priority is already in effect

True failure conditions usually have higher priority than manual commands


Even failure events with priority are not acted upon immediately

to do so would cause unnecessary switches after transient defects

The APS algorithm may maintains several timers, such as

  • Holdoff timers
    • the time between detection of a SF or SD event

and the APS algorithm acting upon this even

    • the algorithm usually used is called “peek twice”

i.e., the condition is checked again after the timer expires

  • Wait To Restore timer
    • for revertive switching, the time between detection of the failure being cleared and the APS algorithm acting upon this event
    • also used in SDH optimized bidirectional 1+1 (nonrevertive)
  • Guard timer
    • for rings – blockout time during which APS messages are ignored (since they may be old and outdated)
aps signaling
APS signaling

In all types except unidirectional 1+1, some APS signaling is needed

APS signaling is used to synchronize between head-end and tail-end

It is critical that head-end and tail-end always be in the same state

Example messages include :

  • No Request (NR)
  • by tail-end to inform head-end of Signal Failure (SF)
  • by head-end to confirm the event’s priority
  • by head-end to report the particular protection channel
  • by head-end to inform tail-end of Reverse (bidirectional) Request (RR)
  • by tail-end after failure cleared to Wait To Restore (WTR)
  • by tail-end after failure cleared to Do Not Revert (DNR) for nonrevertive
aps signaling phases
APS signaling phases

When APS signaling is used, it needs to be as rapid as possible

Depending on the scenario it may be

  • 1-phase tailhead (fastest)
    • tail-end informs head-end of failure
    • both ends uniquely know the protection channel to be used
    • only for 1+1 and unidirectional-(1:1)n (including 1:1)
  • 2-phase 1) tailhead 2) headtail
    • tail-end informs head-end of failure
    • head-end signals that it has switched to protection channel
    • not for bidirectional-1:n or m:n
  • 3-phase 1) tailhead 2) headtail 3) tailhead (slowest)
    • works for all protection types (including m:n)
examples of 1 phase
Examples of 1-phase

Example of when 1-phase signaling is possible is 1:1 or (1:1)n

1. upon detection of failure the tail-end sends SF to the head-end

and immediately changes its selector (blind switch)

upon receipt the head-end changes the bridge setting

(no priority is checked)

1-phase can also be used for bidirectional 1:1

1. upon detection of failure the tail-end sends SF to the head-end

and immediately changes both its selector and bridge

upon receipt the head-end changes its bridge and selector

example of 2 phase
Example of 2-phase

2-phase is useful for unidirectional 1:n with priority checking

1. upon detection of failure the tail-end sends SF to the head-end

but does not change its selector

2. the head-end checks priority

sends confirmation to tail-end (with identity of working channel)

the bridge setting is changed

3. the tail-end changes its selector

example of 3 phase
Example of 3-phase

3-phase signaling is imperative for bidirectional 1:n

1. upon detection of failure the tail-end sends SF to the head-end

but does not change its selector

2. the head-end checks priority, and sends confirmation to tail-end

head-end changes its bridge setting

and also sends a reverse request

3. the tail-end changes selector

checks priority and sends confirmation to head-end

tail-end changes its bridge setting (as head-end of opposite direction)

head-end receives confirmation and changes its selector

for g 805 buffs

protected trail



For G.805 buffs

to add 1+1 trail protection to a trail - expand a trail termination function

we use a special transport processing function - the protection switch

the unprotected TTs report status to the protection switch

sonet protection
SONET protection ?

SONET/SDH networks need to be highly reliable (five nines)

Down-time should be minimal (less than 50 msec)

So systems must repair themselves (no time for manual intervention)

Upon detection of a failure (dLOS, dLOF, high BER)

the network must reroute traffic (protection switching)

from working channel to protection channel

SDH APS is unidirectional

SDH APS may be revertive

working channel

protection channel

tail-end NE

head-end NE

sonet sdh layers

















line (MS section)





SONET/SDH layers

Between regenerators there are sections (regenerator sections)

Between ADMs there are lines (multiplex sections)

Between path terminations there are paths

Protection can be at OC-n level (different physical fibers)

or at STM/VC level

or end-to-end path (trail protection)

line aps
Line APS

90 columns

Synchronous Payload Envelope

3 rows

TOH consists of

  • 3 rows of section overhead - frame sync, trace, EOC, …
  • 6 rows of line overhead - pointers, SSM, FEBE, and

Line APS signaling uses bytes K1 and K2

9 rows

9 rows

6 rows


ho path aps












POH is responsible for type, status, path performance monitoring, VCAT, trace

HO Path APS signaling uses 4 MSBs of byte K3

lo path aps





VC OH is responsible for

Timing, PM, REI, …

LO Path APS signaling is

4 MSBs of byte K4










how does it work

head-end bridge

tail-end bridge

working channel

protection channel

signaling channel

How does it work?

Head-end and tail-end NEs have bridges (muxes)

Head-end and tail-end NEs maintain bidirectional signaling channel

Signaling is contained in K bytes of protection channel

For line APS

  • K1 – tail-end status and requests
  • K2 – head-end status
linear 1 1 protection
Linear 1+1 protection

Can be at OC-n level (different physical fibers)

or at STM/VC level (SubNetworkConnection Protection)

or end-to-end path (called trail protection)

Head-end bridge always sends data on both channels

Tail-end chooses channel to use based on BER, dLOS, etc.

No need for signaling

If non-revertive

there is no distinction between working and protection channels

working channel

protection channel

tail-end NE

head-end NE

linear 1 1 protection40
Linear 1:1 protection

Head-end bridge usually sends data on working channel

When tail-end detects failure it signals (using K1) to head-end

Head-end then starts sending data over protection channel

When not in use

protection channel can be used for (discounted) extra traffic

(pre-emptible unprotected traffic)

May be at any layer (but only OC-n level protects against fiber cuts)

working channel

extra traffic

protection channel

linear 1 n protection

working channels

protection channel

Linear 1:N protection

In order to save BW

we allocate 1 protection channel for every N working channels

N limited to 14

4 bits in K1 byte from tail-end to head-end

  • 0 protection channel
  • 1-14 working channels
  • 15 extra traffic channel
two fiber vs four fiber rings
Two fiber vs. Four-fiber rings

Ring based protection is popular in North America (100K+ rings)

Full protection against physical fiber cuts

Simpler and less expensive than mesh topologies

Protection at line (multiplexed section) or path layer

Four-fiber rings

fully redundant at OC level

can support bidirectional routing at line layer

Two-fiber rings

support unidirectional routing at line layer

2 fibers in opposite directions

unidirectional vs bidirectional












Unidirectional vs. bidirectional

Unidirectional routing

working channel B-A same direction (e.g. clockwise) as A-B

management simplicity: A-B and B-A can occupy same timeslots

Inefficient: waste in ring BW and excessive delay in one direction

Bidirectional routing

A-B and B-1 are opposite in direction

both using shortest route

spatial reuse: timeslots can be reused in other sections

upsr vs blsr ms spring



Path switching

Line switching





Of all the possible combinations, only a few are in use

Unidirectional (routing) Path Switched Rings

protects tributaries

extension of 1+1 to ring topology

Bidirectional (routing) Line Switched Rings (two-fiber and four-fiber versions)

called Multiplex Section Shared Protection Ring in SDH

simultaneously protects all tributaries in STM

extension of 1:1 to ring topology


Working channel is in one direction

protection channel in the opposite direction

All path traffic is “added” in both directions (1+1)

decision as to which to use is made at drop point (no signaling)

Normally non-revertive, so effectively two diversity paths

Good match for access networks

1 access resilient ring

less expensive than fiber pair per customer

Inefficient for core networks

no spatial reuse

every signal in every span

in both directions

node needs to continuously monitor

every tributary to be “dropped”

2 rings



Switch at line level – less monitoring

When failure detected tail-end NE signals head-end NE

Works for unidirectional/bidirectional fiber cuts, and NE failures

Two-fiber version

half of OC-N capacity devoted to protection

only half capacity available for traffic

Four-fiber version

full redundant OC-N devoted to protection

twice as many NEs as compared to two-fiber


2 rings


recovery from unidirectional fiber cut


The original Spanning Tree Protocol automatically removed loops

from arbitrary networks (with loops)

However, its convergence was very slow (about a minute)

STP can not be used as a protection mechanism

since its reconvergence time is very long

due to a cumbersome protocol

and long holdoff timer settings

An evolutionary update called Rapid STP 802.1w

was incorporated into 802.1D-2004 clause 17

that converges in about the same time as STP

but can reconverge after a topology change in less than 1 second

RSTP can be used to detect failures and reconverge

and thus can be used as a primitive protection mechanism

However, the switching time will be many tens of ms to 100s of ms

use of lag
Use of LAG

Ethernet “link aggregation” (AKA bonding, Ethernet trunk, inverse mux, NIC teaming)

enables bonding several ports together as single uplink

Defined by 802.3ad task force and folded into 802.3-2000 as clause 43

Binding of ports to Link Aggregation Groups (LAGs) distributed via

Link Aggregation Control Protocol (LACP)

LACP uses slow protocol frames (up to 5 per second)

Links may be dynamically added/removed from LAG

and LACP continuously monitors to detect if changes needed

Upon link failure LAG delivers traffic at a reduced rate

Thus LAG can be used as a primitive protection mechanism

When used this way it is called worker/standby or N+N mode

The restoration time will be on the order of 1 second

g 8031

Q9 of SG15 in the ITU-T is responsible for protection switching

In 2006 it produced G.8031 Linear Ethernet Protection Switching

G.8031 uses standard Ethernet formats, but is incompatible with STP

The standard addresses

  • point-to-point VLAN connections
  • SNC (local) protection class
  • 1+1 and 1:1 protection types
  • unidirectional and bidirectional switching for 1+1
  • bidirectional switching for 1:1
  • revertive and nonrevertive modes
  • 1-phase signaling protocol

G.8031 uses Y.1731 OAM CCM messages in order to detect failures

G.8031 defines a new OAM opcode (39) for APS signaling messages

Switching times should be under 50 ms (only holdoff timers when groups)

g 8031 signaling
G.8031 signaling

The APS signaling message looks like this :

        • regular APS messages are sent 1 per 5 seconds
        • after change 3 messages are sent at max rate (300 per sec)


  • req/state identifies the message (NR, SF, WTR, SD, forced switch, etc)
  • prot. type identifies the protection type (1+1, 1:1, uni/bidirectional, etc.)
  • requested and bridged signal identify incoming / outgoing traffic

since only 1+1 and 1:1 they are either null or traffic (all other values reserved)













prot. type


requested sig


bridged sig






g 8031 1 1 revertive operation
G.8031 1:1 revertive operation

In the normal (NR) state :

  • head-end and tail-end exchange CCM (at 300 per second rate)

on both working and protection channels

  • head-end and tail-end exchange NR APS messages

on the protection channel (every 5 seconds)

When a failure appears in the working channel

  • tail-end stops receiving 3 CCM messages on working channel
  • tail-end enters SF state
  • tail-end sends 3 SF messages at 300 per second on the APS channel
  • tail-end switches selector (bi-d and bridge) to the protection channel
  • head-end (receiving SF)switches bridge (bi-d and selector) to protection channel
  • tail-end continues sending SF messages every 5 seconds
  • head-end sends NR messages but with bridged=normal

When the failure is cleared

  • tail-end leaves SF state and enters WTR state (typically 5 minutes, 5..12 min)
  • tail-end sends WTR message to head-end (in nonrevertive - DNR message)
  • tail-end sends WTR every 5 seconds
  • when WTR expires both sides enter NR state
ethernet ring aps
Ethernet ring APS




ethernet rings
Ethernet rings ?

Ethernet has become carrier grade :

  • deterministic connection-oriented forwarding
  • OAM
  • synchronization

The only thing missing to completely replace SDH is ring protection

However, Ethernet and ring architectures don’t go together

  • Ethernet has no TTL, so looped traffic will loop forever
  • STP builds trees out of any architecture – no loops allowed

There are two ways to make an Ethernet ring

  • open loop
    • cut the ring by blocking some link
    • when protection is required - block the failed link
  • closed loop
    • disable STP (but avoid infinite loops in some way !)
    • when protection is required - steer and/or wrap traffic
ethernet ring protocols
Ethernet ring protocols

Open loop methods

  • G.8032 (ERPS)
  • rSTP (ex 802.1w)
  • RFER (RAD)
  • ERP (NSN)
  • RRST (based on RSTP)
  • REP (Cisco)
  • RRSTP (Alcatel)
  • RRPP (Huawei)
  • EAPS (Extreme, RFC 3619)
  • EPSR (Allied Telesis)
  • PSR (Overture)

Closed loop methods

  • RPR (IEEE 802.17)
  • CLEER and NERT (RAD)
g 8032

Q9 of SG15 produced G.8032 between 2006 and 2008

G.8032 is similar to G.8031

  • strives for 50 ms protection (< 1200 km, < 16 nodes)
    • but here this number is deceiving as MAC table is flushed
  • standard Ethernet format but incompatible with STP
  • uses Y.1731 CCM for failure detection
  • employs Y.1731 extension for R-APS signaling (opcode=40)
  • R-APS message format similar to APS of G.8031

(but between every 2 nodes and to MAC address 01-19-A7-00-00-01)

  • revertive and nonrevertive operation defined

However, G.8032 is more complex due to

  • requirement to avoid loop creation under any circumstances
  • need to localize failures
  • need to maintain consistency between all nodes on ring
  • existence of a special node (RPL owner)

G.8032v1 defines the Ring Protection Link (RPL)

as the link to be blocked (to avoid closing the loop) in NR state

One of the 2 nodes connected to the RPL

is designated the RPL owner

Unlike RFER

  • there is only one RPL owner
  • the RPL and owner are designated before setup
  • operation is usually revertive

All ring nodes are simultaneously in 1 of 2 modes – idle or protecting

  • in idle mode the RPL is blocked
  • in protecting mode the failed link is blocked and RPL is unblocked
  • in revertive operation

once the failure is cleared the block link is unblocked

and the RPL is blocked again

g 8032 revertive operation
G.8032 revertive operation

In the idle state :

  • adjacent nodes exchange CCM at 300 per second rate (including over RPL)
  • exchange NR RB (RPL Blocked) messages in dedicated VLAN every 5 seconds (but not over RPL)
  • R-APS messages are never forwarded

When a failure appears between 2 nodes

  • node(s) missing CCM messages peek twice with holdoff time
  • node(s) block failed link and flush MAC table
  • node(s) send SF message (3 times @ max rate, then every 5 sec)
  • node receiving SF message will check priority and unblock any blocked link
  • node receiving SF message will send SF message to its other neighbor
  • in stable protecting state SF messages over every unblocked link

When the failure is cleared

  • node(s) detect CCM and start guard timer (blocks acting on R-APS messages)
  • node(s) send NR messages to neighbors (3 times @ max rate, then every 5 sec)
  • RPL owner receiving NR starts WTR timer
  • when WTR expires RPL owner blocks RPL, flushes table, and sends NR RB
  • node receiving NR RB flushes table, unblocks any blocked ports, sends NR RB
g 8032 2010

After coming out with G.8032 in 2008 (G.8032v1)

the ITU came out with G.8032-2010 (G.8032v2) in 2010

This new version is not backwards-compatible with v1

but a v2 node must support v1 as well (but then operation is according to v1)

Major differences :

  • 2 designated nodes – RPL owner node and RPL neighbor node

and for optional flush-optimization “next neighbor node”

  • significant changes to
    • state machine
    • priority logic
    • commands (forced/manual/clear) and protocol
  • new Wait To Block timer
  • supports more general topologies (sub-rings)
    • ladders (For Further Study in v1)
    • multi-ring
  • ring topology discovery
  • virtual channel based on VLAN or MAC address













rpr 802 17



RPR – 802.17

Resilient Packet Rings

  • are compatible with standard Ethernet, but different frame format
  • are robust (lossless, <50ms protection, OAM)
  • are fair (based on client throttling)
  • support QoS (3 classes – A, B, C)
  • are efficient (full spatial reuse)
  • are plug and play (automatic station autodiscovery)
  • extend use of existing fiber rings

counter-rotating add/drop ringlets, running

  • SONET/SDH (any rate, PoS, GFP or LAPS) or
  • “packetPHY” (1 or 10 Gb/s ETH PHY)

developed by 802.17 WG

based on Cisco’s Spatial Reuse Protocol (RFC 2892)

ringlet selection

basic rpr queuing










Basic RPR queuing

traffic going around ring

placed into internal buffer

in dual-transit queue mode

placed into 1 of 2 buffers

according to service class

sent according to fairness

traffic for local sink

placed in output buffer

according to service class

traffic from local source

sent according to fairness

first sent to ringlet selection

Primary/Secondary Transit Queue

rpr service classes
RPR service classes

RPR defines 3 main classes

  • class A : real time (low latency/FDV)
  • class B : near real time (bounded predictable latency/FDV)
  • class C : best effort
rpr class use
RPR Class use

A0 ring BW is reserved – not reclaimed even if no traffic

in dual-transit queue mode:

  • class A frames from the ring are queued in PTQ
  • class B, C in STQ

priority for egress

  • frames in PTQ
  • local class A frames
  • local class B (when no frames in PTQ)
  • frames in STQ
  • local class C (when no PTQ, STQ, local A or B)


class A have minimal delay

class B have higher priority than STQ transit frames, so bounded delay/FDV

classes B and C share STQ, so once in ring have similar delay

rpr protection
RPR - protection

rings give inherent protection against single point of failure

RPR specifies 2 mechanisms

  • steering
  • wrapping (optional)

(implementations may also do wrapping then steering)

steering info


nert and cleer

New Ethernet Ring Technology / Closed Loop Encapsulated Ethernet Ring

Similar to RPR but uses real Ethernet format

NERT and CLEER distinguish between

  • ring nodes
  • switches connected to ring nodes

Traffic in ring is MAC-in-MAC encapsulated

  • External MACs are of ring node
  • Internal MACs are original

Unexpected external MACs discarded

External MACs learned as in 1ah

Ring nodes forward according to table

NERT floods, CLEER never floods

Protection switch only involves changing table

so service restoration is fast

ring nodes


mpls fast reroute
MPLS fast reroute


RFC 4090

ip frr

True protection mechanisms do not exist for connectionless IP

In practice, routing protocols discover breaks and recalculate routes

but this usually takes a long time

Link-state IGPs detect link-down state using hellos

for OSPF - typically every 10 sec, and detection after 40 sec

and then Dijkstra algorithm avoids the failed link

BFD can be used to speed up the detection


  • the information still has to be propagated further (seconds?)
  • and FIBs updated (100s of ms)

Various IP Fast ReRoute (IP FRR) mechanisms have been proposed

but true protection is best done at the MPLS level

mpls fast reroute68
MPLS fast reroute

RSVP-TE enables MPLS traffic engineering by fine control over placement

specifies explicit path using information gathered from IGP

resources may be reserved at LSRs along the way

RFC 4090 defines extensions to RSVP-TE – Fast ReRoute (FRR)

LSRs along the path preconfigure local bypasses (detours)

Upon detection of failure by

  • BFD (specified in microseconds, typically 10s of ms) or
  • RSVP hellos (RFC default is 5 ms) or
  • RESV / PATH messages (driven by IGP)

upstream LSR simply enables the detour

Since this is a local action, it should be fast

RFC 4090 only discusses adding FRR to RSVP-TE network

but its use with LDP is possible if there is a single label generator

not discussed in RFC 4090

plrs and mps
PLRs and MPs

A fundamental entities in MPLS FRR are

  • Point of Local Repair (PLR)
  • Merge Point (MP)

A PLR is the LSR before the failed element (link or node)

All LSRs except the egress LER can be PLRs

The PLR is solely responsible for the FRR (no explicit APS signaling)

During path setup, potential PLRs create detours towards the egress LER

A MP is the LSR where the detour rejoins the LSP

All LSRs except the ingress LER can be MPs








RFC 4090 defines two different protection methods

Usually one or the other is employed in a given network

One-to-one backup

  • each LSP protected separately
  • detour LSP created for each LSP at each potential PLR
  • no labels pushed

Facility backup

  • backup tunnel for multiple LSPs
  • bypass tunnel created at each potential PLR
  • uses label stacking





nhop and nnhop

MPLS FRR can bypass a failed link or a failed node

In order to bypass a single failed link

we need an alternative path to the next hop (NHOP)

In order to bypass a single failed node, we need an alternative path to the next next hop (NNHOP)





mpls tp aps

RFC 6372 (MPLS-TP Survivability Framework)

RFC 6378 (MPLS-TP Linear Protection)


mpls tp resilience
MPLS-TP resilience

Since it strives to be a carrier-grade transport network

TP has strong protection switching requirements

APS has been almost as contentious issue as OAM

and indeed the arguments are inter-related

RFC 6372 gives a general framework

and differentiates between

  • linear
  • shared-mesh and
  • ring protection
linear protection
Linear protection
  • from RFC 6378 (ex draft-ietf-mpls-tp-linear-protection)
  • 1+1, 1:1, 1:n and uni/bidi are supported
  • APS signaling protocol (for all modes except 1+1 uni)
  • is single-phase
  • and called the Protection State Coordination protocol
  • PSC messages are sent over the protection channel
  • APS messages are sent over the GACh with a single channel type
  • message functions identified by a request field
  • 6 states: normal, protecting due to failure, admin protecting,
  • WTR, protection path unavailable, DNR
  • when revertive, a WTR timer is used
psc message format
PSC message format

GAL Label (13) TC S=1 TTL


Request : NR, SF, SD, manual switch, forced switch, lockout, WTR, DNR

PT = Protection Type : uni 1+1, bidi 1+1, bidi 1:1/1:n

R = Revertive

FPath = which path has fault Path = which data path is on protection channel

0001 VER 00000000 PSC channel type


Ver Request PT R Res FPath Path


TLV Length Res

Optional TLVs

psc control logic states
PSC control logic states

Normal state - no trigger events reported

Unavailable state - protection path is unavailable

Protecting failure state –

traffic is being transported on the protection path

Protecting administrative state –

operator issued command switching traffic to protection path

Wait-to-Restore state - recovering from working path SF/SD

WTR timer not up

Do-not-Revert state - recovered from a protecting state

but operator has configured DNR

psc local requests
PSC local requests

In order from highest to lowest priority :

1. Clear (operator command)

2. Lockout of protection (operator command)

3. Forced Switch (operator command)

4. Signal Fail on protection (OAM / control-plane / server indication)

5. Signal Fail on working (OAM / control-plane / server indication)

6. Signal Degrade on working (OAM / control-plane / server indication)

7. Clear Signal Fail/Degrade (OAM / control-plane / server indication)

8. Manual Switch (operator command)

9. WTR Expires (WTR timer)

10. No Request (default)

linear protection itu style
Linear protection – ITU style

from draft-zulr-mpls-tp-linear-protection-switching

Similar to previous, but uses Y.1731/G.8031 format (no surprise!)

GAL Label (13) TC S=1 TTL


0001 VER 00000000 allocated channel type


















ring protection
Ring protection

once again there were two drafts, both supporting

p2p and p2mp, wrapping and steering, link/node failures

draft-ietf-mpls-tp-ring-protection (not yet RFC)

Between any 2 LSRs can define a Sub-Path Maintenance Entity

So between 2 LSRs on a ring there are 2 SPMEs –

we define 1 as the working channel and 1 as the protection channel

Now we re-use the linear protection mechanisms, including the PSC protocol


Both counter-rotating rings carry working and protection traffic

The bandwidth on each ring is divided

X BW is dedicated to working traffic and Y dedicated to protection traffic

The protection bandwidth of one ring is used to protect the other ring

Each node should have information about the sequence of ring nodes

MPLS-TP Ring Protection Switching is G.8032-like, but forwards non-NR msgs