Network Working Group Pierre Francois
Internet-Draft Olivier Bonaventure
Intended status: Standards Track Universite catholique de Louvain
Expires: August 28, 2008 Mike Shand
Stewart Bryant
Stefano Previdi
Cisco Systems
February 25, 2008
Loop-free convergence using oFIB
draft-ietf-rtgwg-ordered-fib-02
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Copyright (C) The IETF Trust (2008).
Abstract
This draft describes a mechanism for use in conjunction with link
state routing protocols which prevents the transient loops which
would otherwise occur during topology changes. It does this by
correctly sequencing the FIB updates on the routers.
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This mechanism can be used in the case of non-urgent link or node
shutdowns and restarts or link metric changes. It can also be used
in conjunction with a FRR mechanism which converts a sudden link or
node failure into a non-urgent topology change. This is possible
where a complete repair path is provided for all affected
destinations.
After a non-urgent topology change, each router computes a rank that
defines the time at which it can safely update its FIB. A method for
accelerating this loop-free convergence process by the use of
completion messages is also described.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The required FIB update order . . . . . . . . . . . . . . . . 4
2.1. Single Link Events . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Link Down / Metric Increase . . . . . . . . . . . . . 4
2.1.2. Link Up / Metric Decrease . . . . . . . . . . . . . . 5
2.2. Multi-link events . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Router Down events . . . . . . . . . . . . . . . . . . 6
2.2.2. Router Up events . . . . . . . . . . . . . . . . . . . 6
2.2.3. Linecard Failure/Restoration Events . . . . . . . . . 6
3. Applying ordered FIB updates . . . . . . . . . . . . . . . . . 6
3.1. Deducing the topology change . . . . . . . . . . . . . . . 6
3.2. Deciding if ordered FIB updates applies . . . . . . . . . 7
4. Computation of the ordering . . . . . . . . . . . . . . . . . 8
4.1. Link or Router Down or Metric Increase . . . . . . . . . . 8
4.2. Link or Router Up or Metric Decrease . . . . . . . . . . . 8
5. Acceleration of Ordered Convergence . . . . . . . . . . . . . 9
5.1. Construction of the waiting list and notification list . . 9
5.1.1. Down events . . . . . . . . . . . . . . . . . . . . . 10
5.1.2. Up Events . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Format of Completion Messages . . . . . . . . . . . . . . 10
6. Fall back to Conventional Convergence . . . . . . . . . . . . 10
7. oFIB state machine . . . . . . . . . . . . . . . . . . . . . . 11
7.1. OFIB_STABLE . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. OFIB_HOLDING_DOWN . . . . . . . . . . . . . . . . . . . . 12
7.3. OFIB_HOLDING_UP . . . . . . . . . . . . . . . . . . . . . 13
7.4. OFIB_ONGOING . . . . . . . . . . . . . . . . . . . . . . . 14
7.5. OFIB_ABANDONED . . . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
With link-state protocols [1][2], each time the network topology
changes, some routers need to modify their Forwarding Information
Base (FIB) to take into account the new topology. Each topology
change causes a convergence phase. During this phase, routers may
transiently have inconsistent FIBs, which may lead to packet loops
and losses, even if the reachability of the destinations is not
compromised after the topology change. Packet losses and transient
loops can also occur in the case of a link down event implied by a
maintenance operation, even if this operation is predictable and not
urgent. When the link state change is a metric update and when a new
link is brought up in the network, there is no direct loss of
connectivity, but transient packet loops and loss can still occur.
For example, in Figure 1, if the link between X and Y is shut down by
an operator, packets destined to X can loop between R and Y when Y
has updated its FIB while R has not yet updated its FIB, and packets
destined to Y can loop between X and S if X updates its FIB before S.
According to the current behaviour of ISIS and OSPF, this scenario
will happen most of the time because X and Y are the first routers to
be aware of the failure, so that they will update their FIBs first.
1
X-------------/-------------Y
| |
| |
| |
| |
1 | | 1
| |
| |
| |
| |
S---------------------------R
2
Figure 1: A simple topology
It should be noted that the loops can occur remotely from the
failure, not just adjacent to it.
The goal of this draft is to define a mechanism which sequences the
router FIB updates to maintain consistency throughout the network.
By correctly setting the FIB change order no looping or packet loss
can occur. As described in [5] this mechanism may be applied to the
case of managed link-state changes, i.e. link metric change, manual
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link down/up, manual router down/up, and managed state changes of a
set of links attached to one router. It may also be applied to the
case where one or more network elements are protected by a fast re-
route mechanism [3] [8]. The mechanisms that are used in the failure
case are exactly the same as those used for managed changes. For
simplicity this draft makes no further distinction between managed
and unplanned changes.
2. The required FIB update order
This section provides an overview of the required ordering of the FIB
updates. A more detailed analysis of the rerouting dynamics and
correctness proofs of the mechanism can be found in [7].
2.1. Single Link Events
For simplicity the correct ordering for single link changes are
described first. The draft then builds on this to demonstrate that
the same principles can be applied to more complex scenarios such as
line card or node changes.
2.1.1. Link Down / Metric Increase
First consider the non-urgent failure of a link or the increase of a
link metric. In this case, a router R MUST NOT update its FIB until
all other routers that send traffic via R and the affected link have
first updated their FIBs.
The following argument shows that this rule ensures the correct order
of FIB change when the link X->Y is shut down or its metric is
increased.
An "outdated" FIB entry for a destination is defined as being a FIB
entry that still reflects the shortest path(s) in use before the
topology change. Once a packet reaches a router R that has an
outdated FIB entry for the packet destination, then, provided the
oFIB ordering is respected, the packet will continue to X only
traversing routers that also have an outdated FIB entry for the
destination. The packet thus reaches X without looping and will be
forwarded to Y via X->Y (or in the case of FRR, the X->Y repair path)
and hence reach its destination.
Since it can be assumed that the original topology was loop-free, Y
will never use the link Y->X to reach the destination and hence the
path(s) between Y and the destination are guaranteed to be unaffected
by the topology change. It therefore follows that the packet
arriving at Y will reach its destination without looping.
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Since it can also be assumed that the new topology is loop-free, by
definition a packet cannot loop while being forwarded exclusively by
routers with an updated FIB entry.
In other words, when the oFIB ordering is respected, if a packet
reaches an outdated router, it can never subsequently reach an
updated router, and cannot loop because from this point on it will
only be forwarded on the consistent path that was used before the
event. If it does not reach an outdated router, it will only be
forwarded on the loop free path that will be used after the
convergence.
According to the proposed ordering, X will be the last router to
update its FIB. Once it has updated its FIB, the link X->Y can
actually be shut down (or the repair removed).
If the link X-Y is bidirectional a similar process must be run to
order the FIB update for destinations using the link in the direction
Y->X. As has already been shown, no packet ever traverses the X-Y
link in both directions, and hence the operation of the two ordering
processes is orthogonal.
2.1.2. Link Up / Metric Decrease
In the case of link up events or metric decreases, a router R MUST
update its FIB BEFORE all other routers that WILL use R to reach the
affected link.
The following argument shows that this rule ensures the correct order
of FIB change when the link X->Y is brought into service or its
metric is decreased.
Firstly, when a packet reaches a router R that has already updated
its FIB, all the routers on the path from R to X will also have
updated their FIB, so that the packet will reach X and be forwarded
along X->Y, ultimately reaching its destination.
Secondly, a packet cannot loop between routers that have not yet
updated their FIB. This proves that no packet can loop.
2.2. Multi-link events
The following sections describe the required ordering for single
events which may be manifest as multiple link events. For example,
the failure of a router may be notified to the rest of the network as
the individual failure of all its attached links. The means of
identifying the event type from the collection of received link
events is described in Section 3.1.
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2.2.1. Router Down events
In the case of the non-urgent shut-down of a router, a router R MUST
NOT update its FIB until all other routers that send traffic via R
and the affected router have first updated their FIBs.
Using a proof similar to that for link failure, it can be shown that
no loops will occur if this ordering is respected [7].
2.2.2. Router Up events
In the case of a router being brought into service, a router R MUST
update its FIB BEFORE all other routers that WILL use R to reach the
affected router.
A proof similar to that for link up, shows that no loops will occur
if this ordering is respected [7].
2.2.3. Linecard Failure/Restoration Events
The failure of a line card involves the failure of a set of links all
of which have a single node in common, i.e. the parent router. The
ordering to be applied is the same as if it were the failure of the
parent router.
In a similar way, the restoration of an entire linecard to service as
a single event can be treated as if the parent router were returning
to service.
3. Applying ordered FIB updates
3.1. Deducing the topology change
As has been described, a single event such as the failure or
restoration of a single link, single router or a linecard may be
notified to the rest of the network as a set of individual link
change events. It is necessary to deduce from this collection of
link state notifications the type of event that has occurred in the
network and hence the required ordering.
When a link change event is received which impacts the receiving
router's FIB, the routers at the near and far end of the link are
noted.
If all events received within some hold-down period have a single
router (R) in common, then it is assumed that the change reflects an
event (line-card or router change) concerning the common router (R).
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In the case of a link change event, the router at the far end of the
link is deemed to be the common router (R).
All ordering computations are based on treating the common router R
as the root for both link and node events.
3.2. Deciding if ordered FIB updates applies
There are some events (for example a subsequent failure with
conflicting repair requirements occurring before the ordered FIB
process has completed) that cannot be correctly processed by this
mechanism. In these cases it is necessary to ensure that convergence
falls back to the conventional mode of operation (see Section 6).
In all cases it is necessary to wait some hold-down period after
receiving the first notification to ensure that all routers have
received the complete set of link state notifications associated with
the single event.
At any time, if a link change notification is received which would
have no effect on the receiving router's FIB, then it may be ignored.
If no other event is received during the hold-down time, the event is
treated as a link event. Note that the reverse connectivity check
means that only the first failure event, or second up event have an
effect on the FIB.
If an event is received within the hold down period which does NOT
reference the common router (R) then in this version of the
specification normal convergence is invoked immediately (see
Section 6).
The sudden failure of a link or a set of links that are not protected
using a FRR mechanism must be processed using the conventional mode
of operation.
In summary an ordered FIB process is applicable iif the set of link
state notifications received between the first event and the hold
down period reference a common router R, and one of the following
assertions is verified :
. The set of notifications refer to link down events concerning
protected links and metric increase events
. The set of notifications refer to link up events and metric
decrease events.
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4. Computation of the ordering
This section describes how the required ordering is computed.
4.1. Link or Router Down or Metric Increase
To respect the proposed ordering, routers compute a rank that will be
used to determine the time at which they are permitted to perform
their FIB update. In the case of a failure event rooted at router Y
or an increase of the metric of link X->Y, router R computes the
reverse Shortest Path Tree in the topology before the failure
(rSPT_OLD) rooted at Y. This rSPT gives the shortest paths to reach Y
before the failure. The branch of the reverse SPT that is below R
corresponds to the set of shortest paths to R that are used by the
routers that reach Y via R.
The rank of router R is defined as the depth (in number of hops) of
this branch. In the case of ECMP, the maximum depth of the ECMP path
set is used.
Router R is required to update its FIB at time
T0 + H + rank * MAX_FIB
where T0 is the arrival time of the link-state packet containing the
topology change, H is the hold-down time and MAX_FIB is a network-
wide constant that reflects the maximum time required to update a FIB
irrespective of the change required. The value of MAX_FIB is network
specific and its determination is out of the scope of this document.
This value must be agreed by all the routers in the network. This
agreement can be performed by using a capability TLV as defined in
[9].
All the routers that use R to reach Y will compute a lower rank than
R, and hence the correct order will be respected. It should be noted
that only the routers that used Y before the event need to compute
their rank.
4.2. Link or Router Up or Metric Decrease
In the case of a link or router up event rooted at Y or a link metric
decrease affecting link Y->W, a router R must have a rank that is
higher than the rank of the routers that it will use to reach Y,
according to the rule described in Section 2. The rank of R is thus
the number of hops between R and Y in its renewed Shortest Path Tree.
When R has multiple equal cost paths to Y, the rank is the length in
hops of the longest ECMP path to Y.
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Router R is required to update its FIB at time
T0 + H + rank * MAX_FIB
It should be noted that only the routers that use Y after the event
have to compute a rank, i.e. only the routers that have Y in their
SPT after the link-state change.
5. Acceleration of Ordered Convergence
The mechanism described above is conservative, and hence may be
relatively slow. The purpose of this section is to describe a method
of accelerating the controlled convergence in such a way that ordered
loop-free convergence is still guaranteed.
In many cases a router will complete its required FIB changes in a
time much shorter than MAX_FIB and in many other cases, a router will
not have to perform any FIB change at all.
This section describes the use of completion messages to speed up the
convergence by providing a means for a router to inform those routers
waiting for it, that it has completed any required FIB changes. When
a router has been advised of completion by all the routers for which
it is waiting, it can safely update its own FIB without further
delay. In most cases this can result in a sub-second re-convergence
time comparable with that of normal convergence.
Routers maintain a waiting list of the neighbours from which a
completion message must be received. Upon reception of a completion
message from a neighbour, a router removes this neighbour from its
waiting list. Once its waiting list becomes empty, the router is
allowed to update its FIB immediately even if its ranking timer has
not yet expired. Once this is done, the router sends a completion
message to the neighbours that are waiting for it to complete. Those
routers are listed in a list called the Notification List.
Completion messages contain an identification of the event to which
they refer.
Note that, since this is only an optimization, any loss of completion
messages will result in the routers waiting their defined ranking
time and hence the loop-free properties will be preserved.
5.1. Construction of the waiting list and notification list
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5.1.1. Down events
Consider a link or node down event rooted at router Y or the cost
increase of the link X->Y. A router R will compute rSPT_OLD(Y) to
determine its rank. When doing this, R also computes the set of
neighbors that R uses to reach the failing node or link, and the set
of neighbors that are using R to reach the failing node or link. The
Notification list of R is equal to the former set and the Waiting
list of R is equal to the latter.
Note that R could include all its neighbors except those in the
Waiting list in the Notification list, this has no impact on the
correctness of the protocol, but would be unnecessarily inefficient.
5.1.2. Up Events
Consider a link or node up event rooted at router Y or the cost
decrease of the link Y->X. A router R will compute its new SPT
(SPT_new(R)). The Waiting list is the set of nexthop routers that R
uses to reach Y in SPT_new(R).
In a simple implementation the notification list of R is all the
neighbours of R excluding those in the Waiting list. This may be
further optimized by computing rSPT_new(Y) to determine those routers
that are waiting for R to complete.
5.2. Format of Completion Messages
The format of completion messages and means of their delivery is
routing protocol dependent and is outside the scope of this document.
An encoding of completion message for IS-IS is proposed in [4].
The following information is required:
. Identity of the sender.
. A list of routing notifications being considered in the
associated FIB change. Each notification is defined as :
. Node ID of the near end of the link
. Node ID of the far end of the link
. Old Metric
. New Metric
6. Fall back to Conventional Convergence
In circumstances where a router detects that it is dealing with
incomplete or inconsistent link state information, or when a further
topology event is received before completion of the current ordered
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FIB update process it may be expedient to abandon the controlled
convergence process. Fall back mechanisms are investigated in [11].
The state machine defined in this version of the draft does not make
an assumption on which fall back mechanism will be used.
7. oFIB state machine
An ofib capable router maintains an ofib state value which can be one
of : OFIB_STABLE, OFIB_HOLDING_DOWN, OFIB_HOLDING_UP, OFIB_ABANDONED,
OFIB_ONGOING.
An ofib capable router maintains a timer, Hold_down_timer. An ofib
capable router is configured with a value refered to as
HOLD_DOWN_DURATION. This configuration can be performed manually or
using [9].
An ofib capable router maintains a timer, rank_timer.
7.1. OFIB_STABLE
OFIB_STABLE is the state of a router which is not currently involved
in any convergence process. This router is ready to process an event
by applying ofib.
EVENT : Reception of a link-state packet describing an event of the
type link X--Y down or metric increase to be processed using oFIB.
ACTION : Set state to OFIB_HOLDING_DOWN. Start Hold_down_timer.
ofib_current_common_set = {X,Y}. Compute rank with respect to the
event, as defined in Section 4. Store Waiting List and Notification
List for X--Y obtained from the rank computation.
EVENT : Reception of a link-state packet describing an event of the
type link X--Y up or metric decrease which to be processed using
oFIB.
ACTION :
Set state to OFIB_HOLDING_UP.
Start Hold_down_timer.
ofib_current_common_set = {X,Y}
Compute rank with respect to the event, as defined in section
Section 4 .
Store Waiting List and Notification List for X--Y obtained from
the rank computation.
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7.2. OFIB_HOLDING_DOWN
OFIB_HOLDING_DOWN is the state of a router that is collecting a set
of link down or metric increase link-state packets to be processed
together using controlled convergence.
EVENT : Reception of a link-state packet describing an event of the
type link up or metric decrease which in itself can be processed
using oFIB.
ACTION :
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism
EVENT : Reception of a link-state packet describing an event of the
type link A--B down or metric increase which in itself can be
processed using oFIB.
ACTION :
ofib_current_common_set =
intersection(ofib_current_common_set,{A,B}).
If ofib_current_common_set is empty, then there is no longer a
node in common in all the pending link-state changes.
Set state to OFIB_ABANDONED
Reset Hold_down_timer
Trigger AAH mechanism.
If ofib_current_common set is not empty, update waiting list and
notification list as defined in Section 4. Note that in the
case of a single link event, the link-state packet received when
the router is in this state describes the state change of the
other direction of the link, hence no changes will be made to
the waiting and notification lists.
EVENT : Hold_down_timer expires.
ACTION :
Set state to OFIB_ONGOING.
Start rank_timer with computed rank.
EVENT : Reception of a completion message
ACTION : Remove the sender from waiting list associated with the
event identified in the completion message.
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7.3. OFIB_HOLDING_UP
OFIB_HOLDING_UP is the state of a router that is collecting a set of
link up or metric decrease link-state packets to be processed
together using controlled convergence.
EVENT : Reception of a link-state packet describing an event of the
type link down or metric increase to be processed using oFIB.
ACTION :
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism.
EVENT : Reception of a link-state packet describing an event of the
type link A--B up or metric decrease to be processed using oFIB.
ACTION :
ofib_current_common_set =
intersection(ofib_current_common_set,{A,B}).
If ofib_current_common_set is empty, then there is no longer a
common node in the set of pending link-state changes.
Set state to OFIB_ABANDONED.
Reset Hold_down_timer.
Trigger AAH mechanism.
If ofib_current_common set is not empty, update waiting list and
notification list as defined in Section 4. Note that in the
case of a single link event, the link-state packet received when
the router is in this state describes the state change of the
other direction of the link, hence no changes will be made to
the waiting and notification lists.
EVENT : Reception of a completion message
ACTION : Remove the sender from the waiting list associated with the
event identified in the completion message.
EVENT : Hold_down_timer expires.
ACTION :
Set state to OFIB_ONGOING.
Start rank_timer with computed rank.
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7.4. OFIB_ONGOING
OFIB_ONGOING is the state of a router that is applying the ordering
mechanism w.r.t. the set of LSP collected when in OFIB_HOLDING_DOWN
or OFIB_HOLDING_UP state.
EVENT : rank_timer expires or waiting list becomes empty.
ACTION :
Perform FIB updates according to the change.
Send completion message to each member of the notification list.
Set State to OFIB_STABLE.
EVENT : Reception of a completion message
ACTION : Remove the sender from the waiting list.
EVENT : Reception of a link-state packet describing a link state
change event.
ACTION :
Set state to OFIB_ABANDONED.
Trigger AAH.
Start Hold_down_timer.
7.5. OFIB_ABANDONED
OFIB_ABANDONED is the state of a router that has fallen back to fast
convergence due to the reception of link-state packets that cannot be
dealt together using oFIB.
EVENT : Reception of a link-state packet describing a link-state
change event.
ACTION : Trigger AAH, reset Hold_down_timer.
EVENT : Hold_down_timer expires.
ACTION : Set state to OFIB_STABLE
8. Acknowledgments
We would like to thank Clarence Filsfils and Jean-Philippe Vasseur
for their useful suggestions and comments.
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9. References
[1] "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless mode Network Service
(ISO 8473). ISO/IEC 10589:2002, Second Edition".
[2] J. Moy, "OSPF Version 2", RFC 2328, April 1998.
[3] M. Shand and S. Bryant, "IP Fast Reroute Framework",
draft-ietf-rtgwg-ipfrr-framework-07.txt (work in progress),
June 2007.
[4] O. Bonaventure, P. Francois, and S. Previdi, "ISIS extensions
for ordered FIB updates", draft-bonaventure-isis-ordered-00.txt
(work in progress), Feb 2006.
[5] S. Bryant and M. Shand, "Applicability of Loop-free
Convergence", draft-bryant-shand-lf-applicability-03.txt (work
in progress), May 2007.
[6] A. Zinin, "Analysis and Minimization of Microloops in Link-
state Routing Protocols",
draft-ietf-rtgwg-microloop-analysis-01.txt (work in progress),
Oct 2005.
[7] P. Francois and O. Bonaventure, "Avoiding transient loops
during IGP convergence in IP Networks", in IEEE/ACM
Transactions on Networking,
http://inl.info.ucl.ac.be/publications, December 2007.
[8] P. Pan et al., "Fast Reroute Extensions to RSVP-TE for LSP
Tunnels", RFC 4090.
[9] S. Bryant, A. Atlas, and M. Shand, "Synchronisation of Loop
Free Timer Values", draft-atlas-bryant-shand-lf-timers-03 (work
in progress), July 2007.
[10] P. Francois, C. Filsfils, J. Evans, and O. Bonaventure,
"Achieving sub-second IGP convergence in large IP Networks",
in ACM SIGCOMM Computer Communication Review,
http://portal.acm.org/citation.cfm?id=1070873.1070877,
July 2005.
[11] S. Bryant, P. Francois, and M. Shand, "Mechanisms for safely
abandoning loop-free convergence (AAH)", February 2008.
Pierre Francois, et al. Expires August 28, 2008 [Page 15]
Internet-Draft Loop-free convergence using oFIB February 2008
Authors' Addresses
Pierre Francois
Universite catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve 1348
BE
URI: http://inl.info.ucl.ac.be/
Olivier Bonaventure
Universite catholique de Louvain
Place Ste Barbe, 2
Louvain-la-Neuve 1348
BE
URI: http://inl.info.ucl.ac.be/
Mike Shand
Cisco Systems
Green Park, 250, Longwater Avenue,
Reading RG2 6GB
UK
Email: mshand@cisco.com
Stewart Bryant
Cisco Systems
Green Park, 250, Longwater Avenue,
Reading RG2 6GB
UK
Email: stbryant@cisco.com
Stefano Previdi
Cisco Systems
Via Del Serafico 200
00142 Roma
Italy
Email: sprevidi@cisco.com
Pierre Francois, et al. Expires August 28, 2008 [Page 16]
Internet-Draft Loop-free convergence using oFIB February 2008
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Pierre Francois, et al. Expires August 28, 2008 [Page 17]
