[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00

LSR Working Group                                               S. Hares
Internet-Draft                                                    Huawei
Intended status: Informational                         February 11, 2019
Expires: August 15, 2019


                IPRAN Grid-Ring IGP convergence problems
              draft-hares-lsr-grid-ring-convergence-00.txt

Abstract

   This draft describes problems with IGP convergence time in some IPRAN
   networks that use a physical topology of grid backbones that connect
   rings of routers.  Part of these IPRAN network topologies exist in
   data centers with sufficient power and interconnections, but some
   network equipment sits in remote sites impacted by power loss.  In
   some geographic areas these remote sites may be subject to rolling
   blackouts.  These rolling power blackouts could cause multiple
   simultaneous node and link failures.  In these remote networks with
   blackouts, it is often critical that the IPRAN phone network re-
   converge quickly.

   The IGP running in these networks may run in a single level of the
   IGP.  This document seeks to briefly describe these problems to
   determine if the emerging IGP technologies (flexible algorithms,
   dynamic flooding, layers of hierarchy in IGPs) can be applied to help
   reduce convergence times.  It also seeks to determine if the
   improvements of these algorithms or the IP-Fast re-route algorithms
   are thwarted by the failure of multiple link and nodes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 15, 2019.





Hares                    Expires August 15, 2019                [Page 1]


Internet-Draft             IPRAN-IGP-Converge              February 2019


Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  IPRAN Topologies  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Requirements language . . . . . . . . . . . . . . . . . .   7
   4.  Problem detection using theoretical IGP Convergence . . . . .   8
     4.1.  Equation applied to Data Center IGP Convergence . . . . .   9
     4.2.  Flooding Problem on the Rings . . . . . . . . . . . . . .  11
     4.3.  Flooding problem on the grid  . . . . . . . . . . . . . .  12
   5.  Multiple simultaneous link and node failures  . . . . . . . .  12
     5.1.  Multiple link failures on Ring  . . . . . . . . . . . . .  13
     5.2.  Multiple link failures on Grid  . . . . . . . . . . . . .  14
   6.  Problem with Flat ISIS areas  . . . . . . . . . . . . . . . .  14
   7.  Problems with Dense Flooding Algorithm  . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References: . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This draft describes problems with IGP convergence time in some IPRAN
   networks.  The physical topologies of these IPRAN networks combine a
   grid backbone topology with a ring topology to support phone networks
   (see figure 1).  Routers are attached to the rings that route traffic
   from the IPRAN devices (see figure 2).  Each of the rings is attached
   to two grid nodes in order to provide redundancy.  All of the routers
   in the IPRAN ring-grid network topology run a single IGP (IS-IS).

   Some current deployments attach 10-30 routers per ring with a 20 by
   20 grid of routers.  In these deployments, a grid of 400 routers
   supports between 10,000 - 15,000 routers on the IPRAN rings.



Hares                    Expires August 15, 2019                [Page 2]


Internet-Draft             IPRAN-IGP-Converge              February 2019


   Convergence of the IGP after a single link failure on one ring router
   is over 1 second for these topologies.  The desired convergence time
   for a single link failure is less than 200 ms for phone networks.

   Initial convergence of the full network may take on the order of
   minutes.

   Part of these IPRAN network topologies exist in data centers with
   sufficient power and interconnections, but some network equipment
   sits in remote sites impacted by power loss.  In some geographic
   regions, these remote sites may be subject to rolling blackouts.
   These rolling power blackouts could cause multiple simultaneous link
   or node failures.  In these remote networks with blackouts, it is
   often critical that the IPRAN network converge quickly to restore
   what mobile phone service it can.  Keeping isolated portions of the
   network working may be critical to keep some phone service working.
   Converging the isolated portions back into the network when repairs
   are made also causes further disruptions.

   Due to the topologies of the IPRAN network, this document examines
   how the flooding of IGP informations causes the longer IGP
   convergence times for single links.  The potential multiple
   simultaneous link and node failures mean that the assumptions in most
   IGP and fast IP-Route algorithms do not apply.

   This document seeks to briefly describe these problems to determine
   if the following emerging IGP technologies an be applied to solve the
   convergence problem:

      flexible algorithms [I-D.ietf-lsr-flex-algo],

      dynamic flooding [I-D.li-lsr-dynamic-flooding],

      Level 1 abstraction for ISIS [I-D.li-area-abstraction]

      hierarchical IS-IS [I-D.li-hierarchical-isis]

2.  IPRAN Topologies

   A bit of background on the IPRan sizes.

   Grid topologies can be any size of square topologies.  Figure 1 shows
   a 3 router by 3 router topologies (3x3) with 9 nodes).  Other sizes
   could be 10 routers by 10 routers (10X10) with 100 nodes, 15 routers
   by 15 routers (15X15) with 225 routers, or 50 nodes by 50 nodes
   (20X20) with 400 routers.  A grid with network topology of a 100x100
   grid would have 10,000 gird-routers (grid only and ring-grid).
   Suppose that for every two grid nodes, 3 rings would be attached and



Hares                    Expires August 15, 2019                [Page 3]


Internet-Draft             IPRAN-IGP-Converge              February 2019


   on each ring there are 50 nodes.  This topology would result in
   750,000 ring routers plus 10,000 grid routers.  The size of this
   topology rivals data center sizes, but the IPRAN network does not
   have the infrastructure advantages of the data center.


            +-----+           +-------+    +-----+
            |Node |===Ring10==| Node  |    |Node |==Ring1======|
            | A   |===Ring11==|  B    |    | C   |==Ring2====| |
            |     |===Ring12==|       |    |     |==Ring3==| | |
            +-+-+-+           +-+-+-+-+    ++--+-+         | | |
              | |               | | |      |   |           | | |
              | +---------------+ |  +-----+   |           | | |
              |                   |            |           | | |
            +-----+           +---+---+    +---+-+         | | |
            |Node |===Ring3===| Node  |    |Node |==Ring3==| | |
            | H   |===Ring4===| G     |----+ I   |==Ring2====+ |
            |     |===Ring5===|       |    |     |==Ring1======+
            +-+-+-+           +-+-+-+-+    +-+-+-+
              | |               | | |        | |
              | +---------------+ | +--------+ |
              |                   |            |
              | +---------------+ | +--------+ |
              | |               | | |        | |
            +-|-+-+           +-+-+-+-+    +-+-+-+
            |Node |===Ring20==| Node  |    |Node |
            | F   |===Ring21==| E     |    | D   |
            |     |===Ring25==|       |    |     |
            +-+-+-+           +-+-+-+-+    +-+-+-+
              | |               | | |        | |

               Figure 1

                Figure 1: Example IPRAN Grid-Ring Topology

















Hares                    Expires August 15, 2019                [Page 4]


Internet-Draft             IPRAN-IGP-Converge              February 2019


                          +----+  +----+     +-----+
                          |Ring|  |Ring|...  |Ring |
                          |Rtr1|  |Rtr2|...  |RTR30|
                          +--+-+  +--+-+     ++-+--+
             +-----+         |       |          |    +-------+
             |Node |==Ring1==+=======+==========+====| Node  |
             |     |                                 |       |
             | A   |==Ring2==+=======+===========+===|  B    |
             |     |         |       |          |    |       |
             +-----+      +--+-+  +--+-+     +--+--+ +-------+
                          |Ring|  |Ring|...  |Ring |
                          |Rtr1|  |Rtr2|...  |RTR50|
                          +--+-+  +--+-+     +--+--+

                     Figure 2


                   Figure 2: Example IPRAN Ring Topology

   One characteristics of a grid is that a basic 3X3 square can be
   overlaid on most grids.  Figure 3 shows a 10 by 10 grid with 3 by 3.
   Notice that the grid squares overlaid on column 10 and row 10 form
   partial squares (see GS4, GS8, GS12, GS13, GS14, GS15, and GS16).

   If additional connections were made most of column 10 could form a
   single Grid (GS4, GS8, and GS12), and most of row 10 could form a
   single grid (GS13, GS14, and GS15).  Alternatively, with a single
   connection, GS16 could merge with GS15 to form a partial grid of 4
   nodes.






















Hares                    Expires August 15, 2019                [Page 5]


Internet-Draft             IPRAN-IGP-Converge              February 2019


                           X = Grid node
                               GS = Grid Square 1

                         GS1       GS2      GS3   GS4
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+
                         GS5       GS6     GS7    GS8
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+
                         GS9      GS10    GS11   GS12
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+
                         GS13     GS14   GS15    GS16
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+

                              Figure 3


              Figure 3: Overlaying Grid Squares on IPRAN Grid

   The grid topology is currently one flat IGP.  However, logical grid
   squares could form Level 1 areas within the IGP.  If one desired to
   create an L1 Area abstraction such as defined
   [I-D.li-area-abstraction], then the grid-square areas could be
   created as L1 areas and connected by 1-3 links to adjacent areas.
   Figure 4 shows a logical topology for grid squares 1-8 from figure 2.














Hares                    Expires August 15, 2019                [Page 6]


Internet-Draft             IPRAN-IGP-Converge              February 2019


                       X = Grid node
                           G  = Grid node G, Area Leader
                          GSn = Grid Square n (1-8)
                            Layer 2 area (1-8)


                     GS1       GS2      GS3   GS4
                    +-------+-------+-------+---+
                    | X X X | X X X-|-E X X | X |
                    | X G E---E G E-|-E G E-|-G |
                    | E E E---E E E | X E E | X |
                    +-|-|---+-|-|-|-+---|-|-+-|-|
                    | E E X | E E E | X E E | E |
                    | X G X | X G X | X G X | G |
                    | X X X | X X X | X X X | X |
                    +-------+-------+-------+---+
                     GS5       GS6     GS7    GS8



                          Figure 4


          Figure 4: Grid Squares Area Leaders and Area Edge Nodes

3.  Definitions

   This section provides definitions for nodes within the IPRAN routing
   infrastructure:

   ring router:   a routing device only attach to a ring in an IPRAN
      topology which routes end-system information

   ring-grid router   routing device attached to ring and the grid
      topology

   grid router:    a routing device which is only attached to the IPRAN
      Grid network

   pseudo-node for grid area:    a pseudo-node which summarizes for an
      IGP a grid area at one level for a higher level.

3.1.  Requirements language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].




Hares                    Expires August 15, 2019                [Page 7]


Internet-Draft             IPRAN-IGP-Converge              February 2019


4.  Problem detection using theoretical IGP Convergence

   Theoretical "best" convergence times for a single link failure on
   ring depths of 30 nodes suggests the flooding time is a major
   component for the flat IGP.  Estimates of theoretical best
   convergence times may be based on set of equations shown in figure 5.
   These equations show how network convergence is the maximum time for
   the information on a link change (down (failure) or up) to spread to
   all routers in the network.  The change travels along a pathway of
   routers from the change to any particular router.  Therefore,
   convergence is really topology dependent on the convergence time in
   each router and the pathways.

   The theoretical convergence equations in figure 5 include updating
   the RIB/FIB (Trib) and forwarding elements (Tdd).  Some IGPS may
   forward IGP traffic after calculating the SPF (Tspf)and updating the
   RIB/FIB, but before updating the FIB line cards (Tdd).  In this case,
   these factors would be zero in the equation.

   If several factors are zero or a constant, then the convergence may
   be determined by one element in the equation that dominates the
   convergence per node.





























Hares                    Expires August 15, 2019                [Page 8]


Internet-Draft             IPRAN-IGP-Converge              February 2019


            CT-Node = Td + To + Tf + Tspf + Trib + Tdd

                CT-Node = Node convergence time
                Td = link failure detection time
                 (or link up detection time)
                To = time to originate LSP
                     describing the new topology

                Tf = Time to flood the change
                     from this node to other nodes
                         that must perform a flood update

                Tspf = Time for shortest path calculation

                Trib = Time to update the RIB and FIB

                Tdd = time to distribute the FIB to line cards

                CT-path(i) = sum [CT-Node(j), .. CT-Node-(n))
                                      where i = path through network
                                            j = nodes on path (1..n)

                CTnetwork = maximum (CT-path(i))
                            where i = 0..n paths
                      Figure 5


                      Figure 5: Convergence equations

   [My first experience with an equation like this was Cengiz
   Alaettinoglu research in IGP around 2000 at NANOG.  (Please let me
   know if you have a good scholarly reference or presentation reference
   for these equations).]

4.1.  Equation applied to Data Center IGP Convergence

   Some early SPF implementations were slow with large IGP topologies.
   In this case, IGP's SPF calculations dominates the convergence time
   for all nodes.  Thus the Tspf dominates the time for each network
   path and the entire networks convergence time.  One might summarize
   the convergence as:

   CT-network = (Tspf + constant) * maximum path-length

   The maximum path length is often called the network depth.  The
   network depth of a full mesh network is 1.  The network depth of a
   dense mesh fat tree in a data center with 3 levels (top of rack,
   aggregate, spine) is 3.  If Tspf dominates the calculation then:



Hares                    Expires August 15, 2019                [Page 9]


Internet-Draft             IPRAN-IGP-Converge              February 2019


      CT-network = (Tspf + constant) * 3

   Centralized algorithms might improve convergence time if Tspf is the
   main factor.  Rather than using routers with typically low
   calculation power, centralized devices could be optimized for the
   calculation.  If the difference in network depth of sending the
   information end-to-end on any network path and sending it to the
   centralized processor and back is minimal, then centralized
   processing may be more effective.

   If flooding (Tf) dominates the per node convergence, the equation is:

      CT-network = (Tf + constant) * 3

   Many of the authors of the IGP flooding enhancements to reduce the
   data flooded understand that the flooding depends on the maximum
   pathway length for pathways in the IGP graph.  (see 802.1aq
   [I-D.allan-lsr-flooding-algorithm], Li et al.
   [I-D.li-lsr-dynamic-flooding], Shen, Ginsberg, and Thyamagundalu
   [I-D.shen-isis-spine-leaf-ext]).  Others mention creating a sub-graph
   of the entire topology to reduce the flooding traffic and reduce
   convergence time (Chen et al.  [I-D.cc-ospf-flooding-reduction]).

   Some of the IGP flooding reductions are identifying and limiting the
   number of global pathways without mentioning their concern for
   length.  (see Chunduri and Eckert [I-D.ce-lsr-ppr-graph]).

   The point behind this is that each algorithm has a set of goals.
   Those goals may impact other things that impact convergence.  Some
   questions one can ask are:

   o  Does the algorithm seek to reduced data flooded and stored?

   o  Does the algorithm seek to reduce convergence time?

   o  If the algorithm tries to both reduce the data flooded and stored,
      what trade-offs did the algorithm make?

   o  what is the impact of the topology?

   If one looks to adapt the algorithms developed for the dense
   interconnections of the 3 tier data center to the IPRAN Grid-ring
   network structure, these questions are important.








Hares                    Expires August 15, 2019               [Page 10]


Internet-Draft             IPRAN-IGP-Converge              February 2019


4.2.  Flooding Problem on the Rings

   Putting 30 or 50 ring routers on a ring may help operational costs.
   Within a city the higher density of rings may allow more cells for
   the phone.  In the rural networks, it may allow the cells to be
   deployed over a larger physical area.

   Every router one puts on a ring increases the network depth of the
   path through a fully operational ring or a partitioned ring that is
   still connected to the network.  The network depth of a ring is

      network depth = (n-ring-nodes + n-grid-ring)/2

      where

         n-ring-nodes = 30 to 50 nodes

         n-grid-nodes = 2 nodes

   A partitioned ring may have the full network depth if the link
   between a grid-router and the ring router attached to it fails.

   This flooding time is only for the on-ring path.  For a network path
   that involves the link failure of a ring router link the pathway is:


              network depth = depth(failed-ring) +
                              depth(grid) +
                              depth(remote-ring)

              depth(failed-ring)= network depth of ring with
                                  failed link.

              depth(grid) = network depth of pathway
                            through Grid

              depth(remote-ring) = network depth of pathway
                                   through remote ring

                              Figure 6


                      Figure 6: Convergence equations

   The worse case IGP convergence time combines the worse case for each
   of these network depths.





Hares                    Expires August 15, 2019               [Page 11]


Internet-Draft             IPRAN-IGP-Converge              February 2019


4.3.  Flooding problem on the grid

   The network depth of grid topologies grows as the size of the grid
   grows from 3X3 to 10X10 to 100X100.  The network depth of the best
   case pathway through the grid is a single hop as it is on the same
   ring-grid router.  The worse case path is the one from x1 to X2 in
   figure 7.  A network pathway that goes from x1 to X2 by using routers
   in the following grid squares: pathway of GS2, GS3, GS4, GS8, GS12
   could take 19 hops.


                           X = Grid node
                               GS = Grid Square 1

                         GS1       GS2      GS3   GS4
                        +-------+-------+-------+---+
                        |X1|X X | X X X | X X X | X |
                        |--+    |       |       |   |
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+
                         GS5       GS6     GS7    GS8
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+
                         GS9      GS10    GS11   GS12
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        | X X X | X X X | X X X | X |
                        +-------+-------+-------+---+
                         GS13     GS14   GS15    GS16
                        +-------+-------+-------+---+
                        | X X X | X X X | X X X | X2|
                        +-------+-------+-------+---+

                              Figure 6


                    Figure 7: Worse Case for 10X10 Grid

5.  Multiple simultaneous link and node failures

   Part of these IPRAN network topologies exist in data centers with
   power and connective, but some do not.  Ring routers are more likely




Hares                    Expires August 15, 2019               [Page 12]


Internet-Draft             IPRAN-IGP-Converge              February 2019


   to be at remote sites where power loss can occur.  However, some
   ring-grid routers or grid-only routers may be in remote sites.

   In some geographic locations, power losses can be rolling blackouts
   that cause multiple link and node outages during the failure.  These
   outages may be unpredictable due to weather or natural disasters, or
   semi-predictable due to brownouts.  Upon attempts to restore power,
   the restorations may have mixed combinations of links and nodes up.
   Multiple simultaneous link and node failures may impact both the ring
   topologies and the grid topologies in the IPRAN network.

   For simplicity of this discussion, I will present the node outages as
   the outages of all links.  A node outage may take far longer if
   rebooting the routers or reconfiguring spare ring routers takes a
   long time.  For this initial pass on this document, I will simply
   treat node outages as failure of all links for a time period that
   clear all valid paths.

   Most fast re-route technology such LFA [RFC5286] or MRT [RFC7812]
   set-up IP backup paths to route around a single link or node failure.
   In fact, the MRT architecture explicitly states that

      "MRT-FRR creates two alternative forwarding trees that ... are
      maximally diverse from one another, providing link and node
      protect for 100% of paths and failures as long as the failures do
      not cut the network into multiple pieces"

5.1.  Multiple link failures on Ring

   Ring routers may be located at sites that may lose connection to the
   ring or to a grid-ring router.  A single link failure may cut the
   ring, but leave all nodes attached if the failed link is between one
   of the ring routers (single on ring) or between the a ring-grid
   routers and a ring router.

   Multiple link failures on a ring will cause the ring to partition,
   isolating some nodes.  One way to handle this is to ignore the
   convergence on the partitioned rings.  Since local phone service
   during these outages may be useful, it may be important for the IGPs
   on the isolated portions of the rings to continue to operate.  During
   the restoration phase, additional links may appear to go up and down
   as the partitions heal.  Several isolated portions of the ring may be
   restored to form a larger isolated portion of the ring.  Eventually,
   the isolated parts should reconnect to a fully connected ring.







Hares                    Expires August 15, 2019               [Page 13]


Internet-Draft             IPRAN-IGP-Converge              February 2019


5.2.  Multiple link failures on Grid

   Multiple link failures can occur on the ring-grid routers or grid-
   only routers.  These failures may dramatically impact the data
   forwarding pathways through the grid and the flooding pathways.  Fast
   convergence of the grid depends on an algorithm tuned for the grid
   topologies.

   The failures on the grid can impact different parts of the IGP
   convergence algorithm.

6.  Problem with Flat ISIS areas

   Abstraction in an IGP can provide a logical means to scale IGPs.
   Creating 2 levels of topology in the IPRAN network based on ISIS
   areas could reduce the network depth and the the size of the topology
   database in level devices.

   However, as Li states in [I-D.li-area-abstraction] the ISIS concepts
   work well if:

   o  "the Level 1 area is tangential to the Level 2 area", or

   o  if "there are a number of routers in both level 1 and level 2 and
      they are adjacent".

   However it does not work well if Level 1 area needs to provide
   transit for level 2 traffic.

   Suppose all ring routers networks were placed in level 1 areas, and
   grid-only routers were in level 2.  The ring-grid routers are in both
   level 1 and 2.  This reduces the current topology to a topology
   similar to the spine-leaf topology.  While this reduces the amount of
   LSP stored, it may not significantly improve IGP convergence.  The
   flooding topology must be examined to determine the maximum network
   depth, and the router operations must be examined to determine the
   per IGP flooding time.

   It also restricts repair of an L2 Grid path via a L1 Ring.  This
   repair might be necessary in the multi-failure scenario.

   The area abstraction described in [I-D.li-area-abstraction] could be
   used to remove these restrictions.

   Additional levels of hierarchy described by Li in
   [I-D.li-hierarchical-isis] could be utilized in the grid to allow
   additional levels of abstractions.  These levels could reduce the
   network depth that IGP flooding passes through.



Hares                    Expires August 15, 2019               [Page 14]


Internet-Draft             IPRAN-IGP-Converge              February 2019


   One difficulty with using abstraction provided by areas and levels is
   the configuration of the appropriate network topology with multiple
   levels, and reconfigurations of these levels.  To be effective for
   100X100 grids, it would be beneficial to automate the configuration
   of areas.

7.  Problems with Dense Flooding Algorithm

   o  spine-leaves - rings may be leaves, but grid is not spine-leave
      topology.

   o  sparse link flooding - Grid may have too little or too much.  Top
      priority is fast convergence not reduced load of LSPF, but fast
      convergence.

   o  preferred path graph - goal is preferred path reduction of the
      number of preferred paths through network.  Fast re-route also
      sets up paths.  The preferred path graph needs to be carefully
      integrated with any fast reroute scheme.

   o  flooding of 802.1aq - is designed for dense mesh.

      *  The algorithm's two tree structure of 802.1aq provide complete
         coverage in the presence of a single link failure while
         constraining the number of LSAs.

      *  Both trees in the two structure have the same convergence
         properties in the IPRAN ring and grid.

8.  References

8.1.  Normative References:

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

8.2.  Informative References

   [I-D.allan-lsr-flooding-algorithm]
              Allan, D., "A Distributed Algorithm for Constrained
              Flooding of IGP Advertisements", draft-allan-lsr-flooding-
              algorithm-00 (work in progress), October 2018.







Hares                    Expires August 15, 2019               [Page 15]


Internet-Draft             IPRAN-IGP-Converge              February 2019


   [I-D.cc-ospf-flooding-reduction]
              Chen, H., Cheng, D., Toy, M., and Y. Yang, "LS Flooding
              Reduction", draft-cc-ospf-flooding-reduction-04 (work in
              progress), September 2018.

   [I-D.ce-lsr-ppr-graph]
              Chunduri, U. and T. Eckert, "Preferred Path Route Graph
              Structure", draft-ce-lsr-ppr-graph-01 (work in progress),
              October 2018.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-01 (work in progress), November 2018.

   [I-D.li-area-abstraction]
              Li, T., "Level 1 Area Abstraction for IS-IS", draft-li-
              area-abstraction-00 (work in progress), June 2018.

   [I-D.li-hierarchical-isis]
              Li, T., "Hierarchical IS-IS", draft-li-hierarchical-
              isis-00 (work in progress), June 2018.

   [I-D.li-lsr-dynamic-flooding]
              Li, T., Psenak, P., Ginsberg, L., Przygienda, T., Cooper,
              D., Jalil, L., and S. Dontula, "Dynamic Flooding on Dense
              Graphs", draft-li-lsr-dynamic-flooding-02 (work in
              progress), December 2018.

   [I-D.shen-isis-spine-leaf-ext]
              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-07 (work in progress), October 2018.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC7812]  Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for
              IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-
              FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016,
              <https://www.rfc-editor.org/info/rfc7812>.








Hares                    Expires August 15, 2019               [Page 16]


Internet-Draft             IPRAN-IGP-Converge              February 2019


Author's Address

   Susan Hares
   Huawei
   Saline
   US

   Email: shares@ndzh.com











































Hares                    Expires August 15, 2019               [Page 17]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/