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Fast Reroute Using Alternative Shortest Path <draft-tian-frr-alt-shortest-path-01.txt>

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Fast Reroute Using Alternative Shortest Path Albert J. Tian, tian_at_redback.com Naiming Shen, naiming_at_redback.com – PowerPoint PPT presentation

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Date added: 30 September 2020
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Title: Fast Reroute Using Alternative Shortest Path <draft-tian-frr-alt-shortest-path-01.txt>


1
Fast Reroute Using Alternative Shortest
Pathltdraft-tian-frr-alt-shortest-path-01.txtgt
  • Albert J. Tian, tian_at_redback.com
  • Naiming Shen, naiming_at_redback.com

2
Overview
  • A new way to calculate repair path using
    alternative shortest path which provides 100
    repair coverage
  • Proposed an explicit path with loose segments
    model to characterize the repair paths
  • Asserting that a valid loose segments in a repair
    path will not be changed by the failure of the
    protected link or node

3
Repair Path Termination Point
  • Link Protection terminate at nexthop
  • Node Protection
  • If nexthop N is not a primary egress, terminate
    at the next-nexthop
  • If nexthop N is a primary egress but is not the
    only primary egress, terminate at another primary
    egress
  • If nexthop N is the only primary egress and there
    is an alternative egress, terminate at one of the
    alternative egresses.
  • If nexthop N is the only egress, terminate at the
    nexthop and attempt link protection

4
Repair Path Calculation
  • Use alternative shortest path as repair path
  • Link protection take out the link being
    protected and recalculate shortest path to
    nexthop
  • Node protection take out the node being
    protected and recalculate shortest paths to
    termination points(usually next-nexthops)

5
Repair Path Implementation
  • Pure IP solution
  • IP TE Route Switched Path (RSP)
  • ltdraft-shen-ip-te-rsp-01.txtgt
  • MPLS based solutions
  • MPLS Source Route with Domain Wide Label
  • ltdraft-tian-mpls-lsp-source-route-01.txtgt
  • RSVP-TE ltRFC3209gt
  • RSVP-TE with loose segment optimization
  • ltdraft-tian-rsvp-loose-seg-opt-00.txtgt

6
Loose Segments Optimization
  • General reduce the number segments and simplify
    implementation
  • For unicast traffic protection, when the nexthop
    is not the only primary egress, the last loose
    segment can be optimized out
  • Gave out an algorithm to identify loose segments
    in an alternative shortest path to simplify
    repair path implementation

7
Example
Y
1
3
1
I
1
Z
X
E
1
2
8
Data Plane
  • Reroute link protection
  • Assign traffic to each link, and create repair
    path for that link
  • In case of link failure, switch traffic assigned
    to the link to repair path
  • Reroute node protection
  • Assign traffic to each next-nexthop, and create
    repair path for each next-nexthop
  • In case of node failure, switch traffic assigned
    to each next-nexthop affected by the node failure
    to repair paths

9
Standardization
  • None

10
Repair Coverage
  • 100 repair coverage

11
Control Plane Complexity Link Protection
  • L is the number of links
  • L SPFs needed to compute repair paths for L links
  • If link metrics are all symmetrical, no
    additional SPF for loose segment optimization
  • One additional reverse metric SPF for loose
    segment optimization if some link metrics are
    asymmetrical

12
Control Plane Complexity Node Protection
  • N is the number of nexthops
  • N SPFs Repair paths without loose segment
    optimization
  • 2N SPFs Repair paths with loose segment
    optimization when link metrics are symmetrical
  • 3N SPFs Repair paths with loose segment
    optimization when some link metrics are
    asymmetrical

13
Data Plane Complexity
  • Require a mechanism that can support arbitrary
    repair paths
  • Simple source routed MPLS Source Route with
    Domain Wide Label
  • RSVP based
  • IP TE Route Switched Path (RSP)
  • RSVP-TE ltRFC3209gt
  • RSVP-TE with loose segment optimization

14
Comparison
  • Commonalities
  • Explicit Path with Loose Segments
  • Properties of the last loose segment and
    conditions under which it can be optimized out
  • Inter-area/inter-domain repair paths
  • Differences
  • Support arbitrary repair path hence 100 coverage
  • Repair paths terminate at nexthop or next-nexthop
    resulting in much less repair paths to manage
  • Can protect multicast traffic(with other
    extensions)
  • What can be borrowed
  • Handling of LAN and pseudo nodes
  • Loop free transition

15
Repair Path Classification
  • Classification
  • SL Downstream, ECMP
  • LL/SLL/LSL/SLSL Tunnel approach
  • SSL (subset) Uturn
  • Arbitrary LSP Source Route, RSVP-TE (possibly
    with loose segment optimization), IP-TE-RSP
  • A common abstraction can help separate repair
    path calculation from implementation

16
Conclusion
  • A solution for IP fast reroute that provide 100
    coverage
  • Request to become a working group document
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