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

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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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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

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

4
Repair Path Calculation
• Use alternative shortest path as repair path
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
• Assign traffic to each link, and create repair
• 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
• 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