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IP Fast reroute

IP Fast reroute. Russ White russw@riw.us. Motivation. Routing Proto-cols. VoIP call. Signalling. Video. TCP Session dies. Tunnels go down. L1 / L2 Transport Conver-gence. 50 msec. 500 msec. 1 sec. 5 secs. 30 secs. 1 Minute. Motivation.

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IP Fast reroute

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  1. IP Fast reroute Russ White russw@riw.us

  2. Motivation Routing Proto-cols VoIP call Signalling Video TCP Session dies Tunnels go down L1/L2 Transport Conver-gence 50 msec 500 msec 1 sec 5 secs 30 secs 1 Minute

  3. Motivation Range of viable engineering options may vary by type of application Cost and Complexity Number of possible approaches, or combinations of approaches. Loss (Impairments/Time) Re-engineering Required Viable- Engineering Potential Over- Engineering

  4. Techniques • Preinstalled Backup • Supports the other mechanisms • Loop Free Alternates • Not-Via • Disjoint Topologies

  5. Preinstalled Backups • Use whatever technique to find an alternate path • Various techniques are described in the following slides • Install the best path and an alternate path • When the primary path fails, instantly move to the alternate path • Saves the time of installing a new route from the routing table into the forwarding table (CEF on cisco) • Saves on the order of 25-50ms, perhaps longer • Provides hardware specific support for fast reroute

  6. Loop Free Alternate • Calculate the best path • If my path through C is less than B’s cost, C’s path is loop free • This is the loop free alternate • The LFA can be installed in the forwarding table as a backup path A B C Shortest Path D Loop Free Alternate E

  7. Loop Free Alternate • B fails… • Switch to the path through C • Wait for the new topology information to be flooded • Recalculate SPF A B C Shortest Path D Loop Free Alternate E

  8. Not-Via • D advertises a route to B “not via” B • A unique host route advertised only to C, not to B • A knows it’s best path is through B • Install the route with a next hop of D • A must tunnel to D A B C Shortest Path Notvia Advertisement D E

  9. Not-Via • B fails… • A installs the backup tunnel into the forwarding table • Wait for the new topology information to be flooded • Recalculate SPF A B C Shortest Path Notvia Advertisement D E

  10. Disjoint Topologies • A calculates two paths to E • One through B, one through C • These use a different virtual topology to reach the same destination A B C Shortest Path Disjoint Path D E

  11. Disjoint Topologies • F B fails… • Switch all traffic to the second topology • Wait for the new topology information to be flooded • Recalculate the topologies A B C Shortest Path Disjoint Path D E

  12. Disjoint Topologies • There are a number of different ways to build disjoint topologies • MPLS/TE with constrained SPF • Maximally Disjoint Trees • Essentially detects rings and marks opposite directions as different topologies • Various clever numbering mechanisms

  13. Summary • There are three different types of IP/FRR techniques • LFAs, already implemented by most routing vendors • Not-via, not likely to ever be implemented • Disjoint Trees, current research area but possible future solution • These are all supported by fast data plane switchover techniques

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