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Published byLaura Lyons Modified over 9 years ago
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Russ White russw@riw.us
IP Fast reroute Russ White
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L1/L2 Transport Conver-gence
Motivation 1 Minute TCP Session dies 30 secs Routing Proto-cols Tunnels go down 5 secs 1 sec VoIP call 500 msec Video 50 msec L1/L2 Transport Conver-gence Signalling
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Motivation Cost and Complexity Loss
(Impairments/Time) Cost and Complexity Re-engineering Required Potential Over- Engineering Viable- Number of possible approaches, or combinations of approaches. Range of viable engineering options may vary by type of application
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Techniques Preinstalled Backup Loop Free Alternates Not-Via
Supports the other mechanisms Loop Free Alternates Not-Via Disjoint Topologies
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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
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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
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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
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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
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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
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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 D Disjoint Path E
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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 D Disjoint Path E
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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
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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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