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

2. 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

3. 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

4. Techniques Preinstalled Backup Loop Free Alternates Not-Via
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 D
Disjoint Path 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 D
Disjoint Path 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