This document discusses various techniques for improving IP routing convergence times, including fast convergence (FC) which aims to provide convergence in 1 second or better without pre-computed repair paths. Fast re-route (FRR) pre-populates forwarding tables with pre-computed alternate paths using techniques like loop-free alternates (LFA) to allow rerouting in milliseconds. Prefix independent convergence (PIC) provides rerouting independently of the number of routing table prefixes in milliseconds for node or link failures. The document also lists common causes of unscheduled network downtime and principles for improving convergence like graceful restart, non-stop routing, and in-service software upgrades.

1. IP Routing Fast Convergence Cheat Sheet
© 2012 Cisco Systems, Inc. and/or its affiliates. Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco’s trademarks can be found at www.cisco.com/go/trademarks. Third-party trademarks
mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1007R) C45-697761-00 1/12
The Components of IPFRR?
Historical Routing
Convergence Artifacts
It is a common belief that for an IGP (ISIS,
OSPF, EIGRIP, RIP) there is 10 to 30 seconds
convergence time, while for BGP it may take
up to few minutes.
What is Fast Convergence
(FC)?
Fast Convergence is about improving
protocols to provide best convergence
(down to 1 second or better). Pre-computed
repair paths are not part of this design.
What is Fast Re-Route
(FRR)?
FRR is about adding mechanism to
pre-populate TCAM with a repair path
pre-computed by some technique like LFA
to protect from a node failure or from a link
failure within milliseconds per prefix in the
routing table.
What is Prefix Independent
Convergence (PIC)?
PIC is an FRR reroute improvement to
protect from a node failure or from a link
failure within milliseconds independent to the
number of prefixes in the routing table.
Core Failures
(PIC)
error repair
Next Hops in the IGP
include: ISIS/OSPF, BGP,
BFD, LDP, TE/FRR
Edge Failures
to an IP destination:
Deploy BGP PIC
an IP destination: Converge
BGP as quickly as possible
include: ISIS/OSPF, BGP,
BFD, LDP, TE/FRR
Network Operations Failures
Physical Link Failures
Network Hardware Failures
Network Software Failures
Customer Premises Equipment Failure
Physical Environment Failures
Congestion/Overload
Unknown
Acts of Nature
Malicious Damage
87%
87%
79%
67%
67%
62%
44%
37%
25%
37%
0% 20% 40% 60% 80% 100%
Source: Sage Research, IP Service Provide Downtime Study: Analysis of Downtime Causes, Costs and
Containment Strategies, August 17, 2001, Prepared for Cisco SPLOB
% of Respondents
Causes of Unscheduled Downtime
Principles
Scheduled Outage
SSO: Dual control plane with stateful
switch-over
Graceful Restart (GR): To implement
Non-Stop-Forwarding (NSF) during
control plane switch-over
Non-Stop-Routing (NSR): To implement
routing continuity to reduce the scope of
the downtime upon the network routing
In-Service Software Upgrades (ISSU):
Reduce the impact for software upgrades
upon the network
Non-scheduled Outage
BGP PIC (Prefix Independent Convergence)
Core: A technology to converge within the
Core a network issue
Bi-directional Forwarding detection (BFD):
Lightweight mechanism to detect
bi-directional link problems
BGP PIC Edge: If there is more as one path
to the customer site, then pre-program the
alternate path in the forwarding ASIC
Loop Free Alternate (LFA): A technology
used by an IGP to pre-calculate an alternate
path during link or node failures