[c-nsp] Minimizing convergence time/failover in IP networks

[Bruce Pinsky]

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Shishir Belbase wrote:
> As a newcomer to this list, I apologize in advance if I have not followed
> the protocol in posting this message.
> 
>  
> 
> Does anyone have any experience tweaking the following timers in the network
> (consisting of all the following protocols)? If yes, what minimal settings
> do you have these running with and if you have seen improved performance in
> the IP network converging faster?
> 
>  
> 
> BGP keepalive/hold timers 
> 
> OSPF Hello and Dead Intervals
> 
> HSRP Hello and Dead Intervals
> 
> Any other that might help?
> 
>  
> 
> Currently we are running all of the above with its defaults.  Thanks!
> 


The number one key to faster convergence is faster fault detection.
Configuring for that means knowing more about the media attached to your
router and the underlying substrate that implements that media. Eliminating
the "carrier-delay", introducing additional POS triggers, and the use of
Bidirectional Forwarding Detection (BFD) are just a few of the tools that
are available to help aid in faster failure detection.

Your routing protocol hello/keepalive timers should be tweaked only as a
last resort to catch failures that are "not well-behaved".

Improvements in OSPF convergence can be achieved by using LSA and SPF
throttling mechanisms.  However, introducing these features into your
network should only be done after a thorough review of your existing
routing infrastructure and topology to insure that the features don't
create undue stress or instability on your network and that proper setting
values for your network are configured.

For BGP, Nexthop Tracking can help reduce the delay in converging to a new
bestpath when a nexthop failure occurs.  Combined with OSPF fast
convergence, new paths can be selected in just a few seconds and
potentially in sub-second timeframes.  BGP Fast Fallover can help reduce
the time takes to teardown external peering sessions.

I would suggest working with a Cisco SE, TAC Engineer, or other individual
who is familiar with these topics to properly assess the best way to
improve convergence in your specific network.

- --
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bep

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