Subject: Re: question on link state routing protocols

From: Curtis Villamizar <[email-masked]>
Date: Sat, 27 Jul 2013 12:09:00 -0400

In message <[email-masked]>
Tony Li writes:
 
>  
> On May 8, 2013, at 2:34 PM, [email-masked] (Noel Chiappa) wrote:
>  
> >> The CSNP mechanism is FAR more robust than a LSP-by-LSP
> >> acknowledgement. It is a continuing and ongoing cross-checksum between
> >> adjacencies. With IS-IS, if an LSP is lost in flight, then it is caught
> >> in the next CSNP cycle and retransmitted.
> > 
> > Perhaps "robust" was poor word choice on my part. Let's substitute "timely".
> > 
> > A lost LSP is not caught until the next CSNP cycle, right? Why is it OK to
> > wait until then?
>  
>  
> P[packet loss] ~= 10^-18
>  
> Doing more would be over-design.
>  
> Tony

Tony,

Old thread, but I Just noticed this.  That figure applies to very
small packets (1 bit) on very clean links.  A lot of ultra long haul
fiber does not acheive that low a bit rate even after FEC.  Also, very
few networks carry one bit packets.  :-)

ISIS packets are full MTU so not 1 bit but between 10^4 or 10^5 bits.
Error rates for these big packet could be 10^-6 to 10^-8 and maybe
higher on trans-oceananic links.

Your point is still valid.  Its a very low number.

Noel,

Good to hear from you from time to time.

Also note that most routers have multiple adjacencies so if they miss
an update on one of them, they don't need a CSNP cycle to get the
update from another adjacency.

Also if ISIS is used to carry information for MPLS-TE, then transient
inconsistencies have no harmful effect.  The only harmful effect of
delayed news would be an LSR not knowing a link is down.  If a link
goes down, some LSR will send packets into a black hole until they are
aware that the link is down.  Some may fall back to IP forwarding
where applicable (which might get there, but also might loop, so
fallback to IP is IMHO a very bad idea) or may make use of RSVP-TE
TEAR and notification mechanisms to route around a failed link even if
ISIS has not yet heard the news.  Of course, protection, such as FRR,
makes any single link failure almost a non-event even if ISIS
propogation were to get delayed by a CSNP cycle now and then.

Curtis