Re: Why recalculation from scratch?

["Manral, Vishwas" <[e-mail]>]

Hi Yasu,

The idea is this. When we are under high-congestion, we are in a state where
the topology information on the router is incomplete/out of date, doing
incremental SPF only further loads the CPU. So instead of doing incremental
SPF with every change, we instead club changes and do the entire SPF after a
longer period of time, which anyway is best effort(because of databse
discrepancies).

Also check ISO10589, it states that the CPU load for two incremental SPF's
can be as much as a single SPF, I dont remember the section though(probably
in the Annex somewhere). However I  remember conflicting numbers being
presented at NANOG.

Thanks,
Vishwas

-----Original Message-----
From: Yasuhiro Ohara [mailto:[e-mail]]
Sent: Thursday, August 15, 2002 12:52 PM
To: [e-mail]
Subject: Re: Why recalculation from scratch?


Hi,

Related to this, I wonder why the congestion-control draft saying "do
not do incremental SPF when congested" (in 4.2.2.4 Reduce the Rate of
SPF Computation). Could you give me some more explanation or a
reference pointer, authors ?

IMHO simply we're just negative to have a lot of state informations,
though there are some ways to avoid recalculation from scratch
(i.e. incremental SPF calculation).

regards.
yasu

jshen> Bin Liu,
jshen>
jshen> The first, router does not maintains all information as human does.
jshen> the second, each router computes its routing table on its view of
network.
jshen> When state of some links varies, the logical view of network changes
jshen> and the shortest path tree of the graph may become a totally new one;
jshen> the third, as link state propagates by relaying hop by hop, it can
not
jshen> be expected that every router update their routing table
simulataneously,
jshen> so each time a new link state is received the routing table must be
jshen> recomputed
jshen> to guarantee the convergence.
jshen>
jshen> Of course, in a network with thousands of prefix such computing need
a lot
jshen> of
jshen> CPU time but it's just one of the key reasons. IMO, Route flapping
and
jshen> looping is
jshen> the factors attracting more attention.
jshen>
jshen> Cheers
jshen>
jshen> Jing Shen