RE: gigabit router (was Re: Getting a "portable" /19 or /20)

["David Schwartz" <davids () webmaster com>]

> >     Why do you think central fowarding is superior to
> > distributed forwarding?
>
> Because you will have consistency problem. You are nearly 100%
> guaranteed to
> have them.
>
> Alex

        Ahh, so that's what you're thinking.

        If you have forwarding table F(X) at time X and forwarding table F(X+1) at
time X+1, a packet that arrives between times X and X+2 can reasonably be
forwarded by any of the tables. There is no special sequencing present or
required between the packets that involve routing protocols and the data
packets.

        Suppose a router received a packet that causes it to modify its routing
table in some way. If another packet is received in close time proximity to
the first packet, it can be reasonably routed by either policy. Even a
router with a central table could still route it either way, depending upon
when the routing process get scheduled in relation to when the interface
interrupt is services. (Or for other reasons, depending upon the hardware
you are dealing with.)

        The only way to sure this type of consistency is to centrally process every
single packet in strict sequence, fully applying all routing changes the
packet may require. There is no benefit to this added effort, after all, the
router would still have to work even if the packet with the routing data was
dropped.

        We misroute packets between routers because routing table updates don't
happen fast enough. It's not a problem -- IP is designed to tolerate packet
losses and has never guaranteed sequencing.

        The added occasional misroutes due to inconsistency will be proportional to
the ratio of the average network transport time for a routing protocol
packet to the average delay in propogating forwarding table changes to a
linecard. You do the math.

        DS