see comments inline...
----- Original Message -----
From: "Howard C. Berkowitz" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Wednesday, January 24, 2001 9:14 AM
Subject: Re: But isn't that the routers job???
> >"Guy Tal" <[EMAIL PROTECTED]> wrote,
> >
> >
> >> >Plus routing of packets is done more quickly when done at the Switch
> >level
> >> >rather than having to go through the router for every packet.
> >>
> >> What's wrong with "going through the router," and how does routing
> >> through a switch differ from routing through a router?
> >
> ><snip>
> >
> >> Making forwarding decisions on layer 3 information is routing. Period.
> >
> >I actually have to disagree here with your terminology I guess.
Forwarding
> >decisions are being made with Layer 3 information. The first time a
packet
> >hits that router, a decision is made as far as which exit interface the
> >packet should be sent to and the best route for the packet to hit its
> >destination, based on whatever policy/protocol the router is using to
make
> >that decision in the first place. It is only subsequent packets that are
> >heading to the same destination that are spared the whole lookup process
> >again.
>
> What you are describing is a special case of using a RIB as first
> lookup and a cache for subsequent lookup. That is indeed the case
> for fast and silicon switching, and probably silicon. It is not the
> case for CEF (there is no cache, only a full FIB synchronized
> one-to-one with the RIB), and is not the case for process switching
> (everything goes through the RIB).
I apologize here, I should have specified that I was not talking about
process switching. And I know you are being technically correct about CEF,
but the bottom line is that the lookup is off the RIB.
>
> >Maybe my last email didn't send properly, but I replied to this one
> >last night that bypassing the RP is akin to an arp cache.
>
> Not all routers have RPs. If you're talking about a specific
> platform, be specific about that platform. You're making
> generalizations about all Cisco platforms and switching modes, much
> less non-Cisco products. If you have quantitative information that
> route lookup is a significant issue, please share it.
>
> Look at some Tolly group reports.
I apologize, but I am not familiar with these reports. I went to tolly.com
and found a terrific site. Thank you very much for pointing this out. I'll
look into it more soon.
>
> >Without an arp
> >cache, your device would overload looking up mac addresses. While your
> >router may not actually be crippled without this feature, and anyone that
> >has worked with enough 7500s knows that VIP cards are not the most stable
> >animals out there, it is a great feature if reduced latency is more
> important to you than money, which is a point you made earlier.
>
> >
> >
> >> There are more and less hardware intensive ways to make routing
> >> decisions. But the actual lookup time is rarely a limiting factor.
> >
> >I would have to disagree here as well. Perhaps lookup time isn't so bad
if a
> >router is sitting on a T1 somewhere, but when you have multiple oc48s
tied
> >into your router, processing time adds up, *real* quick.
>
> Again I ask, how do you know that lookup time is the problem? I work
> with gigabit routers, and indeed work on designing next-generation
> routers. Believe me, to run at line rate, destination lookup is not
> nearly the concern that filtering, traffic shaping, internal
> blocking, accounting, etc. are.
Well, one specific example would be for the 75xx series of Cisco router that
uses the VIP cards. If you attach the VIP card (from enable mode, do a
if-con <slot#>) and then run sh proc cpu from there, and you will see that
alot of the memory is getting tied up with routing lookups and interrupt
requests. Add a few of these to a router and you will suffer from latency
due to not enough memory to do lookups. I am not a big fan of VIP cards in
practice because it seems like on a decent sized network, one will crash per
week. But that is a specific Cisco example. What is your opinion of Juniper
routers that are reported to achieve true line rate?
>
> Any commercial router that thinks about handling multiple OC-48's or
> more is multiprocessor, with separate forwarding and path
> determination processors. The processor types involved in the two
> areas may be different. A router with those speeds is almost
> certainly meant for ISP applications, and we are very concerned with
> keeping the routing protocol processing clean.
>
> >
> >
>
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