>"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).
>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.
>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.
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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