One of the things that confuses this discussion is that "switch" is
more a marketing than a technical term. Certainly "layer 3 switch"
is a marketing term.
There seems to be an assumption in this discussion that fastest is
always best. No. Taking off my Cisco stockholder hat, cheapest that
will do the job is best.
Regardless of the vendor, routing has two distinct functions. Path
determination builds the "routing table," or, more properly, the
Routing Information Base (RIB). The RIB is what you see when you do a
"show IP route." RIBs are optimized for updating by dynamic routing.
Incidentally, the OSPF database, BGP Adj-RIB, etc., are not part of
the RIB, but are inputs to it.
Typically, the first packet to a destination must go through the RIB
to get the FIB set up.
From the RIB is derived the Forwarding Information Base (FIB), which
the second function, packet forwarding, examines to select the
outgoing interface to which the packet is to be sent, based minimally
on destination address.
In process switching, the RIB and FIB are the same data structure.
There is no true FIB.
In fast switching, there is a FIB, which still is in main RAM, and
forwarding is done by the CPU.
In autonomous and silicon switching on the AGS+ and 7000, the FIB was
in a separate memory, and the bus controller (AGS) or Silicon Switch
Processor (7000) did the forwarding. The FIB was on the same board as
the forwarding engine. FIB memory was small, so if the particular
destination was not present (i.e., new or not recently used), there
could be "cache misses". On a cache miss, the FIB was invalidated and
rebuilt from the RIB.
In optimum switching, the FIB and RIB are both on the RSP card, but
in separate physical memories. One processor/memory set does path
determination, and one does forwarding.
In distributed switching (CEF and NetFlow) on router platforms,
simplifying slightly, there is one RIB but multiple copies of the FIB
are distributed onto the VIPs, each of which runs a separate
forwarding process. VIPs have large memories, so the FIB and RIB (at
least in CEF) are in 1:1 correspondence, and there are no cache
misses. Still, the first packet to a destination goes through the
RIB.
In distributed/layer3 switching on "switch" platforms, there remains
a single route determination engine. This can be in the same physical
chassis (e.g., RSM in a 5000), or in a separate chassis (the
"external router"). A Cisco proprietary protocol transfers the FIB
information to a NFFC on a 5000 series or to a forwarding board on
the higher-speed distributed switches.
Using an external router platform as the source of the FIB, or even
using an external router for all inter-VLAN routing, is simply a
design choice. A very real-world situation is having your clients in
one place and servers in another, but on the same VLAN (or using
VLAN-aware NICs). In such a situation, the actual requirement for
inter-VLAN forwarding may be limited to management (e.g., pinging
from the management station) or perhaps email. The function of
routing is important, but not the speed.
In this case, to keep costs low, I'd consider, in order,
1. Use a 2600 to do all inter-VLAN routing
2. Use a 3600 to do it a little faster
3. I don't know the most recent support for external path determination --
used to be that the 4500/4700 was the lowest platform. Probably a 3600
can these days. Use a 3600 as path determination engine and an NFFC
or equivalent on the switch platform(s).
4. Use an RSM/NFFC or equivalent.
5. Use a 10000/12000, etc., for very heavy routing loads with multiple
WAN interfaces. A 7200 or 7500 might be appropriate in some cases
"What Problem are you trying to solve?"
***send Cisco questions to the list, so all can benefit -- not
directly to me***
Howard C. Berkowitz [EMAIL PROTECTED]
Technical Director, CertificationZone.com
Senior Product Manager, Carrier Packet Solutions, NortelNetworks (for ID only)
but Cisco stockholder!
"retired" Certified Cisco Systems Instructor (CID) #93005
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