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Posted by Ugarchina on November 7, 2006, 1:04 pm
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hello.
could anyone please briefly explain what does this mean for Cisco
Catalyst 2950T-24 switch:
6.6 Mpps wire-speed forwarding rate
8.8 Gbps switching fabric
what's more important when comparing 2 different devices with
different values?
and 1 offtopic question:
can any type of OM2 fiber optic work on 10Gb?
thanks a lot.
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Posted by Walter Roberson on November 7, 2006, 1:43 pm
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>could anyone please briefly explain what does this mean for Cisco
>Catalyst 2950T-24 switch:
>6.6 Mpps wire-speed forwarding rate
>8.8 Gbps switching fabric
Each packet that travels through the 2950 internally has headers
added reflecting the output ports, vlan, prioritization, and so on.
Those internal headers take up internal bandwidth.
The switching fabric rate reflects the internal bandwidth. In systems
with multiple line cards, the fabric rate deals with how quickly
the (internally augmented) data can be shipped around. The forwarding
rate deals with how quickly (aggregate total) the device can pull
data onto and off of the wires, and takes into account internal
queue sizes, internal classification and prioritization and routing
or time to decide upon the destination ports.
>what's more important when comparing 2 different devices with
>different values?
If the switching fabric rate is not comfortably more than the
forwarding rate, then it implies that there is are "choke points",
traffic paths that cannot operate at full speed.
A good example of this is the 6500 series architecture, which has gone
through several generations of backplane connections for the
peripherals, some of which were zippy in their day but now pretty much
crawl, relatively speaking. It is not uncommon on the 6500 to find
line cards that can switch or route very quickly to other ports on the
same card, but because the switching fabric is relatively limited,
routing or switching to other cards might be much more limited.
You can get line cards for the 6500 which have 48 gigabit ports,
but which can only attach to the backplane (switching fabric) at
6 Gb/s, so you if you did not know the details of the buffering
and controller arrangements, you could end up "oversubscribed"
by 8:1 if groups of ports happened to need the switching fabric
simultaneously -- the switching fabric would "choke" the performance
down to 6 Gb/s when travelling off-card.
(Note that this is not necessarily a "massive design flaw": if those
48 ports are connected to desktops, chances are excellent that only
a few of them are actually trying to use the switch for any kind of
sustained gigabit transfer.)
>and 1 offtopic question:
>
>can any type of OM2 fiber optic work on 10Gb?
Let's see...
OM2: 50 um or 62.5 um multimode, 500 MHz* km modal bandwidth at
850 nm or 1300 nm
Glancing at a draft for 10000Base-SX it appears the above would be
within spec for the following combinations:
50 um MMF, 500 MHz*km modal @ 850 nm, 2 to 86 metres
62.5 um MMF, 200 MHz*km modal @ 850 nm, 2 to 35 metres.
Glancing at a draft for 10000Base-LX, it appears the above would
be within spec for the following conditions:
50 um MMF, 500 MHz*km modal @ 1300 nm, 2 to 300 metres
62.5 um MMF, 500 MHz*km modal @ 1300 nm, 2-300 meters
You should crosscheck that those made it into the final specs.
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