From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Sat May 15 2004 - 11:27:39 GMT-3
At 8:57 AM -0400 5/15/04, CCIE 2004 wrote:
> Thanks for your reply Chris. Does anyone have any other thoughts. Thanks ---
The first thought would be whether you are talking about bridges
versus routers, which are well-defined technical terms, or layer 2
switches versus layer 3 switches, which are largely marketing terms
dating back to several vendors' slogans "switch when you can, route
when you must."
There's actually more of a distinction that really can be made
between a traditional bridge and even a basic layer 2 switch. The
switch, in this case, is intended to microsegment -- use one physical
port to connect to each end device, removing, as long as the
connectivity is full duplex, collisions. Switches also are more
likely to support VLAN trunking than traditional bridges, which are
interconnected simply as part of a homogeneous spanning tree.
>On Tue 05/11, Chris Larson < clarson52@comcast.net >
>wrote:From: Chris Larson [mailto: clarson52@comcast.net]To:
>ccie2004@excite.comDate: Tue, 11 May 2004 06:59:47 -0400Subject: Re:
>Difference between Layer 2 and Layer 3 switches There is a lot too
>it. But in a pure layer 2 switch, to route between vlansyou will
>need an external router. In a layer 3 switch, routing can be
>donewithin the switch. Also, a layer 3 switch is probably going to
>give you QoSabilities like traffic marking and cvlassificagtion
>based on layer 3 andlayer 4 info in the packet.
Actually, it's perfectly common and useful to have layer 4, but not
layer 3, capabilities in something one might consider a layer 2
switch. An increasingly common carrier application, when metro
Ethernet services are deployed, is to put such a switch on the
customer premises. In this application, the switch will have a port
for each VPN and for Internet connectivity, as well as a port for
voice (and possibly video). All of these ports are mapped into
802.1q VLANs for transmission to the carrier POP, where they are then
mapped to routed VPNs. At the customer premises switch, 802.1p QoS
bits are set on the voice/video VLANs, which variously are mapped to
traffic-engineered VPNs at the POP L3 function, and/or have 802.1p
mapped to DSCP in IP packets.
>----- Original Message ----- From: "CCIE 2004" To: Sent: Tuesday,
>May 11, 2004 12:54 AMSubject: OT: Difference between Layer 2 and
>Layer 3 switches> Hi, I just wanted to get a brief overview of
>the general concept of layer2 and layer 3 switches. I did read a
>thread earlier on groupstudy and hadseen a recent article but cannot
>remember where I saw !
> the article. I thinklayer 3 switches are mainly routers that are
>capable of caching and doing alookup on network layer addresses (ip,
>ipx etc) in Asic.
This is a really old definition that Cisco persists in continuing.
ASIC is not a very precisely defined characteristic. I can build a
router with intelligent forwarding cards, each with a general purpose
RISC processor, that will hammer a single-forwarding-ASIC router into
the ground.
An ASIC may be a pure hardware device like a fixed gate array, but is
probably more likely to be at least somewhat flexible, such as a
field-programmable (e.g., through EEPROM) gate array (FPGA), a
microcode sequencer (again with code either burned into ROM or in an
EEPROM), or even a general-purpose RISC processor with the
instructions prefetched, decoded, and in ROM/EEPROM.
>Layer 2 switchesare mainly devices that build their table on source
>mac address.
Bridges and layer 2 switches make decisions on _destination_ MAC
addresses. If VLANs are used, layer 2 switches may make their first
decisions, on a trunk port, on the VLAN ID field.
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