Switching fabrics and Router Architecture

Posted on 2006-03-25
Medium Priority
Last Modified: 2013-11-29
I was wondering if anyone can explain me switching fabrics in a router architecture. A conventional router uses a BUS architecture but when switching fabric is introduced, what happens to the BUS and the architecture in general?

Im considering part of the BUS architecture:
a processor,
memory (buffers and routing table),
Network Ports.

Thanks in advance!
Question by:u0369527
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Expert Comment

ID: 16294445
Hi u0369527,

The below urls gives you in techno jargon the information as described by Cisco.



The quick and dirty translation is you have an operational topology that is based on a layer2 switching. It's unified by the connection of the layer 2 devices (hopefully in a nice hierarchical structure of access, distribution and core).  In the core, there should be nothing but fast switching (the delivery of data frames from source to destination).  On the distribution and access levels are where all the other devices (PCs, routers, printers, etc) and ACLs (rules) are installed.  This is your switch fabric.  

VLANs are created to securely segregate the L2 traffic.  Routers and/or L3 devices are used route IP (IPX, or Appletalk) traffic between (or to and from) VLANs, physically segregated L2 switches and the internet (or another site).

BUS architecture ??? In an extended network, routers provide the connections to create a hierarchical structure to enable the transition of data packets from one network to another.   The closest thing to a bus architecture would be the joining of 2 or more peer routers connected almost sequentially to provide network routing services.  If that is the case, the models are independent of each other but are not exclusive.  Per the design of the network, inferences of either can be drawn out.  


Author Comment

ID: 16295046
Are you saying that the distribution level (routers) + access level is a switching fabric? I thought the switching fabric was some sort of huge router containing several interfaces (network cards) inside..
Please correct me if i'm wrong.
Also, by architecture I mean the layout of the inside of a router (CPU, RAM, ROM, NVRAM...).

Author Comment

ID: 16295079
I mean, I thought switching fabric was a component of a modern router?
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Accepted Solution

ECNSSMT earned 1200 total points
ID: 16296964
Hi u0369527,

> Are you saying that the distribution level (routers) + access level is a switching fabric?
Sorry, let me reclarify.  Routers in the classical sense are not a component of a switch fabric.  

The description of the core, distribution and access layers is a Cisco visualization of how your switching devices should be set up to permit data to traverse across your switching fabric.  As an example, at the core (center); maybe you have a 6509. From the 6509 via GBIC cards you may have several 3550-12gs (distribution) and from the 3550-12g you may have several 3550-48s (access).

at the access level, you may have PCs, routers and other devices connected.  As well you can create rules governing packet transmissions (ACLs).

at the distribution layer, you have access level switches attached, you may possibility have routers attached as well as additional ACLs

at the core layer, you want to ensure that if the packets reach this layer (or device) it traverses from source to destination as quickly as possible, the only thing that should communicate with this device is distribution switches or other switches used as switches in the core; example multiple 6509s connected to form a multi-switch core.

> I thought the switching fabric was some sort of huge router containing several interfaces (network cards) inside..
well now we may be getting into semantics, but that also describes the 6509 which has layer 3 switching capabilities; it can route packets at a very fast rate.  

>I mean, I thought switching fabric was a component of a modern router?
Its just a description of your topology.  Some sites may for example have one Cisco 6509 with several cards; each of them containing up to 48 switch ports to support your network.  That is the description you are thinking about.*  

Another possibility could be several 3550-48s (with a varying number of ports) all trunked to a 3550-12g.  These switches acting in unison can also be described as a switch fabric.**
*(Also note that its a relative description.  In this case, if this is the ONLY switching service; then it is performing all 3 functions of core distribution and access.  The backplane; or the entire device may be considered core, and the distribution and access layer functions may be described as being performed by the switch card.)

** (Here the 3550-12g could be consider core or core and distribution, the 3550-48 could then be considered access or access and distribution)


Author Comment

ID: 16297056
thanks. everything is starting to make sense now, its just confusing to see many people using different semantics
LVL 14

Expert Comment

ID: 16297171
Yikes, you will find that Cisco ( along with every major company) will have specifc ways of describing something and it is just getting accustom to the lingo.  If anything else, if you have some free time, check out the Cisco website (or actually any vendor site) to see if they have seminars (sales pitches mostly) in your area.  Check them out, check out their mind set, etc.


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