what is arbitrated loop and fabric topologies?

[stephen peng]

dear all,
i could not quite understand the meaning of arbitrated loop and fabric topologies, could anyone offer me some explanation or document?

thanks a lot

P.S, This thread has been moved from General to Storage Area Networks (SAN) (Enterprise). - Hp Forum Moderator

[Uwe Zessin · Accepted solution]

Fibre Channel Arbitrated Loop is a 'ring' of devices, a bit similar to FDDI or token ring where data goes _through_ each device.

Of course, if a device goes down, the entire loop will be dead, so a Fibre Channel hub is used which provides some bypass logic. It moves the data across the device which does not work.


Fabric topologies describe how fabric switches are interconnected. E.g. cascaded means that multiple switches are connected, but there are not redundant links - it looks more or less like a tree.

Other topologies are ring, mesh (lots of redundant links) and trees.

Fabric topologies are explained in the SAN design guide:
http://www.hp.com/go/sandesignguide

arbitrated loop is not much interesting for the end users these days, but it still lives on in the back-end of storage arrays, because Fibre Channel disk drives use FC_AL.

[stephen peng]

zessin,
hi! now i want to know the difference between fabric topologies and private loop, could you offer me some esplanation?
thanks

[Uwe Zessin]

Fibre Channel fabric protocol uses a 24-bit address which allows all devices in a fabric to be able to communicate with each other (leaving zoning and such things aside...).

FC_AL private loop uses an 8-bit address and is limited within that loop.

FC_AL public loop uses an 8-bit address within the loop and a 16-bit "loop address" to form a 24-bit address. The enables a device anywhere in the fabric to address a device within a public loop.
A Fabric Switch connects with one of its ports 'into' the loop and provides services to insert data into the loop or pick up data and send it into the fabric.

[stephen peng]

Zessin,
i could not quite catch your explanation, so could you please produce some examples to illustrate the upper items? or document to present detaied explain.

thanks a lot

[Uwe Zessin]

Fabric topologies describe how Fibre Channel switches are connected - see above.

Fibre Channel Arbitrated (private) Loop means that _devices_ (computers, disks / disk arrays, tape storage) are connected in a 'ring'.

Public loop means that such a 'ring' is connected to a Fibre Channel switch and the devices are capable to address devices outside the loop/ring.