private loop and public loop

[Mridul Dutta]

Hi Friends ,

I m little bit of confused in private loop and public loop defination. i m working in a SAN (Mc data) environment. We have 4 SAN switches (director) . SAN is connected to tape library , XP storage , some hosts also . so i wana know , is it private loop or in public loop . Any simple defination of private and public loop.

Mridul

[Vivekjaiswal001]

Hi,
Loops come into picture when we use FC loop switches (FC HUB).
as far as ur scenerio is concerned there wont be loops occuring as there are no FC hub connected to ur fabric.

If all the devices are connected to a fc hubs hub then such a loop is known as private loop.(max 126 devices)

If a HC Hub is connected to a fabric(FC switch) then loop becomes a public loop (max127 devies)

So as far as u dont connect a HFC Hub to ur FC switches..ur fabric doesnt have any loop. and once connected ...its a public loop.

[Uwe Zessin]

> i m working in a SAN (Mc data) environment.

In that case, the components are usually working with fabric protocol, not Fibre Channel Arbitrated Loop (FC_AL).


FC_AL has two different addressing modes:

- private loop means that a device uses one out of 126 valid loop addresses (AL_PAs) and can talk to other devices in that loop. The interconnect can be a direct cable between two devices, a FC_AL hub, a FC_AL 'loop switch' or a Brocade switch in 'Quick Loop' mode which even allows extension between two switches.

- public loop means that the device uses a full 24-bit Fibre Channel address. The lower 8 bits are still used for 1-out of 126 loop addresses. The upper 16 bits are the 'loop address' which is assigned by the Fibre Channel switch. The switch 'injects' a 127th address into the loop. This enables a public loop device to talk to other devices outside its own loop.

There is still more stuff possible, but it becomes more complicate and you were asking for 'simple' ;-)