Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

WWN vs FCID

I understand WWNs are 64 bits and FCIDs are 24 bits and I understand the fabric assigns FCIDs when a device logs into the fabric (FLOGI).

Here's what seems confusing:

Since a device is communicating via FC and that protocol uses FCIDs, what is the purpose of a WWN? If devices (disregarding HP-UX and AIX) point to WWNs but the fabric uses FCIDs, where is this mapping stored on the server and how does it work exactly? Is there a table like a ARP cache? If so, when does it time out?

My point is, there is no field in the FCID frame for WWNs - only source and destination FCIDs. So, how do WWNs play into this picture?

5 REPLIES
Cisco Employee

Re: WWN vs FCID

You can think of WWN's as MAC addresses and FCID as the layer3 IP address. So, the WWN is burned into the HBA firmware. HPUX and AIX take the FCID of the storage they are talking to and enter it into a persistent file on the /etc filesystem. The wwn to FCID mapping can be seen with show flogi database and show fcns database.

New Member

Re: WWN vs FCID

Also, the part of the operation of the HBA on the host is to store and retain a Login map of all targets, it stores both FCID's and PWWN's of devices on the SAN so it can build frames for commands that are sent to storage devices, FC devices learn PWWN of target port from PLOGI ACC , the 24 bit FCID is used to route frame through the SAN using FSPF, 24 bit lookup is much faster then 64 bit lookup, but remember access control to storage is many times controlled by PWWN, both for zoning on Switch and also the controller port on the array. This PWWN is checked at PLOGI for access control.

New Member

Re: WWN vs FCID

So, if the initiator HBA learns the target's FCID during PLOGI, how long is the FCID retained by the server? Is the FCID retained until a logout occurs or does it time out (like an arp entry)?

One reason I ask is I'd like to fully understand the ramifications of changing FCIDs.

New Member

Re: WWN vs FCID

Host learns target's fcid thro' Plogi into NameServer,if it is zoned together. It is retained

as long as storage is in the fabric with same fcid

and host has PLOGIed onto the storage.

FCID of storage is retained in the host until

LOGO (explicit).

If the storage is not in fabric, then RSCN

is generated and host requeries the name server

for the storage, if it is not there, then

HBA cleans up resources related to that FCID.

New Member

Re: WWN vs FCID

Thanks for all the replies!

10288
Views
0
Helpful
5
Replies
CreatePlease login to create content