Storage Area Networks For Dummies®

Chapter 7: Put ting It All Together
167
the 1-2-3 ports where you’ll plug in your hosts). Plug the other end into
Switch-A2’s last port (16) and watch your telnet session on the console for
update messages from the switch:
Switch-A1:>
11/17/02-15:24:00 [FABRIC EVENT] - Domain:1 (Switch-A1) E-Port Connect Port:16
11/17/02-15:24:02 [FABRIC EVENT] - Domain:2 (Switch-A2) Discovered Port:16
Checking Fabric Compatibility...Done
Merging Fabrics...
11/17/02-15:25:12 [FABRIC EVENT] - Fabric Merge (Domain:1
What you see here is Switch-A2 connecting to Switch-A1 (via the cable on
port 16). As soon as a switch connects to something — anything — it asks
the device what it is and what it wants to do. In this case, the two switches
said “Hello” to each other, realized that they were two fabric switches, and
decided to join forces. Two SAN switches connect via a special port: an
Extended Port (E_Port). It gets its name because the fabric is extended outside
this switch to another switch.
When two E_Ports are created, a SAN fabric merge takes place in which the
zoning information unique to both switches is compared. If no errors, duplications,
or other problems are found, the switches copy their configurations
to each other and make one big combined copy of the zones that they use.
A fabric merge is like introducing two groups of friends to each other, only
some of them might have the same name. If, for instance, you have two friends
named Bill, you can introduce Bill from work as Bill A and Bill your college
buddy as Bill B to use their last names as qualifiers. Unique names help everyone
keep track of who you’re talking about. The same goes for SAN fabrics: If
you have two zones named Zone1 (one on each switch), but they have different
entries, the fabrics would fail to merge. You need to change the name of
one of them to Zone2 or something unique to fix the duplication. But for the
example merge here, no zones have been defined, so the two blank zone configurations
combine into one big empty list.
From now on, any zone configuration change made to either switch will propagate
(be copied) from one switch to the other (or to any other switches that
are connected and have merged into the same fabric.)
As mentioned earlier in this chapter, the connection between the two
switches is an ISL. If any data from a host goes into Switch-A1 and is destined
for a port on Switch-A2, the data is sent across the ISL to the other switch to
its destination. Figure 7-4 shows the path that the data takes between ports in
a fabric over the ISL to its destination.