McDATA StorageWorks 2/140 - Director Switch Planning Manual page 161

Implementing SAN Internetworking Solutions
4
During SAN router configuration, each R_Port is assigned (through
the SANvergence manager application) a unique Fabric_ID between
1 and 12. Although the theoretical limit is 12 Fabric_IDs per mSAN,
the supported limit is six. As shown in Table 4-1, four Area_IDs are
available to each Fabric_ID. Therefore, the combination of domain,
area, and fabric IDs creates a theoretical limit of 1,024 devices per
fabric (although the supported number is far less).
When a fabric element encounters a device with a Fibre Channel
network address starting with Domain_ID 30 or 7E, the associated
device is physically connected to a different fabric. In addition,
routing communication between the fabric element and device is
provided through FC_NAT technology. Fibre Channel network
addresses are not unique to each routed fabric (and require router
translation for cross-fabric communication) because the Domain_ID
space is reused across fabrics. Although device network addresses are
router translated, device WWNs are not translated and remain
consistent across the entire routed fabric.
Router Name Servers
Each SAN router in an mSAN (up to two) maintains an mSNS
database. With one SAN router installed, the router maintains a
primary simple name server (pSNS) database with information about
all fabric-attached or router-attached devices in the mSAN (and
across iSANs). The pSNS, using the router fabric manager R_Port
as a conduit, interfaces with the fabric SNS to form a complete name
server database.
With two SAN routers installed, one router maintains a pSNS
database and the second router maintains a secondary simple name
server (sSNS) database. Each mSAN always has one pSNS. The
sSNS contains information only about devices directly attached to the
second router and is a client to the pSNS. The pSNS router is user-
selected or assigned during the build fabric process on the basis of the
lowest WWN.
The secondary router sSNS transmits connectivity information to the
primary router pSNS as required. Because SNS databases use unicast
and subnet broadcasts to communicate, the pSNS and sSNS routers
must be configured on the same subnet. If the mFCP IRL between the
routers segments, different information exists in the SNS databases.
Implementing SAN Internetworking Solutions
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