Internetworking for FC SAN extension

Posted on June 01, 2003

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As storage networking demands increase, IT organizations may need to connect geographically distributed networks. "Internetworking" is one option.

By Tom Clark

The maturity of the Fibre Channel storage area network (SAN) market is in part affirmed by the growing requirement of enterprises to connect their existing SAN islands together. Be it for disaster recovery, campus storage connectivity, consolidating remote tape backup, or content sharing over long distances, linking SANs together is the next evolutionary step once departmental and application-specific storage networks have been deployed.

Unlike connecting Ethernet switches or routers, however, connecting Fibre Channel fabrics is not as simple as cabling ports together. The fabric services that the Fibre Channel architecture provides require special attention to ensure interoperability and stability of the storage network as more Fibre Channel switches are combined into a single network.

This article examines Fibre Channel fabric extension and reviews an alternative approach to SAN connectivity: SAN internetworking.

Fibre Channel basics

Fibre Channel fabrics are self-configuring. When a Fibre Channel end device such as a storage array is attached to a Fibre Channel switch, it must log on to the switch to receive a unique 24-bit network address, which is used for routing data through the fabric from one device to another. As shown in the table, the three-byte address is composed of Domain, Area, and Port fields. For the purposes of fabric building, the most significant byte (Byte 0) typically identifies a single fabric switch. After subtracting reserved addresses, there are 239 possible Domain identifiers for individual fabric switches, which is the maximum number of switches that could theoretically be joined in a single Fibre Channel network.


In Fibre Channel, a three-byte network address includes Domain, Area, and Port fields.
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With a unique Domain identifier, a fabric switch can allocate the remaining two bytes (Area and Port) to provide approximately 64,000 addresses (256 x 256) for individual devices. Because the fabric switch is responsible for issuing unique network addresses to end devices, the end devices do not require manual address administration. In practice, however, SAN administrators are often forced to verify or assign preferred addresses due to specific requirements of particular host bus adapters (HBAs) or storage arrays.

If each Fibre Channel switch is responsible for automatically assigning unique addresses to its end devices, there must also be some mechanism to ensure each fabric switch has a unique Domain identifier. This is accomplished through a process known as principal switch selection. This process determines which switch will act as master in the newly configured SAN and oversee the allocation of unique blocks of addresses to each switch. Without a principal switch selection process, two switches could inadvertently maintain the same Domain identity, which would result in duplicate addressing in the network and misrouting of data.

During the principal switch selection procedure, one of two types of fabric building will occur. A non-disruptive Build Fabric event will try to preserve existing Domain identities for each switch and thus not require reassignment of address blocks. A disruptive Build Fabric event, by contrast, may cause some switches to acquire a different Domain identifier. This disruptive fabric reconfiguration, in turn, mandates that each attached end device re-login to the fabric to acquire new network addresses. Any time two or more fabric switches with identical Domain identifiers are connected together, a disruptive fabric reconfiguration is necessary to avoid address duplication.

Ironically, the automatic addressing and self-governing functionality that a Fibre Channel architecture implements to simplify network convergence is making storage networking more problematic. Disruptive fabric reconfiguration has become an issue for customers who are attempting to build large Fibre Channel fabrics or who are extending Fibre Channel fabrics over distance.

Fibre Channel SAN extension

The principal switch selection process is invoked whenever a potential addressing conflict occurs between two or more joined fabric switches. Whether two switches are separated by a few feet within a data center or by hundreds of miles between remote sites, establishing a connection between their expansion ports may trigger a fabric reconfiguration. A stretched fabric connection such as E_Port connectivity over a wide area network, however, is vulnerable to both disruptive fabric reconfigurations initiated by local site events and by those initiated by disruptions in the WAN transport.


Figure 1: With SAN extension techniques using DWDM or FCIP tunneling, a fabric reconfiguration event at one site will propagate to another site.
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Traditional SAN extension builds a single Fibre Channel fabric between two remote locations. As shown in Figure 1 (top), the E_Port connection may be directly through dedicated fiber for metropolitan distances or through dense wavelength-division multiplexing (DWDM) over a fiber-optic cable plant. Native Fibre Channel extension over fiber may support metropolitan distances to a radius of 50 or more miles. To extend further, the Fibre Channel over IP (FCIP) tunneling protocol and IP network services may be used (bottom). FCIP simply wraps each Fibre Channel frame in an IP datagram for transport over an IP network. At the receiving end, the IP datagram is removed and native Fibre Channel frames are delivered to the remote SAN.

Whether via direct Fibre Channel connection, DWDM, or use of the FCIP protocol, standard Fibre Channel fabric building occurs. The stretched E_Port connection between Fibre Channel switches at either end triggers traditional principal switch protocols between them.

As illustrated in Figure 1, a fabric reconfiguration event at one local site will necessarily propagate to the other site. For example, if an operational Fibre Channel switch with a conflicting Domain identity is inserted into one site, the entire stretched fabric must undergo fabric reconfiguration. Likewise, a disruption in the wide area link may cause the single extended SAN to devolve into separate SAN islands until stable service is restored. In either case, storage transactions are disrupted for the duration. For mission-critical storage over distance applications such as disaster recovery, an extended SAN may defeat the purpose of the installation itself. Created to ensure data availability, the extended SAN may inadvertently introduce instabilities that subvert highly reliable data access.

Because traditional SAN extension creates one large, flat Fibre Channel network, it has been difficult to create stable configurations for large deployments within data centers and between geographically remote sites. Analogous to Layer 2 (link layer) bridged LANs of the 1980s, extended SANs are vulnerable to flooding or broadcast storms that interfere with normal data traffic. However, a Layer 3 routing function can provide connectivity between SAN islands while preserving the autonomy of each local SAN. This solution is sometimes referred to as SAN internetworking, based on the Internet Fibre Channel Protocol (iFCP) and IP storage switches, which can eliminate fabric reconfiguration issues.

SAN internetworking provides interoperable E_Port connectivity to each local SAN fabric. Unlike conventional SAN extension, however, SAN internetworking terminates the E_Port connection at each site. Fabric building is thus restricted to each location, and Fibre Channel switch-to-switch protocols are not passed across the IP network. If two or more sites are connected by IP storage switches with iFCP (see Figure 2), each site remains a separate SAN. Only authorized (zoned) connections between storage devices and servers are allowed across the IP network.

By preserving the autonomy of each local SAN, a SAN internetworking solution ensures disruptions at one site will be isolated and not allowed to propagate to other locations. As shown in Figure 2, a fabric reconfiguration event at SAN C would not affect storage transactions occurring between SAN A and SAN B. This provides stability for the connected SANs and high availability for disaster recovery, consolidated tape backup, and other storage applications.

Analogous to Layer 3 network routing, SAN internetworking facilitates high-performance transactions for each SAN segment but does not expose the storage network as a whole to potential disruption. This enables IT organizations to deploy complex, multi-SAN solutions while leveraging the cost benefits of IP network services.


Figure 2: If multiple sites are connected by IP storage switches with iFCP, each site remains a separate SAN. Only authorized connections between storage devices and servers are allowed across the IP network.
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In addition to fault isolation, SAN internetworking streamlines SAN connectivity by eliminating address overlap issues. Internetworking permits use of duplicate Domain addressing on separate Fibre Channel switches without incurring routing errors. In Figure 2, for example, a fabric switch in SAN A could have the same Domain identity as a fabric switch in SAN C. Since the switches at both sites remain in separate SANs, address conflicts between the SANs do not occur. The SAN internetworking engine provides the appropriate address translation only for the devices that have been authorized to communicate across the network. The administrator, therefore, does not have to constantly monitor or manipulate switch addresses to prevent Domain ID conflicts.

Multi-vendor interoperability

One of the persistent problems associated with large Fibre Channel fabrics has been the issue of multi-vendor interoperability. Due to delays in establishing common standards for switch-to-switch communications and the Fabric Shortest Path First (FSPF) protocol, Fibre Channel switch vendors must now support multiple interoperability modes. In addition to standards-compliant E_Port mode, a vendor may also support one or more proprietary modes. Furthermore, ongoing enhancements to both proprietary and open systems switch-to-switch protocols result in various microcode versions for a particular switch model. A switch resold through an OEM is typically one microcode version level behind the same switch sold directly by the manufacturer. This makes it difficult for users to directly connect fabric switches to build larger SANs, even if the switches are made by the same vendor. Simply upgrading to a common microcode version may not be possible, either due to disruption of product traffic or for fear of violating the OEM's warranty.

To address this long-standing interoperability issue, IP storage switches may provide concurrent support for multiple E_Port compatibility modes, including vendor-specific and open systems versions. Thus, in addition to the benefits of fault isolation and stability provided by SAN internetworking, end users can leverage their existing Fibre Channel switch assets that otherwise would be a source of potential interoperability conflicts.

Internetworking in the data center

SAN internetworking provides a solution for connecting existing SAN islands together. By maintaining the autonomy of each SAN site and restricting propagation of potential faults, SAN internetworking is especially suited to storage-over-distance applications such as disaster recovery and remote tape backup. In some data-center situations, however, it is desirable to link separate SANs to share the tape subsystem or other storage assets. To avoid building one large Fibre Channel fabric with its accompanying fabric reconfiguration issues, SAN internetworking can be used to provide connectivity and fault isolation.


Figure 3: A single IP storage switch can be used to connect multiple SANs within a data center, building, or campus.
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As shown in Figure 3, a single IP storage switch can be used to connect multiple SANs within a data center, building, or campus. Each SAN continues to function as a separate fabric, and yet each has access to authorized storage resources through the IP storage switch. This provides the benefits of shared access while eliminating interoperability and fabric building issues. A disruption in the Development SAN, for example, would be restricted to the Development group, while Production, Finance, and Engineering could continue normal storage operations. This solution is also scalable and can accommodate additional SANs and IP storage switches to support a larger population of devices over time.

By providing a SAN routing function for Fibre Channel fabrics, SAN internetworking enables IT organizations to build very large storage networks and extend SAN traffic to virtually any distance and any number of sites. This provides new opportunities for end users to implement enterprise-wide SAN strategies that are stable, interoperable, and scalable.

Tom Clark is director of technical marketing at Nishan Systems (www.nishansystems.com) and is the author of IP SANs and Designing Storage Area Networks Second Edition (Addison-Wesley).


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