FlexFabric – Small Step, Right Direction

Note: I’ve added a couple of corrections below thanks to Stuart Miniman at Wikibon (http://wikibon.org/wiki/v/FCoE_Standards)  See the comments for more.

I’ve been digging a little more into the HP FlexFabric announcements in order to wrap my head around the benefits and positioning.  I’m a big endorser of a single network for all applications, LAN, SAN, IPT, HPC, etc. and FCoE is my tool of choice for that right now.  While I don’t see FCoE as the end goal, mainly due to limitations on any network use of SCSI which is the heart of FC, FCoE and iSCSI, I do see FCoE as the way to go for convergence today.  FCoE provides a seamless migration path for customers with an investment in Fibre Channel infrastructure and runs alongside other current converged models such as iSCSI, NFS, HTTP, you name it.  As such any vendor support for FCoE devices is a step in the right direction and provides options to customers looking to reduce infrastructure and cost.

FCoE is quickly moving beyond the access layer where it has been available for two years now.  That being said the access layer (server connections) is where it provides the strongest benefits for infrastructure consolidation, cabling reduction, and reduced power/cooling.  A properly designed FCoE architecture provides a large reduction in overall components required for server I/O.  Let’s take a look at a very simple example using standalone servers (rack mount or tower.)

imageIn the diagram we see traditional Top-of-Rack (ToR) cabling on the left vs. FCoE ToR cabling on the right.  This is for the access layer connections only.  The infrastructure and cabling reduction is immediately apparent for server connectivity.  4 switches, 4 cables, 2-4 I/O cards reduced to 2, 2, and 2.  This is assuming only 2 networking ports are being used which is not the case in many environments including virtualized servers.  For servers connected using multiple 1GE ports the savings is even greater.

Two major vendor options exist for this type of cabling today:


  • Brocade 8000 – This is a 1RU ToR CEE/FCoE switch with 24x 10GE fixed ports and 8x 1/2/4/8G fixed FC ports.  Supports directly connected FCoE servers. 
    • This can be purchased as an HP OEM product.
  • Brocade FCoE 10-24 Blade – This is a blade for the Brocade DCX Fibre Channel chassis with 24x 10GE ports supporting CEE/FCoE.  Supports directly connected FCoE servers.

Note: Both Brocade data sheets list support for CEE which is a proprietary pre-standard implementation of DCB which is in the process of being standardized with some parts ratified by the IEEE and some pending.  The terms do get used interchangeably so whether this is a typo or an actual implementation will be something to discuss with your Brocade account team during the design phase.  Additionally Brocade specifically states use for Tier 3 and ‘some Tier 2’ applications which suggests a lack of confidence in the protocol and may suggest a lack of commitment to support and future products.  (This is what I would read from it based on the data sheets and Brocade’s overall positioning on FCoE from the start.)


  • Nexus 5000 – There are two versions of the Nexus 5000:
    • 1RU 5010 with 20 10GE ports and 1 expansion module slot which can be used to add (6x 1/2/4/8G FC, 6x 10GE, 8x 1/2/4G FC, or 4x 1/2/4G FC and 4x 10GE)
    • 2RU 5020 with 40 10GE ports and 2 expansion module slots which can be used to add (6x 1/2/4/8G FC, 6x 10GE, 8x 1/2/4G FC, or 4x 1/2/4G FC and 4x 10GE)
    • Both can be purchased as HP OEM products.
  • Nexus 7000 – There are two versions of the Nexus 7000 which are both core/aggregation Layer data center switches.  The latest announced 32 x 1/10GE line card supports the DCB standards.  Along with support for Cisco Fabric path based on pre-ratified TRILL standard.

Note: The Nexus 7000 currently only supports the DCB standard, not FCoE.  FCoE support is planned for Q3CY10 and will allow for multi-hop consolidated fabrics.

Taking the noted considerations into account any of the above options will provide the infrastructure reduction shown in the diagram above for stand alone server solutions.

When we move into blade servers the options are different.  This is because Blade Chassis have built in I/O components which are typically switches.  Let’s look at the options for IBM and Dell then take a look at what HP and FlexFabric bring to the table for HP C-Class systems.


  • BNT Virtual Fabric 10G Switch Module – This module provides 1/10GE connectivity and will support FCoE within the chassis when paired with the Qlogic switch discussed below.
  • Qlogic Virtual Fabric Extension Module – This module provides 6x 8GB FC ports and when paired with the BNT switch above will provide FCoE connectivity to CNA cards in the blades.
  • Cisco Nexus 4000 – This module is an DCB switch providing FCoE frame delivery while enforcing DCB standards for proper FCoE handling.  This device will need to be connected to an upstream Nexus 5000 for Fibre Channel Forwarder functionality.  Using the Nexus 5000 in conjunction with one or more Nexus 4000s provides multi-hop FCoE for blade server deployments.
  • IBM 10GE Pass-Through – This acts as a 1-to-1 pass-through for 10GE connectivity to IBM blades.  Rather than providing switching functionality this device provides a single 10GE direct link for each blade.  Using this device IBM blades can be connected via FCoE to any of the same solutions mentioned under standalone servers.

Note: Versions of the Nexus 4000 also exist for HP and Dell blades but have not been certified by the vendors, currently only IBM supports the device.  Additionally the Nexus 4000 is a standards compliant DCB switch without FCF capabilities, this means that it provides the lossless delivery and bandwidth management required for FCoE frames along with FIP snooping for FC security on Ethernet networks, but does not handle functions such as encapsulation and de-encapsulation.  This means that the Nexus 4000 can be used with any vendor FCoE forwarder (Nexus or Brocade currently) pending joint support from both companies.


  • Dell 10GE Pass-Through – Like the IBM pass-through the Dell pass-through will allow connectivity from a blade to any of the rack mount solutions listed above.

Both Dell and IBM offer Pass-Through technology which will allow blades to be directly connected as a rack mount server would.  IBM additionally offers two other options: using the Qlogic and BNT switches to provide FCoE capability to blades, and using the Nexus 4000 to provide FCoE to blades. 

Let’s take a look at the HP options for FCoE capability and how they fit into the blade ecosystem.


  • 10GE Pass-Through – HP also offers a 10GE pass-through providing the same functionality as both IBM and Dell.
  • HP FlexFabric – The FlexFabric switch is a Qlogic FCoE switch OEM’d by HP which provides a configurable combination of FC and 10GE ports upstream and FCoE connectivity across the chassis mid-plane.  This solution only requires two switches for redundancy as opposed to four with FC and Ethernet configurations.  Additionally this solution works with HP FlexConnect providing 4 logical server ports for each physical 10GE link on a blade, and is part of the VirtualConnect solution which reduces the management overhead of traditional blade systems through  software.

On the surface FlexFabric sounds like the way to go with HP blades, and it very well may be, but let’s take a look at what it’s doing for our infrastructure/cable consolidation.


With the FlexFabric solution FCoE exists only within the chassis and is split to native FC and Ethernet moving up to the Access or Aggregation layer switches.  This means that while reducing the number of required chassis switch components and blade I/O cards from four to two there has been no reduction in cabling.  Additionally HP has no announced roadmap for a multi-hop FCoE device and their current offerings for ToR multi-hop are OEM Cisco or Brocade switches.  Because the HP FlexFabric switch is a Qlogic switch this means any FC or FCoE implementation using FlexFabric connected to an existing SAN will be a mixed vendor SAN which can pose challenges with compatibility, feature/firmware disparity, and separate management models.

HP’s announcement to utilize the Emulex OneConnect adapter as the LAN on motherboard (LOM) adapter makes FlexFabric more attractive but the benefits of that LOM would also be recognized using the 10GE Pass-Through connected to a 3rd party FCoE switch, or a native Nexus 4000 in the chassis if HP were to approve and begin to OEM he product.


As the title states FlexFabric is definitely a step in the right direction but it’s only a small one.  It definitely shows FCoE commitment which is fantastic and should reduce the FCoE FUD flinging.  The main limitation is the lack of cable reduction and the overall FCoE portfolio.  For customers using, or planning to use VirtualConnect to reduce the management overhead of the traditional blade architecture this is a great solution to reduce chassis infrastructure.  For other customers it would be prudent to seriously consider the benefits and drawbacks of the pass-through module connected to one of the HP OEM ToR FCoE switches.

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FCoE multi-hop; Do you Care?

There is a lot of discussion in the industry around FCoE’s current capabilities, and specifically around the ability to perform multi-hop transmission of FCoE frames and the standards required to do so.  A recent discussion between Brad Hedlund at Cisco and Ken Henault at HP (http://bit.ly/9Kj7zP) prompted me to write this post.  Ken proposes that FCoE is not quite ready and Brad argues that it is. 

When looking at this discussion remember that Cisco has had FCoE products shipping for about 2 years, and has a robust product line of devices with FCoE support including: UCS, Nexus 5000, Nexus 4000 and Nexus 2000, with more products on the road map for launch this year.  No other switching vendor has this level of current commitment to FCoE.  For any vendor with a less robust FCoE portfolio it makes no sense to drive FCoE sales and marketing at this point and so you will typically find articles and blogs like the one mentioned above.  The one quote from that blog that sticks out in my mind is:

“Solutions like HP’s upcoming FlexFabric can take advantage of FCoE to reduce complexity at the network edge, without requiring a major network upgrades or changes to the LAN and SAN before the standards are finalized.”

If you read between the lines here it would be easy to take this as ‘FCoE isn’t ready until we are.’  This is not unusual and if you take a minute to search through articles about FCoE over the last 2-3 years you’ll find that Cisco has been a big endorser of the protocol throughout (because they actually had a product to sell) and other vendors become less and less anti-FCoE as they announce FCoE products.

It’s also important to note that Cisco isn’t the only vendor out there embracing FCoE: NetApp has been shipping native FCoE storage controllers for some time, EMC has them road mapped for the very near future, Qlogic is shipping a 2nd generation of Converged Network adapter, and Emulex has fully embraced 10Gig Ethernet as the way forward with their OneConnect adapter (10GE, iSCSI, FCoE all in one card.)  Additionally support for FCoE switching of native Fibre Channel storage is widely supported by the storage community.

Fibre Channel over Ethernet (FCoE) is defined in IEEE FC-BB5 and requires the switches it traverses to support the IEEE Data Center Bridging (DCB)standards for proper traffic treatment on the network.  For more information on FCoE or DCB see my previous posts on the subjects (FCoE: http://www.definethecloud.net/?p=80, DCB: http://www.definethecloud.net/?p=31.)

DCB Has four major components, and the one in question in the above article is Quantized Congestion Notification (QCN) which the article states is required for multi-hop FCoE.  QCN is basically a regurgitation of FECN and BECN from frame relay.  It allows a switch to monitor it’s buffers and push congestion to the edge rather than clog the core. In the comments Brad correctly states that QCN is not required for FCoE, the reason for this is that Fibre Channel operates today without any native version of QCN, therefore when placing it on Ethernet you will not need to add functionality that wasn’t there to begin with, remember Ethernet is just a new layer 1-2 for native FC layers 2-4, the FC secret sauce remains unmodified.  Remember that not every standard defined by a standards body has to be adhered to by every device, some are required, some are optional.  Logical SANs are a great example of an optional standard.

Rather than discuss what is or isn’t required for multi-hop FCoE I’d like to ask a more important question that we as engineers tend to forget: Do I care?  This question is key because it avoids having us argue the technical merits of something we may never actually need, or may not have a need for today.

Do we care?

First let’s look at why we do multi-hop anything: to expand the port-count of our network.  Take TCP/IP networks and the internet for example, we require the ability to move packets across the globe through multiple routers (hops.)  This is in order to attach devices on all corners of the globe.

Now let’s look at what we do with FC today: typically one or two hop networks (sometimes three) used to connect several hundred devices (occasionally but rarely more.)  It’s actually quite common to find FC implementations with less than 100 attached ports.  This means that if you can hit the right port count without multiple hops you can remove complexity and decrease latency, in Storage Area Networks (SAN) we call this the collapsed core design.

The second thing to consider is a hypothetical question: If FCoE were permanently destined for single hop access/edge only deployments (it isn’t) should that actually stop you from using it?  The answer here is an emphatic no, I would still highly recommend FCoE as an access/edge architecture even if it were destined to connect back to an FC SAN and Ethernet LAN for all eternity.  Let’s jump to some diagrams to explain.  In the following diagrams I’m going to focus on Cisco architecture because as stated above they are currently the only vendor with a full FCoE product portfolio.


In the above diagram you can see a fairly dynamic set  of FCoE connectivity options.  Nexus 5000 can be directly connected to servers, or to Nexus 4000 in IBM BladeCenter to pass FCoE.  It can also be connected to 10GE Nexus 2000s to increase its port density. 

To use the nexus 5000 + 2000 as an example it’s possible to create a single-hop (2000 isn’t an L2 hop it is an extension of the 5000) FCoE architecture of up to 384 ports with one point of switching management per fabric.  If you take server virtualization into the picture and assume 384 servers with a very modest V2P ratio of 10 virtual machines to 1 physical machine that brings you to 3840 servers connected to a single hop SAN.  That is major scalability with minimal management all without the need for multi-hop. The diagram above doesn’t include the Cisco UCS product portfolio which architecturally supports up to 320 FCoE connected servers/blades.

The next thing I’ve asked you to think about is whether or not you should implement FCoE in a hypothetical world where FCoE stays an access/edge architecture forever.  The answer would be yes.  In the following diagrams I outline the benefits of FCoE as an edge only architecture.


The first benefit is reducing the networks that are purchased, managed, power, and cooled from 3 to 1 (2 FC and 1 Eth to 1 FCoE.)  Even just at the access layer this is a large reduction in overhead and reduces the refresh points as I/O demands increase.

image The second benefit is the overall infrastructure reduction at the access layer.  Taking a typical VMware server as an example we reduce 6x 1GE ports, 2x 4GFC ports and the 8 cables required for them to 2x 10GE ports carrying FCoE.  This increases total bandwidth available while greatly reducing infrastructure.  Don’t forget the 4 top-of-rack switches (2x FC, 2x GE) reduced to 2 FCoE switches.

Since FCoE is fully compatible with both FC and pre-DCB Ethernet this requires 0 rip-and-replace of current infrastructure.  FCoE is instead used to build out new application environments or expand existing environments while minimizing infrastructure and complexity.

What if I need a larger FCoE environment?

If you require a larger environment than is currently supported extending your SAN is quite possible without multi-hop FCoE.  FCoE can be extended using existing FC infrastructure.  Remember customers that require an FCoE infrastructure this large already have an FC infrastructure to work with.


What if I need to extend my SAN between data centers?

FCoE SAN extension is handled in the exact same way as FC SAN extension, CWDM, DWDM, Dark Fiber, or FCIP.  Remember we’re still moving Fibre Channel frames.



FCoE multi-hop is not an argument that needs to be had for most current environments.  FCoE is a supplemental technology to current Fibre Channel implementations.  Multi-hop FCoE will be available by the end of CY2010 allowing 2+ tier FCoE networks with multiple switches in the path, but there is no need to wait for them to begin deploying FCoE.  The benefits of an FCoE deployment at the access layer only are significant, and many environments will be able to scale to full FCoE roll-outs without ever going mutli-hop. 

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FCoE initialization Protocol (FIP) Deep Dive

In an attempt to clarify my future posts I will begin categorizing a bit.  The following post will be part of a Technical Deep Dive series.

Fibre Channel over Ethernet (FCoE) is a protocol designed to move native Fibre Channel over 10 Gigabit Ethernet and above links, I’ve described the protocol in a previous post (http://www.definethecloud.net/?p=80.)  In order for FCoE to work we need a mechanism to carry the base Fibre Channel port / device login mechanisms over Ethernet.  These are the processes for a port to login and obtain a routable Fibre Channel Address.  Let’s start with some background and definitions:

DCB Data Center Bridging
FC Native Fibre Channel Protocol
FCF Fibre Channel Forwarder (an Ethernet switch capable of handling Encapsulation/De-encapsulation of FCoE frames and some or all FC services)
FCID Fibre Channel ID (24 Bit Routable address)
FCoE Fibre Channel over Ethernet
FC-MAP A 24-Bit value identifying an individual fabric
FIP FCoE Initialization Protocol
FLOGI FC Fabric Login
FPMA Fabric Provided MAC Address
PLOGI FC Port Login
PRLI Process Login
SAN Storage Area Network (switching infrastructure)
SCSI Small Computer Systems Interface
Now for the background, you’ll never grasp FIP properly if you don’t first get the fundamentals of FC:
N_Port Initialization


When a node comes online it’s port is considered an N_port.  When an N_port connects to the SAN it will connect to a switch port defined as a Fabric Port F_Port (this assumes your using a switched fabric.)  All N_ports operate the same way when they are brought online:

  1. FLOGI – Used to obtain a routable FCID for use in FC frame exchange.  The switch will provide the FCID during a FLOGI exchange.
  2. PLOGI – Used to register the N_Port with the FC name server

At this point a targets (disk or storage array) job is done, they can now sit and wait for requests.  An initiator (server) on the other hand needs to perform a few more tasks to discover available targets:

  1. Query – Request available targets from the FC name server, zoning will dictate which targets are available.
  2. PLOGI – A 2nd port Login, this time into the target port.
  3. PRLI – Process login to exchange supported upper layer protocols (ULP) typically SCSI-3.

Once this process has been completed the initiator can exchange frames with the target, i.e. the server can write to disk.


The reason the FC login process is key to understanding FIP is that this is the process that FIP is handling for FCoE networks.  FIP allows an Ethernet attached FC node (Enode) to discover existing FCFs and supports the FC login procedure over 10+GE networks.  Rather than just providing an FCID, FIP will provide an FPMA which is a MAC address comprised of two parts: FC-MAP and FCID.

48 bit FCMAP (Mac Address)




So FIP provides an Ethernet MAC address used by FCoE to traverse the Ethernet network which contains the FCID required to be routed on the FC network.  FIP also passes the query and query response from the FC name server.  FIP uses a separate Ethertype from FCoE and its frames are standard Ethernet size (1518 Byte 802.1q frame) whereas FCoE frames are 2242 Byte Jumbo Frames.

FIP Snooping:

FIP snooping is used in multi-hop FCoE environments.  FIP snooping is a frame inspection method that can be used by FIP snooping capable DCB devices to monitor FIP frames and apply policies based on the information in those frames.  This allows for:

  • Enhanced FCoE security (Prevents FCoE MAC spoofing.)
  • Creates FC point-to-point links within the Ethernet LAN
  • Allows auto-configuration of ACLs based on name server information read in the FIP frames

FIP Snooping



FIP snooping uses dynamic Access Control Lists to enforce Fibre Channel rules within the DCB Ethernet network.  This prevents Enodes from seeing or communicating with other Enodes without first traversing an FCF.

Feedback, corrections, updates, questions?

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Fibre Channel over Ethernet

Fibre Channel over Ethernet (FCoE) is a protocol standard ratified in June of 2009.  FCoE provides the tools for encapsulation of Fibre Channel (FC) in 10 Gigabit Ethernet frames.  The purpose of FCoE is to allow consolidation of low-latency, high performance FC networks onto 10GE infrastructures.  This allows for a single network/cable infrastructure which greatly reduces switch and cable count, lowering the power, cooling, and administrative requirements for server I/O.

FCoE is designed to be fully interoperable with current FC networks and require little to no additional training for storage and IP administrators. FCoE operates by encapsulating native FC into Ethernet frames.  Native FC is considered a ‘lossless’ protocol, meaning frames are not dropped during periods of congestion.  This is by design in order to ensure the behavior expected by the SCSI payloads.  Traditional Ethernet does not provide the tools for lossless delivery on shared networks so enhancements were defined by the IEEE to provide appropriate transport of encapsulated Fibre Channel on Ethernet networks.  These standards are known as Data Center Bridging (DCB) which I’ve discussed in a previous post (http://www.definethecloud.net/?p=31.)  These Ethernet enhancements are fully backward compatible with traditional Ethernet devices, meaning DCB capable devices can exchange standard Ethernet frames seamlessly with legacy devices.  The full 2148 Byte FC frame is encapsulated in an Ethernet jumbo frame avoiding any modification/fragmentation of the FC frame.

FCoE itself takes FC layers 2-4 and maps them to Ethernet layers 1-2, this replaces the FC-0 Physical layer, and FC-1 Encoding Layer.  This mapping between Ethernet and Fibre Channel is done through a Logical End-Point (LEP) which can by thought of as a translator between the two protocols.  The LEP is responsible for providing the appropriate encoding and physical access for frames traveling from FC nodes to Ethernet nodes and vice versa.  There are two devices that typically act as FCoE LEPs: Fibre Channel Forwarders (FCF) which are switches capable of both Ethernet and Fibre Channel, and Converged Network Adapters (CNA) which provide the server-side connection for a FCoE network.  Additionally the LEP operation can be done using a software initiator and traditional 10GE NICs but this places extra workload on the server processor rather than offloading it to adapter hardware.

One of the major advantages of replacing FC layers 0-1 when mapping onto 10GE is the encoding overhead.  8GB Fibre Channel uses an 8/10 bit encoding which adds 25% protocol overhead, 10GE uses a 64/64 bit encoding which has about 2% overhead, dramatically reducing the protocol overhead and increasing throughput.  The second major advantage is that FCoE maintains FC layers 2-4 which allows seamless integration with existing FC devices and maintains the Fibre Channel tool set such as zoning, LUN masking etc.  In order to provide FC login capabilities, multi-hop FCoE networks, and FC zoning enforcement on 10GE networks FCoE relies on another standard set known as Fibre Channel initialization Protocol (FIP) which I will discuss in a lter post.

Overall FCoE is one protocol to choose from when designing converged networks, or cable-once architectures.  The most important thing to remember is that a true cable-once architecture doesn’t make you choose your Upper Layer Protocol (ULP) such as FCoE, only your underlying transport infrastructure.  If you choose 10GE the tools are now in place to layer any protocol of your choice on top, when and if you require it.

Thanks to my colleagues who recently provided a great discussion on protocol overhead and frame encoding…

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Data Center Bridging Exchange

Data Center Bridging Exchange (DCBX) is one of the components of the DCB standards.  These standards offer enhancements to standard ethernet which are backwards compatible with traditional Ethernet and provide support for I/O Consolidation (http://www.definethecloud.net/?p=18.)  The three purposes of DCBX are:

Discovery of DCB capability:

The ability for DCB capable devices to discover and identify capabilities of DCB peers as well as identify non-DCB capable legacy devices.  You can find more information on DCB in a previous post (http://www.definethecloud.net/?p=31.)

Identification of misconfigured DCB features:

The ability to discover misconfiguration of features that require symmetric configuration between DCB peers.  Some DCB features are asymmetric meaning they can be configured differently on each end of a link, other features must match on both sides to be effective (symmetric.)  This functionality allows detection of configuration errors for these symmetric features.

Configuration of Peers:

A capability allowing DCBX to pass configuration information to a peer.  For instance a DCB capable switch can pass Priority Flow Control (PFC) information on to a Converged Network Adapter (CNA) to ensure FCoE traffic is appropriately tagged and pause is enabled for the chosen Class of Service (CoS) value.  This PFC exchange is a symmetric exchange and must match on both sides of the link.  DCB features such as Enhanced Transmission Selection (ETS) otherwise known as bandwidth management can be configured asymmetrically (different on each side of the link.)

DCBX relies on Link Level Discovery Protocol (LLDP) in order to pass this information and configuration.  LLDP is an industry standard version of Cisco Discovery Protocol (CDP) which allows devices to discover one another and exchange information about basic capabilities.  Because DCBX relies on LLDP and is an acknowledged protocol (2 way communication) any link which is intended to support DCBX must have LLDP enabled on both sides of the link for Tx/Rx.  When a port has LLDP disabled for either Rx or Tx DCBX is disabled on the port and DCBX Type Length Values (TLV) within received LLDP frames will be ignored.

DCBX capable devices should have DCBX enabled by default with the ability to administratively disable it.  This allows for more seamless deployments of DCB networks with less tendency for error.

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Data Center Bridging

Data Center Bridging (DCB) is a group of IEEE standard protocols designed to support I/O consolidation.  DCB enables multiple protocols with very different requirements to run over the same Layer 2 10 Gigabit Ethernet infrastructure.  Because DCB is currently discussed along with Fibre Channel over Ethernet (FCoE) it’s not uncommon for people to think of them as part of FCoE.  This is not the case, while FCoE relies on DCB for proper treatment on a shared network, DCB enhancements can be applied to any protocol on the network.  DCB support is being built into data center hardware and software from multiple vendors and is fully backwards compatible with legacy systems (no forklift upgrades.)  For more information on FCoE see my post on the subject (http://www.definethecloud.net/?p=80.)

Network protocols typically have unique requirements in regards to latency, packet/frame loss, bandwidth, etc.  These differences have a large impact on the performance of the protocol in a shared environment.  Differences such as flow-control and frame loss are the reason Fibre Channel networks have traditionally been separate physical infrastructures from Ethernet networks.  DCB is the set of tools that allows us to converge these networks without sacrificing performance or reliability.

Lets take a look at the DCB suite:

Priority Flow Control (PFC) 802.1Qbb:

PFC is a flow control mechanism.  PFC is designed to eliminate frame loss for specific traffic types on Ethernet networks.  Protocols such as Small Computer System Interface (SCSI) which is used for block data storage are very sensitive to data loss.  SCSI protocol is the heart of Fibre Channel which is a tool used to extend SCSI from internal disk to centralized storage across a network.  In its native form on dedicated networks Fibre Channel has tools to ensure that frames are not lost as long as the network is stable.  In order to move Fibre Channel across Ethernet networks that same ‘lossless’ behavior must be guaranteed, PFC is the tool to do that.

PFC uses a pause mechanism to allow a receiving device to signal a pause to the directly connected sending device prior to buffer overflow and packet loss.  While Ethernet has had a tool to do this for some time (802.3x pause) it has always been at the link level.  This means that all traffic on the link would be paused, rather than just a selected traffic type.  Pausing a link carrying various I/O types would be a bad thing, especially for traffic such as IP Telephony and streaming video.  Rather than pause an entire link PFC sends a pause signal for a single Class of Service (CoS) which is part of an 802.1Q Ethernet header.  This allows up to 8 classes to be defined and paused independent of one another.

Congestion Management (802.1Qau):

When we begin pausing traffic in a network we have the potential to spread network congestion by causing choke points.  Imagine trying to drive past a football stadium (football or American football pick your flavor) when the game is about to start.  You’re stuck in dead lock traffic even though you’re not going to the game, if you’ve got that image your on the right track.  Congestion management is a set of signaling tools used to push that congestion out of the network core to the network edge (if you’re thinking old school FECN and BECN you’re not far off.)

Bandwidth Management (802.1Qaz):

Bandwidth management is a tool for simple consistent application of bandwidth controls at Layer 2 on a DCB network.  Bandwidth management allows specific traffic type to be guaranteed a percentage of available bandwidth based on its CoS.  For instance on a 10GE network access port utilizing FCoE you could guarantee 40% of the bandwidth to FCoE.  This provides a 4Gb tunnel for FCoE when needed but allows other traffic types to utilize that bandwidth when not in use for FCoE.

Data Center bridging Exchange (DCBX):

DCBX is a Layer 2 communication protocol that allows DCB capable devices to communicate and discover the edge of the DCB network, i.e. legacy devices.  DCBX not only allows passing of information but provides tools for passing configuration.  This is key to the consistent configuration of DCB networks.  For instance a DCB switch acting as a Fibre Channel over Ethernet Forwarder (FCF) can let an attached Converged Network Adapter (CNA) on a server know to tag FCoE frames with a specific CoS and enable pause for that traffic type.

All in all the DCB features are key enablers for true consolidated I/O.  They provide a tool set for each traffic type to be handled properly independent of other protocols on the wire.  For more information on Consolidated I/O see my previous post Consolidated IO (http://www.definethecloud.net/?p=67.)

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