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Understanding DCBX Application Protocol TLV Exchange

 

Data Center Bridging Capability Exchange protocol (DCBX) discovers the data center bridging (DCB) capabilities of connected peers. DCBX also advertises the capabilities of applications on interfaces by exchanging application protocol information through application type, length, and value (TLV) elements. DCBX is an extension of Link Layer Discovery Protocol (LLDP). LLDP must remain enabled on every interface on which you want to use DCBX.

Note

LLDP and DCBX are enabled by default on all interfaces.

Setting up application protocol exchange consists of:

  • Defining applications

  • Mapping the applications to IEEE 802.1p code points in an application map

  • Configuring classifiers to prioritize incoming traffic and map the incoming traffic to the application by the traffic code points

  • Applying the application maps and classifiers to interfaces

You need to explicitly define the applications that you want an interface to advertise. The FCoE application is a special case (see Applications) and only needs to be defined on an interface if you want DCBX to exchange application protocol TLVs for other applications in addition to FCoE on that interface.

You also need to explicitly map all of the defined applications that you want an interface to advertise to IEEE 802.1p code points in an application map. The FCoE application is a special case that only requires inclusion in an application map when you want an interface to use DCBX for other applications in addition to FCoE, as described later in this topic (see Application Maps).

This topic describes:

Applications

Before an interface can exchange application protocol information, you need to define the applications that you want to advertise. The exception is the FCoE application. If FCoE is the only application that you want the interface to advertise, then you do not need to define the FCoE application. You need to define the FCoE application only if you want interfaces to advertise other applications in addition to FCoE.

Note

If FCoE is the only application that you want DCBX to advertise on an interface, DCBX exchanges FCoE application protocol TLVs by default if the interface:

  • Carries FCoE traffic (traffic mapped by CoS configuration to the FCoE forwarding class and applied to the interface)

  • Has a congestion notification profile with PFC enabled on the FCoE priority (IEEE 802.1p code point)

  • Does not have an application map

If you apply an application map to an interface, then all applications that you want DCBX to advertise must be defined and configured in the application map, including the FCoE application.

If no CoS configuration for FCoE is mapped to an interface, that interface does not exchange FCoE application protocol TLVs.

You can define:

  • Layer 2 applications by EtherType

  • Layer 4 applications by a combination of protocol (TCP or UDP) and destination port number

The EtherType is a two-octet field in the Ethernet frame that denotes the protocol encapsulated in the frame. For a list of common EtherTypes, see http://standards.ieee.org/develop/regauth/ethertype/eth.txt on the IEEE standards organization website. For a list of port numbers and protocols, see the Service Name and Transport Protocol Port Number Registry at http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xml on the Internet Assigned Numbers Authority (IANA) website.

You must explicitly define each application that you want to advertise, except FCoE. The FCoE application is defined by default (EtherType 0x8906).

Application Maps

An application map maps defined applications to one or more IEEE 802.1p code points. Each application map contains one or more applications. DCBX includes the configured application code points in the protocol TLVs exchanged with the connected peer.

To exchange protocol TLVs for an application, you must include the application in an application map. The FCoE application is a special case:

  • If you want DCBX to exchange application protocol TLVs for more than one application on a particular interface, you must configure the applications, define an application map to map the applications to code points, and apply the application map to the interface. In this case, you must also define the FCoE application and add it to the application map.

    This is the same process and treatment required for all other applications. In addition, for DCBX to exchange FCoE application TLVs, you must enable priority-based flow control (PFC) on the FCoE priority (the FCoE IEEE 802.1p code point) on the interface.

  • If FCoE is the only application that you want DCBX to advertise on an interface, then you do not need to configure an application map and apply it to the interface. By default, when an interface has no application map, and the interface carries traffic mapped to the FCoE forwarding class, and PFC is enabled on the FCoE priority, the interface advertises FCoE TLVs (autonegotiation mode). DCBX exchanges FCoE application protocol TLVs by default until you apply an application map to the interface, remove the FCoE traffic from the interface (you can do this by removing the or editing the classifier for FCoE traffic), or disable PFC on the FCoE priority.

    If you apply an application map to an interface that did not have an application map and was exchanging FCoE application TLVs, and you do not include the FCoE application in the application map, the interface stops exchanging FCoE TLVs. Every interface that has an application map must have FCoE included in the application map (and PFC enabled on the FCoE priority) in order for DCBX to exchange FCoE TLVs.

Mapping an application to code points does two things:

  • Maps incoming traffic with the same code points to that application

  • Allows you to configure classifiers that map incoming application traffic, by code point, to a forwarding class and a loss priority, in order to apply class of service (CoS) to application traffic and prioritize application traffic

You apply an application map to an interface to enable DCBX application protocol exchange on that interface for each application specified in the application map. All of the applications that you want an interface to advertise must be configured in the application map that you apply to the interface, with the previously noted exception for the FCoE application when FCoE is the only application for which you want DCBX to exchange protocol TLVs on an interface.

Classifying and Prioritizing Application Traffic

When traffic arrives at an interface, the interface classifies the incoming traffic based on its code points. Classifiers map code points to loss priorities and forwarding classes. The loss priority prioritizes the traffic. The forwarding class determines the traffic output queue and CoS service level.

When you map an application to an IEEE 802.1p code point in an application map and apply the application map to an interface, incoming traffic on the interface that matches the application code points is mapped to the appropriate application. The application receives the loss priority and the CoS associated with the forwarding class for those code points, and is placed in the output queue associated with the forwarding class.

You can use the default classifier or you can configure a classifier to map the application code points defined in the application map to forwarding classes and loss priorities.

Enabling Interfaces to Exchange Application Protocol Information

Each interface with the fcoe forwarding class and PFC enabled on the FCoE code point is enabled for FCoE application protocol exchange by default until you apply an application map to the interface. If you apply an application map to an interface and you want that interface to exchange FCoE application protocol TLVs, you must include the FCoE application in the application map. (In all cases, to achieve lossless transport, you must also enable PFC on the FCoE code point or code points.)

Except when FCoE is the only protocol you want DCBX to advertise on an interface, interfaces on which you want to exchange application protocol TLVs must include the following two items:

  • The application map that contains the application(s)

  • A classifier

Note

You must also enable PFC on the code point of any traffic for which you want to achieve lossless transport.

Disabling DCBX Application Protocol Exchange

To disable DCBX application protocol exchange for all applications on an interface, issue the set protocols dcbx interface interface-name applications no-auto-negotiation command.

You can also disable DCBX application protocol exchange for applications on an interface by deleting the application map from the interface, or by deleting a particular application from the application map. However, when you delete an application from an application map, the application protocol is no longer exchanged on any interface which uses that application map.

On interfaces that use IEEE DCBX mode to exchange DCBX parameters, you can disable sending the enhanced transmission selection (ETS) Recommendation TLV to the peer if you want an asymmetric ETS configuration between the peers.