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Understanding DCBX Application Protocol TLV Exchange on EX Series Switches

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.

LLDP and DCBX are enabled by default on all 10-Gigabit Ethernet interfaces of EX4500 CEE-enabled switches.


This topic applies only to DCBX on EX Series switches that do not support the Enhanced Layer 2 Software (ELS) configuration style. EX4500 and EX4550 switches are the only non-ELS EX Series switches that support DCBX.

DCBX TLV exchange on ELS EX Series switches and QFX Series switches is described in Understanding DCBX Application Protocol TLV Exchange.

This topic describes:

Basic Steps for Setting Up Application Protocol TLV Exchange

Setting up application protocol exchange for FCoE applications consists of:

  • Configuring the fcoe forwarding class for IEEE 802.1p code point 011

  • Configuring PFC for IEEE 802.1p code point 011

We recommend that you use code point 011 for the fcoe forwarding class, because this is the conventional IEEE 802.1p code point for FCoE traffic. We recommend that you configure PFC to use the same code point. See Example: Configuring an FCoE Transit Switch.

Setting up application protocol exchange for non-FCoE applications consists of:

  • Defining applications

  • Mapping the applications to IEEE 802.1p code points

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

  • Applying the application maps and classifiers to interfaces

Except for FCoE applications, you must explicitly define and map all applications that you want an interface to advertise.


Do not explicitly configure an FCoE application map, because doing that generates a commit error.


Before an interface can exchange application protocol information, you must define the applications that you want to advertise, except for the FCoE application, which is defined by default. You can define:

  • Layer 2 applications by EtherType

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

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 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 on the Internet Assigned Numbers Authority (IANA) website.

The switch automatically defines the FCoE application as 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 (with the exception of the FCoE application).

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 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. Applications that you want an interface to advertise must be configured in the application map that you apply to the interface (except the FCoE application). Do not explicitly configure an FCoE application map, because doing that generates a commit error.

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 its trafffic 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.

Traffic for the FCoE application is classified and prioritized by your configuration of the fcoe forwarding class.

Requirements for Interfaces in Non-FCoE Applications to Exchange Application Protocol Information

For non-FCoE applications, interfaces on which you want to exchange application protocol TLVs must include the following two items:

  • The application map that contains the application

  • A classifier

See Defining an Application for DCBX Application Protocol TLV Exchange and Configuring an Application Map for DCBX Application Protocol TLV Exchange.