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Example: Configuring DCBX to Support an iSCSI Application

Data Center Bridging Capability Exchange protocol (DCBX) support for the application protocol type, length, and value (TLV) enables you to implement DCBX for various Layer 2 and Layer 4 applications. Internet small computer system interface (iSCSI) is a Layer 4 storage application that can benefit from DCBX. Implementing iSCSI over data center bridging (DCB) reduces latency in networks that are oversubscribed and provides a predictable and certain application responsiveness, eliminating Ethernet’s dependence on TCP/IP for the retransmission of dropped Ethernet frames. Although DCBX is not a requirement for such applications, it adds the reliability required for enterprise data storage.


You can configure and apply priority flow control (PFC) for any DCBX interfaces, but it is not a requirement for applications other than Fiber Channel over Ethernet (FCoE).

This example shows how to configure DCBX to support an iSCSI application:


This example uses the following hardware and software components:

  • One EX4500 switch (CEE-capable model)

  • Junos OS Release 12.1 or later for EX Series switches

Overview and Topology

You can use the same switch to support your LAN traffic and your storage area network (SAN) traffic—including both FCoE and iSCSI traffic. The DCBX application protocol TLV allows you to associate a specific DCBX interface with a specific application map.

DCBX discovers the DCB capabilities of peers by exchanging feature configuration information, detects feature misconfiguration and mismatches, and can configure DCB on peers. DCBX is an extension of Link Layer Discovery Protocol (LLDP). LLDP must remain enabled on every interface for which you want to use DCBX. If you attempt to enable DCBX on an interface on which LLDP is disabled, the configuration commit fails.

The switch supports DCBX information exchange for other applications, such as iSCSI, as specified in your configuration by EtherType or by the destination port and protocol.

To take advantage of this feature for non-FCoE applications, you must configure the application and application map and associate the application map with the interface that is carrying the application’s traffic. This configuration includes specifying the 802.1 code points to be used for this application.

When you configure an iSCSI application, you must always designate destination-port 3260.


DCBX is enabled by default on all 10-Gigabit Ethernet interfaces on EX4500 switches (CEE-capable models).

This example shows how to configure an iSCSI application on a DCBX interface of the EX4500 switch that is connected to an iSCSI storage device.


The components of the topology for this example are shown in Table 1.

Table 1: Components of the DCBX iSCSI Topology
Properties Settings

Switch hardware

One EX4500 switch (CEE capable model)



Application map code points


Interface for iSCSI application


Destination port


In this example, the switch has already been configured as follows:

  • DCBX is enabled by default on all 10-Gigabit Ethernet interfaces.


To configure DCBX to support an iSCSI application, perform these tasks:


CLI Quick Configuration

To quickly configure a DCBX interface for an iSCSI application, copy the following commands and paste them into the switch terminal window:

Step-by-Step Procedure

Configure a DCBX interface for an iSCSI application:

  1. Create the application:

  2. Create the application map:

  3. Apply the application map to the DCBX interface that you want to use for iSCSI:


Check the results of the configuration:


To confirm that the configuration is working properly:

Verifying That the iSCSI Application Is Advertised and That the Switch Interface and DCB Peer Are Using the Same 802.1p Code Points


Verify that both the switch and the DCB peer are using a DCBX iSCSI application configured for the same 802.1p code points.


Send some requests from the switch to the DCB peer.

Display the DCBX information advertised by DCBX interface (xe-0/0/37) and detected by the switch:


Check the status for Local-Advertisement in the section Feature: Application.

If there is misconfiguration between the switch and the DCB peer, the status displays Error: Yes.

In this example, there is no error. The output for Feature: Application, Protocol-State, displays a list of DCBX applications under Appl-Name.

This field displays information for the user-configured application iscsi. When you configure an iSCSI application, you must always designate the destination port as 3260. The output displays this as the Socket-Number .

The Priority-Map for the iSCSI application reflects the 802.1p code points that were specified in this example for the iSCSI-map. The example specified 101 for the iSCSI application map code points. The Priority-Map is an 8-bit code point format of the 802.1p code points; thus, 0010000.

The Status of the iSCSI application is Enabled, because the switch and the DCB are using the same code points for the iSCI application.