Brocade Communications Systems 53-1002163-02 manual Enhanced Transmission Selection ETS

9 DCBX overview

DCBX uses LLDP to exchange parameters between two link peers; DCBX is built on the LLDP infrastructure for the exchange of information. DCBX-exchanged parameters are packaged into organizationally specific TLVs. The DCBX protocol requires an acknowledgement from the other side of the link, therefore LLDP is turned on in both transmit and receive directions. DCBX requires version number checking for both control TLVs and feature TLVs.

DCBX interacts with other protocols and features as follows:

•LLDP—LLDP is run in parallel with other Layer 2 protocols such as RSTP and LACP. DCBX is built on the LLDP infrastructure to communicate capabilities supported between link partners. The DCBX protocol and feature TLVs are treated as a superset of the LLDP standard.

•QoS management—DCBX capabilities exchanged with a link partner are passed down to the QoS management entity to set up the Brocade FCoE hardware to control the scheduling and priority-based flow control in the hardware.

The DCBX standard is subdivided into two features sets:

•“Enhanced Transmission Selection (ETS)”

•“Priority Flow Control (PFC)”

Enhanced Transmission Selection (ETS)

In a converged network, different traffic types affect the network bandwidth differently. The purpose of ETS is to allocate bandwidth based on the different priority settings of the converged traffic. For example, Inter-process communications (IPC) traffic can use as much bandwidth as needed and there is no bandwidth check; LAN and SAN traffic share the remaining bandwidth. Table 20 displays three traffic groups: IPC, LAN, and SAN. ETS allocates the bandwidth based on traffic type and also assigns a priority to the three traffic types as follows: Priority 7 traffic is mapped to priority group 0 which does not get a bandwidth check, priority 2 and priority 3 are mapped to priority group 1, priorities 6, 5, 4, 1 and 0 are mapped to priority group 2.

The priority settings shown in Table 20 are translated to priority groups in the Brocade FCoE hardware.

TABLE 20	ETS priority grouping of IPC, LAN, and SAN traffic
Priority	Priority group	Bandwidth check
7	0	No
6	2	Yes
5	2	Yes
4	2	Yes
3	1	Yes
2	1	Yes
1	2	Yes
0	2	Yes

Priority Flow Control (PFC)

With PFC, it is important to provide lossless frame delivery for certain traffic classes while maintaining existing LAN behavior for other traffic classes on the converged link. This differs from the traditional 802.3 PAUSE type of flow control where the pause affects all traffic on an interface.

90	Converged Enhanced Ethernet Administrator’s Guide
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