BROCADE IP PRIMER

Data Center Bridge eXchangeData Center Bridge eXchangeIn order for PFC and ETS to work as intended, both devices on a link must usethe same rules. (PFC and ETS are implemented only in point-to-point, fullduplextopologies.) The role of Data Center Bridge eXchange (DCBX) is to allowtwo adjacent nodes to exchange information about themselves and what theysupport. If both nodes support PFC and ETS, then a lossless link can be establishedto support the requirements of FCoE.As with other protocols, an implementation goal of DCBX is that it must be possibleto add DCBX-equipped devices to legacy networks without breakingthem. That's the only way to build a lossless Ethernet sub-cloud inside a largerEthernet data center deployment. This is discussed in more detail in a followingsection.Today, nearly all Ethernet devices are equipped to support the Link Layer DiscoveryProtocol (LLDP), a mechanism whereby each switch periodicallybroadcasts information about itself to all of its neighbors. It's a one-way protocol,meaning that there is no acknowledgement of any of the data transmitted.Broadcasted information includes a chassis identifier, a port identifier, a timeto-live(TTL) field, and other information about the state and configuration ofthe device.Information in an LLDP data unit (LLDPDU) is encoded using a type-lengthvalue(TLV) convention:• Each unit of information in the LLDPDU starts with a type field, which tellsthe receiver what that information block contains.• The next field, the length field, allows a receiver to determine where thenext unit of information begins. By using this field, a receiver can skipover any TLVs that it either doesn't understand or doesn't want to process.• The third element of the TLV is the value of that information unit.The LLDP standard defines a number of required and optional TLVs. It alsoallows for a unique TLV type, which permits organizations to define their ownTLVs as required. By taking advantage of this feature, DCBX can build on LLDPto allow two nodes to exchange information about their ability to support PFCand ETS. nodes that do not support PFC and ETS are not negatively impactedby the inclusion of this information in the LLDPDU, and they can just skip overit. The absence of DCBX-specific information from an LLDPDU informs an adjacentnode that it’s not capable of supporting those protocols.DCBX also enhances the capabilities of LLDP by including additional informationthat allow the two nodes to be better informed about what the other nodehas learned to keep the two nodes in sync. For example, the addition ofsequence numbers in the DCBX TLV allows each of the two nodes to know thatit has received the latest information from its peer and that its peer hasreceived the latest information from it.Brocade IP Primer 83