Understanding MC-LAGs on an FCoE Transit Switch

Supported MC-LAG Topology

To support lossless transport of FCoE traffic across an MC-LAG, you must configure the appropriate class of service (CoS) on both of the switches with MC-LAG port members. The CoS configuration must be the same on both of the MC-LAG switches because MC-LAGs do not carry forwarding class and IEEE 802.1p priority information.

Switches that are not directly connected to FCoE hosts and that act as pass-through transit switches support MC-LAGs for FCoE traffic in an inverted-U network topology. Figure 1 shows an inverted-U topology using QFX3500 switches.

Figure 1: Supported Topology for an MC-LAG on an FCoE Transit Switch

Standalone switches support MC-LAGs. QFabric system Node devices do not support MC-LAGs. Virtual Chassis and mixed-mode Virtual Chassis Fabric (VCF) configurations do not support FCoE. Only pure QFX5100 VCFs (consisting of only QFX5100 switches) support FCoE.

Ports that are part of an FCoE-FC gateway configuration (a virtual FCoE-FC gateway fabric) do not support MC-LAGs. Ports that are members of an MC-LAG act as pass-through transit switch ports.

The following rules and guidelines apply to MC-LAGs when used for FCoE traffic. The rules and guidelines help to ensure the proper handling and lossless transport characteristics required for FCoE traffic.

• The two switches that form the MC-LAG (Switches S1 and S2) cannot use ports that are part of an FCoE-FC gateway fabric. The MC-LAG switch ports must be pass-through transit switch ports (used as part of an intermediate transit switch that is not directly connected to FCoE hosts).

• MC-LAG Switches S1 and S2 cannot be directly connected to the FCoE hosts.

• The two switches that serve as access devices for FCoE hosts (FCoE Transit Switches TS1 and TS2) use standard LAGs to connect to MC-LAG Switches S1 and S2. FCoE Transit Switches TS1 and TS2 can be standalone switches or they can be Node devices in a QFabric system.

• Transit Switches TS1 and TS2 must use transit switch ports for the FCoE hosts and for the standard LAGs to MC-LAG Switches S1 and S2.

• Enable FIP snooping on the FCoE VLAN on Transit Switches TS1 and TS2. You can configure either VN_Port to VF_Port (VN2VF_Port) FIP snooping or VN_Port to VN_Port (VN2VN_Port) FIP snooping, depending on whether the FCoE hosts need to access targets in the FC SAN (VN2VF_Port FIP snooping) or targets in the Ethernet network (VN2VN_Port FIP snooping).

  FIP snooping should be performed at the access edge and is not supported on MC-LAG switches. Do not enable FIP snooping on MC-LAG Switches S1 and S2. (Do not enable FIP snooping on the MC-LAG ports that connect Switches S1 and S2 to Switches TS1 and TS2 or on the LAG ports that connect Switch S1 to S2.)

  Note:

  Juniper Networks QFX10000 aggregation switches do not support FIP snooping, so they cannot be used as FIP snooping access switches (Transit Switches TS1 and TS2) in this topology.

• The CoS configuration must be consistent on the MC-LAG switches. Because MC-LAGs carry no forwarding class or priority information, each MC-LAG switch needs to have the same CoS configuration to support lossless transport. (On each MC-LAG switch, the name, egress queue, and CoS provisioning of each forwarding class must be the same, and the priority-based flow control (PFC) configuration must be the same.)

• Transit Switches (Server Access)
• MC-LAG Switches (FCoE Aggregation)

FIP Snooping and FCoE Trusted Ports

To maintain secure access, enable VN2VF_Port FIP snooping or VN2VN_Port FIP snooping at the transit switch access ports connected directly to the FCoE hosts. FIP snooping should be performed at the access edge of the network to prevent unauthorized access. For example, in Figure 1, you enable FIP snooping on the FCoE VLANs on Transit Switches TS1 and TS2 that include the access ports connected to the FCoE hosts.

Do not enable FIP snooping on the switches used to create the MC-LAG. For example, in Figure 1, you would not enable FIP snooping on the FCoE VLANs on Switches S1 and S2.

Configure links between switches as FCoE trusted ports to reduce FIP snooping overhead and ensure that the system performs FIP snooping only at the access edge. In the sample topology, configure the Transit Switch TS1 and TS2 LAG ports connected to the MC-LAG switches as FCoE trusted ports, configure the Switch S1 and S2 MC-LAG ports connected to Switches TS1 and TS2 as FCoE trusted ports, and configure the ports in the LAG that connects Switches S1 to S2 as FCoE trusted ports.

CoS and Data Center Bridging (DCB)

The MC-LAG links do not carry forwarding class or priority information. The following CoS properties must have the same configuration on each MC-LAG switch or on each MC-LAG interface to support lossless transport:

• FCoE forwarding class name—For example, the forwarding class for FCoE traffic could use the default fcoe forwarding class on both MC-LAG switches.

• FCoE output queue—For example, the fcoe forwarding class could be mapped to queue 3 on both MC-LAG switches (queue 3 is the default mapping for the fcoe forwarding class).

• Classifier—The forwarding class for FCoE traffic must be mapped to the same IEEE 802.1p code point on each member interface of the MC-LAG on both MC-LAG switches. For example, the FCoE forwarding class fcoe could be mapped to IEEE 802.1p code point 011 (code point 011 is the default mapping for the fcoe forwarding class).

• Priority-based flow control (PFC)—PFC must be enabled on the FCoE code point on each MC-LAG switch and applied to each MC-LAG interface using a congestion notification profile.

You must also configure enhanced transmission selection (ETS) on the MC-LAG interfaces to provide sufficient scheduling resources (bandwidth, priority) for lossless transport. The ETS configuration can be different on each MC-LAG switch, as long as enough resources are scheduled to support lossless transport for the expected FCoE traffic.

Link Layer Discovery Protocol (LLDP) and Data Center Bridging Capability Exchange Protocol (DCBX) must be enabled on each MC-LAG member interface (LLDP and DCBX are enabled by default on all interfaces).