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Understanding FCoE-FC Gateway Functions

 

When it functions as a Fibre Channel over Ethernet (FCoE)-Fibre Channel (FC) gateway, the QFX3500 switch provides the following functions:

Login and Logout

Each of the native FC interfaces on the gateway performs a fabric login (FLOGI) to the FC switch when each interface initializes. This establishes the link between each gateway FC interface and the FC switch.

When FCoE devices on the Ethernet network send an FCoE Initialization Protocol (FIP) login (FIP FLOGI) or FIP discovery (FIP FDISC) request to the gateway, the gateway acts on behalf of those devices and converts their FIP FLOGI and FIP FDISC requests to FC FDISC requests. The gateway then sends the FC FDISC requests to the FC switch. When the FC switch responds to an FDISC request, the gateway converts the FC response into a FIP response and sends it to the appropriate FCoE device.

The gateway also converts FIP logout (LOGO) requests from FCoE devices into FC LOGO requests to the FC switch, and converts the FC switch response into a FIP response for the FCoE device.

FCoE and FC Frame Handling

When it receives FCoE frames from FCoE devices, the gateway strips away the Ethernet encapsulation from the FC frame before sending the native FC frame to the FC switch.

When it receives native FC frames from the FC switch, the gateway encapsulates the native FC frames in Ethernet before sending the resulting FCoE frames to the appropriate VN_Port.

Data Center Bridging

The Ethernet ports connected to the FCoE devices are 10-Gbps Ethernet ports and support data center bridging (DCB) specifications:

  • Priority-based flow control (PFC, described in IEEE 802.1Qbb)

  • Data Center Bridging Capability Exchange protocol (DCBX), which is an extension of Link Layer Data Protocol (LLDP, described in IEEE 802.1AB)

  • Enhanced transmission selection (ETS, described in IEEE 802.1Qaz)

  • 10-Gigabit Ethernet ports

Disabling the Fabric WWN Verification Check

The gateway connects to a SAN fabric using the gateway NP_Ports (native FC ports). When the NP_Ports initialize, each port sends a FLOGI to the FC switch to which it is connected in the SAN fabric. The FC switch sends a FLOGI accept (FLOGI-ACC) message back to each NP_Port. The FLOGI-ACC message includes the SAN fabric worldwide name (WWN). The gateway uses the SAN fabric WWN in the multicast discovery advertisement (MDA) that the gateway sends to the ENodes in the FCoE network.

Some FC switches substitute their own WWN (often the FC switch’s virtual WWN) for the SAN fabric WWN in the FLOGI-ACC message. When the FC switch substitutes its own WWN for the fabric WWN, gateway NP_Ports that log in to the same SAN fabric might receive different fabric WWNs in the FLOGI-ACC messages if the NP_Ports are connected to different FC switches in that SAN fabric. This creates a problem, because different fabric WWNs indicate different SAN fabrics. But in this scenario, the different fabric WWNs come from different FC switches in the same SAN fabric.

If the gateway receives different fabric WWNs on NP_Ports that are connected to the same SAN fabric, the gateway uses the first fabric WWN it receives in the MDA it sends to the ENodes. The gateway isolates the NP_Ports connected to that fabric that receive a different fabric WWN in the FLOGI-ACC message. No ENode sessions are assigned to the isolated NP_Ports. FC traffic is assigned only to NP_Ports that receive a fabric WWN that matches the fabric WWN received by the first NP_Port to log in to the FC fabric. (If an NP_Port receives a fabric WWN that does not match the fabric WWN received by the first NP_Port to log in to the FC fabric, it does not carry traffic to the SAN fabric.)

In summary, the scenario is:

  1. The gateway has multiple NP_Ports connected to more than one FC switch in a SAN fabric.
  2. When the NP_Ports initialize, each NP_Port sends a FLOGI to the FC switch to which it is connected.
  3. The FC switches substitute their own WWNs for the fabric WWN in the FLOGI-ACC message, so different NP_Ports receive different fabric WWNs.
  4. In the MDA the gateway sends to FCoE devices, the gateway uses the fabric WWN that the first NP_Port to log in to the fabric receives in the FLOGI-ACC message. If other NP_Ports receive a different fabric WWN from other FC switches in the SAN fabric, that fabric WWN is not advertised.
  5. NP_Ports that receive a fabric WWN that does not match the first received fabric WWN are isolated, and the ENode sessions cannot use those ports.

To prevent this from happening, you can disable the gateway fabric WWN verification check so that all NP_Ports connected to a SAN fabric are used to carry traffic between the gateway and the FC switch, regardless of the fabric WWN the NP_Port receives in the FLOGI-ACC message.

Note

Disabling or enabling the fabric WWN verification check logs out all FCoE sessions.

Load Balancing

The switch performs automatic link load balancing for the connections between the gateway and the FC SAN and can also perform load balancing for the connections between the gateway and the FCoE devices in the Ethernet network. On the native FC links (NP_Ports) between the gateway and the FC SAN, the gateway can use one of the following three load-balancing algorithms:

  • Simple load balancing—The switch assigns each ENode FLOGI session and VN_Port FDISC session to the least-loaded link. The switch can place FDISC sessions on a different link than the parent FLOGI session (an ENode FLOGI session and its subsequent FDISC sessions can be placed on different links). Simple load balancing is the default load-balancing algorithm. Rebalancing the link load disrupts only selected sessions to minimize the impact (the switch uses an algorithm to log out only the sessions that need to be moved to other links to balance the load when those sessions log in again).

  • ENode-based load balancing—When an ENode logs in to the fabric, the switch places all subsequent VN_Port FDISC sessions associated with that ENode on the same link as the ENode FLOGI session, regardless of the link load. New ENode FLOGIs are placed on the least-loaded link. The switch calculates the link load based on the combined total of FLOGIs and FDISCs on each NP_Port link. Rebalancing the link load disrupts all sessions (all sessions log out and then log in again).

  • FLOGI-based load balancing—Similar to ENode-based load balancing; when an ENode logs in to the fabric, the switch places all subsequent VN_Port FDISC sessions associated with that ENode on the same link as the ENode FLOGI session, regardless of the link load. New ENode FLOGIs are placed on the least-loaded link.

Note

Changing the load-balancing algorithm when FCoE sessions are running forces the FCoE sessions to log out, then log in again.