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Understanding CoS IEEE 802.1p Priority Remapping on an FCoE-FC Gateway

 

When the QFX Series acts as an FCoE-FC gateway, it connects an Ethernet network that carries Fibre Channel over Ethernet (FCoE) traffic to a Fibre Channel (FC) network. Ethernet interfaces connect to the FCoE network. Native FC interfaces (NP_Ports) connect to the FC network.

FCoE traffic typically uses IEEE 802.1p priority 3 (code point 011). The QFX Series default configuration maps priority 3 traffic to the FCoE forwarding class. If your FCoE network uses priority 3 for FCoE traffic, you do not need to remap priorities, because the default configuration maps priority 3 to the FCoE forwarding class. (But you do need to enable PFC on IEEE 802.1p code point 3 on the Ethernet interfaces to achieve lossless behavior.)

However, if the FCoE network uses a different IEEE 802.1p priority than priority 3 for FCoE traffic, then you can use priority remapping to classify FCoE traffic into a lossless forwarding class mapped to that priority (and classified to that priority on the FCoE Ethernet interfaces in the ingress classifier). You specify the lossless forwarding class used for the FCoE traffic by configuring a fixed classifier and applying it to the native FC (NP_Port) interface. All traffic received from the FC SAN on that NP_Port interface is classified into the forwarding class specified in the fixed classifier.

When native FC interfaces on the FCoE-FC gateway encapsulate incoming FC traffic in Ethernet to create FCoE frames, by default they assign IEEE 802.1p code point 011 to the FCoE traffic, forward the traffic internally to the gateway Ethernet interfaces, and then forward the traffic to the FCoE network. Setting a rewrite value for the IEEE 802.1p code point configures the gateway native FC interface to assign the rewrite value priority to the FCoE frames when the native FC interface forwards the FCoE frames to the gateway Ethernet interface. Instead of a priority of 3, the FCoE frames use the priority specified in the rewrite value.

You can configure one rewrite value for each local FCoE-FC gateway fabric. All of the native FC interfaces in a particular fabric must use the same rewrite value. Native FC interfaces that belong to different FCoE-FC gateway fabrics can use different rewrite values.

Priority Remapping Configuration

Native FC interfaces on an FCoE-FC gateway receive native FC traffic from the FC SAN and encapsulate it in Ethernet to create FCoE frames. Priority remapping enables you to map the encapsulated FC traffic (the FCoE traffic) to any IEEE 802.1p priority. (This is similar to the rewrite rules you can configure to remap forwarding classes to code points on Ethernet egress interfaces, but the rewrite takes place at the ingress FC interface so that the QFX Series uses the correct priority for FCoE traffic on the converged Ethernet network.)

To support lossless traffic flows, you must configure the remapped priority correctly on the native FC interfaces and also on the Ethernet interfaces that connect to the FCoE network. Achieving lossless behavior for FCoE traffic when you remap the FCoE priority requires configuring:

  • A lossless forwarding class for FCoE traffic (or using the default fcoe forwarding class)

  • A behavior aggregate (BA) classifier on the FCoE Ethernet interfaces to map the FCoE forwarding class to the IEEE 802.1p code points (priority) used for FCoE traffic on the FCoE network (the ingress classifier priority for the forwarding class must be the same as the rewrite value priority)

  • A fixed classifier on the FCoE-FC gateway FC interface that maps all traffic from the FC network into the lossless FCoE forwarding class (the forwarding class must be lossless)

  • A priority rewrite value that remaps the IEEE 802.1p code point on the FCoE-FC gateway FC interface to the priority used for FCoE traffic on the FCoE network

  • An input congestion notification profile (CNP) to enable priority-based flow control (PFC) on the FCoE code point (the code point used as the rewrite value) at the Ethernet ingress interfaces

The ingress and egress configurations must match to achieve lossless behavior. The priority and the forwarding class specified in the BA classifier and in the CNP on the Ethernet ingress interfaces must match the fixed classifier and rewrite value on the FC interfaces. You must specify the same lossless FCoE forwarding class in each configuration and use the same IEEE 802.1p code point (priority) so that the FCoE traffic is properly classified into flows and so that those flows receive lossless treatment.

For example, if you configure a lossless forwarding class named my_fcoe_fc and your Ethernet network uses IEEE 802.1p priority 5 (code point 101) for FCoE traffic, then:

  • The forwarding class configuration, the BA classifier, and the fixed classifier all specify my_fcoe_fc as the forwarding class

  • The BA classifier, the input CNP, and the rewrite value all specify the IEEE 802.1p code point 101

Configuration Rules

The following configuration rules apply when you remap priorities on an FCoE-FC gateway:

  • Each native FC interface (NP_Port) supports one IEEE 802.1p priority value. The interface rewrites the IEEE 802.1p code point of all incoming traffic on the interface to the rewrite value. (The FC interface uses either the default value of 3 or the rewrite value for all incoming traffic.)

  • Ports in the same FCoE-FC gateway local fc-fabric must use the same rewrite value. For example, if ports fc-0/0/0 and fc-0/0/1 are in the same local FCoE-FC gateway fabric, they must use the same rewrite value. If you attempt to commit a configuration that uses different IEEE 802.1p priority rewrite values, the system returns a commit error.

  • Ports in different FCoE-FC gateway local fc-fabrics can use different rewrite values. An example scenario is:

    • Interfaces fc-0/0/0 and fc-0/0/1 are in FCoE-FC gateway fc-fabric my_fc_fab1.

    • Interfaces fc-0/0/4 and fc-0/0/5 are in FCoE-FC gateway fc-fabric my_fc_fab2.

    In this scenario, interfaces fc-0/0/0 and fc-0/0/1 must use the same rewrite value because they belong to the same local FC fabric on the gateway. Interfaces fc-0/0/4 and fc-0/0/5 also must use the same rewrite value because they belong to the same local FC fabric. However, the rewrite value you use for interfaces fc-0/0/0 and fc-0/0/1 can be different than the rewrite value you use for interfaces fc-0/0/4 and fc-0/0/5 because the interfaces belong to different local FC fabrics.

  • You can apply the rewrite value only to native FC interfaces; you cannot apply the rewrite value configuration to Ethernet interfaces.

  • The forwarding class specified in the fixed classifier on the native FC interface must be a lossless forwarding class. You cannot apply a fixed classifier to a native FC interface unless the associated forwarding class is lossless. (The forwarding class must be one of the two default lossless forwarding classes, or you must explicitly configure the forwarding class with the no-loss drop attribute.)

  • The lossless forwarding class and IEEE 802.1p priority configuration must match on the FCoE-FC gateway native FC interfaces and Ethernet interfaces:

    • The same IEEE 802.1p priority (code point) must be enabled for PFC on the Ethernet ingress interfaces, classified to the lossless forwarding class used in the native FC interface fixed classifier, and set as the rewrite value on the native FC interfaces.

    • The same lossless forwarding class must be used in the fixed classifier on the native FC interfaces and in the classifier configuration on the Ethernet interfaces.

Fate Sharing

To ensure that congestion on one interface does not affect the fate of traffic on a native FC interface on which you remap priorities, avoid fate sharing (different traffic flows receiving the same CoS treatment) configurations.

You can avoid fate sharing by ensuring that the remapping priority (code point) on the native FC interface is classified only to the forwarding class used in the fixed classifier on all other interfaces. For example, if you configure a fixed classifier on an FC interface that classifies all of the traffic into lossless forwarding class myfcoe1 and remaps the priority to priority 5 (IEEE 802.1p code point 101), then in all other classifier configurations on all other interfaces, priority 5 should always be classified to forwarding class myfcoe1. If you classify priority 6 on another interface to forwarding class myfcoe1, then congestion on priority 6 traffic affects priority 5 traffic unfairly.