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Understanding BA Traffic Classification on Transparent Mode Security Devices


A BA classifier checks the header information of an ingress packet. The resulting traffic classification consists of a forwarding class (FC) and packet loss priority (PLP). The FC and PLP associated with a packet specify the CoS behavior of a hop within the system. For example, a hop can place a packet into a priority queue according to its FC, and manage queues by checking the packet's PLP. Junos OS supports up to eight FCs and four PLPs.


MPLS EXP bit-based traffic classification is not supported.

BA classification can be applied within one DiffServ domain. BA classification can also be applied between two domains, where each domain honors the CoS results generated by the other domain. Junos OS performs BA classification for a packet by examining its Layer 2 and Layer 3 CoS-related parameters. Those parameters include the following:

  • Layer 2—IEEE 802.1p: User Priority

  • Layer 3—IPv4 Precedence, IPv4 DSCP, IPv6 DSCP

On SRX Series devices in transparent mode, a BA classifier evaluates only Layer 2 parameters. On SRX Series devices in Layer 3 mode, a BA classifier can evaluate Layer 2 and Layer 3 parameters; in that case, classification resulting from Layer 3 parameters overrides that of Layer 2 parameters.

On SRX Series devices in transparent mode, you specify one of four PLP levels—high, medium-high, medium-low, or low—when configuring a BA classifier.