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    Packet Flow on ACX Series Routers

    The class-of-service (CoS) architecture for ACX Series routers is in concept similar to that of MX Series routers. The general architecture for ACX Series routers is shown in Figure 1.

    Figure 1: ACX Series Router Packet Forwarding and Data Flow

    ACX Series Router
Packet Forwarding and Data Flow

    Based on the model, ACX routers contain a built-in Routing Engine and Packet Forwarding Engine and can contain both T1/E1 and Gigabit Ethernet Ports.

    The Packet Forwarding Engine has one or two “pseudo” Flexible PIC Concentrators. Because there is no switching fabric, the single Packet Forwarding Engine takes care of both ingress and egress packet forwarding.

    Fixed classification places all packets in the same forwarding class, or the usual multifield (MF) or behavior aggregate (BA) classifications can be used to treat packets differently. BA classification with firewall filters can be used for classification based on IP precedence, DSCP, IEEE, or other bits in the frame or packet header.

    However, the ACX Series routers can also employ multiple BA classifiers on the same physical interface. The physical interfaces do not have to employ the same type of BA classifier. For example, a single physical interface can use classifiers based on IP precedence as well as IEEE 802.1p. If the CoS bits of interest are on the inner VLAN tag of a dual-tagged VLAN interface, the classifier can examine either the inner or outer bits. (By default, the classification is done based on the outer VLAN tag.)

    Eight queues per egress port support scheduling using the weighted deficit round-robin (WDRR) mechanism, a form of round-robin queue servicing. The supported priority levels are strict-high and default (low). The ACX series router architecture supports both weighted random early detect (WRED) and weighted tail drop (WTD).

    All CoS features are supported at line rate.

    The packet pipeline through an ACX Series router is shown in Figure 2. Note that the rate limiting is done with an integrated architecture along with all other CoS functions. Scheduling and shaping are supported on the output side.

    Figure 2: ACX Series Router Packet Handling

    ACX Series Router Packet Handling

    Published: 2013-04-04