Packet Flow on Juniper Networks T Series Core Routers
On T Series Core Routers, CoS actions are performed in several locations: the incoming and outgoing Switch Interface ASICs, the T Series router Internet Processor ASIC, and the Queuing and Memory Interface ASICs. These locations are shown in Figure 1.
This topic describes the packet flow through the following components in more detail:
Incoming Switch Interface ASICs
When a data packet is passed from the receiving interface to its connected FPC, it is received by the incoming Switch Interface ASIC on that specific FPC. During the processing of the packet by this ASIC, the information in the packet’s header is examined by a BA classifier. This classification action associates the packet with a particular forwarding class. In addition, the value of the packet’s loss priority bit is set by this classifier. Both the forwarding class and loss priority information are placed into the notification cell, which is then transmitted to the T Series router Internet Processor ASIC.
T Series Routers Internet Processor ASIC
The T Series router Internet Processor ASIC receives notification cells representing inbound data packets and performs route lookups in the forwarding table. This lookup determines the outgoing interface on the router and the next-hop IP address for the data packet. While the packet is being processed by the T Series router Internet Processor ASIC, it might also be evaluated by a firewall filter, which is configured on either the incoming or outgoing interface. This filter can perform the functions of a multifield classifier by matching on multiple elements within the packet and overwriting the forwarding class settings, loss priority settings, or both within the notification cell. Once the route lookup and filter evaluations are complete, the notification cell, now called the result cell, is passed to the Queuing and Memory Interface ASICs.
Queuing and Memory Interface ASICs
The Queuing and Memory Interface ASICs pass the data cells to memory for buffering. The data cells are placed into a queue to await transmission on the physical media. The specific queue used by the ASICs is determined by the forwarding class associated with the data packet. The configuration of the queue itself helps determine the service the packet receives while in this queued state. This functionality guarantees that certain packets are serviced and transmitted before other packets. In addition, the queue settings and the packet’s loss priority setting determine which packets might be dropped from the network during periods of congestion.
In addition to queuing the packet, the outgoing I/O Manager ASIC is responsible for ensuring that CoS bits in the packet’s header are correctly set before it is transmitted. This rewrite function helps the next downstream router perform its CoS function in the network.
The Queuing and Memory Interface ASIC sends the notification to the Switch Interface ASIC facing the switch fabric, unless the destination is on the same Packet Forwarding Engine. In this case, the notification is sent back to the Switch Interface ASIC facing the outgoing ports, and the packets are sent to the outgoing port without passing through the switch fabric. The default behavior is for fabric priority queuing on egress interfaces to match the scheduling priority you assign. High-priority egress traffic is automatically assigned to high-priority fabric queues.
The Queuing and Memory Interface ASIC forwards the notification, including next-hop information, to the outgoing Switch Interface ASIC.
Outgoing Switch Interface ASICs
The destination Switch Interface ASIC sends bandwidth grants through the switch fabric to the originating Switch Interface ASIC. The Queuing and Memory Interface ASIC forwards the notification, including next-hop information, to the Switch Interface ASIC. The Switch Interface ASIC sends read requests to the Queuing and Memory Interface ASIC to read the data cells out of memory, and passes the cells to the Layer 2 or Layer 3 Packet Processing ASIC. The Layer 2 or Layer 3 Packet Processing ASIC reassembles the data cells into packets, adds Layer 2 encapsulation, and sends the packets to the outgoing PIC interface. The outgoing PIC sends the packets out into the network.