3troubleshootingjunos

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Troubleshooting JUNOS Platforms

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Troubleshooting JUNOS Platforms

The Buffer Manager 1 ASIC also extracts the key information, which is normally the fi st 64-bytes of a Layer 3 packet, and passes this information to the Internet

P cessor II ASIC in the form of a notification cell. The Internet Processor II performs a l ngest-match route lookup against the forwarding table to identify the packet’s

utg ing interface and forwarding next hop.

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After the Inte net Processor II ASIC determines the packet’s egress interface and rwa ding next hop, it amends the notification cell with this information and passes

the n tificati n cell to the second Distributed Buffer Manager ASIC. The second Distributed Buffer Management ASIC then passes the notification cell to the I/O Manager ASIC on the egress FPC (as identified by the modified contents of the notification cell). The Distributed Buffer Manager 2 ASIC acts as an agent for the FPC’s I/O Manager ASIC. Once the I/O Manager ASIC receives a notification cell indicating hat a packet is waiting for servicing, it issues read requests to the Buffer Manager 2 ASIC for the J-cells associated with the packet. As the I/O Manager receives the J-cells, it transmits them to the PIC Controller ASIC, which in turn transmits them out the appropriate port.

In the case of a multicast packet, multiple outgoing interfaces might exist, in which case the notification cell is directed to multiple FPCs or to the same FPC multiple times—once for each outgoing interface served by that FPC.

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Troubleshooting JUNOS Platforms

ABC ChipsReproductiont Pack Flow: Part 5

When the gr ss FPC is ady to service the packet, the I/O Manager ASIC issues read

requests for the 64-byte J-c lls that comprise the packet. In response, the Distributed Buffer Manager 2 ASIC retrieves the J-cells from shared memory and feeds them to

the I/O Manager ASIC. The I/O Manager ASIC reassembles the packet, decrements forthe packet’s TTL, adds the Layer 2 framing, and then sends the bit stream to the

eg ess PIC.

The I/O Manager ASIC is responsible for class-of-service (CoS)-related queuing, scheduling, and congestion avoidance operations at packet egress. Note that the packet itself never queues on the FPC; rather, a pointer to the packet, in the form of a notification cell, queues on the egress FPC. Each output port on a given PIC associates with four forwarding classes (or queues). You configure schedulers to provide each forwarding class with some share of the port’s bandwidth.

Note that traffic classification, which associates traffic with one of the defined forwarding classes, occurs at the ingress FPC. Once identified at ingress, the traffic is handled in accordance with the parameters configured for that traffic class by the I/O Manager on the egress FPC. The I/O Manager implements the random early detection (RED) algorithm during egress processing to avoid tail drops and the resulting risk of global synchronization of TCP retransmissions. A full coverage of JUNOS Software CoS capabilities is beyond the scope of this class.