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If there is no CEF entry for the destination, the packet is dropped.

5.2: Fast switchingIf CEF is not enabled or the packet cannot be CEF switched, the Cisco IOS

software attempts to fast−switch the packet. If there is a valid fast cache entry for this destination, the

Cisco IOS software rewrites the MAC header information and begins to transmit the packet (see Step

8). If there is no valid fast cache entry, the packet is queued for process switching (see Step 6).

Step 6: Process switchingIf both CEF and fast switching fail, the Cisco IOS software falls back to process

switching. The packet goes in the queue of the appropriate process (for instance, an IP packet is placed in the

queue for the IP Input process), and the receive interrupt is dismissed.

Step 7: Eventually the packet switching process runs, and switches the packet and rewrites the MAC header

as needed. Notice that the packet still has not moved from the buffer into which it was originally copied. After

the packet is switched, the Cisco IOS software continues to the packet transmit stage.

3 − Transmit the Packet

Step 8: If the packet was CEF or fast switched, the Cisco IOS software checks to see if there are packets on

the output queue of the outbound interface, while still in receive interrupt context.

8.1: If there are packets already on the output hold queue for the interface, the Cisco IOS software

places the packet on the output hold queue instead of directly into the transmit ring to reduce the

possibility of out−of−order packets, and then proceeds to Step 8.3.

8.2: If the output hold queue is empty, the Cisco IOS software places the packet on the transmit ring

of the output interface. To do so, it links the packet buffer to a transmit ring descriptor. The receive

interrupt is dismissed, and processing continues with Step 11. If there is no room on the transmit ring,

the packet is placed on the output hold queue instead, and the receive interrupt is dismissed.

8.3: If the output hold queue is full, the packet is dropped, the output drop counter value increases,

and the receive interrupt is dismissed.

Step 9: If the packet was process−switched, the packet is placed on the output queue for the input interface. If

the output queue is full, the packet is dropped and the output drop counter value increases.

Step 10: The Cisco IOS software attempts to find a free descriptor in the output interface transmit ring. If a

free descriptor exists, the Cisco IOS software removes the packet from the output hold queue and links the

buffer to the transmit ring. If the ring is full, the Cisco IOS software leaves the packet in the output hold queue

until the media controller transmits a packet from the ring and frees a descriptor.

Step 11: The outbound interface media controller polls its transmit ring periodically for packets that need to

be transmitted. As soon as the media controller detects a packet, it copies the packet onto the network media

and raises a transmit interrupt to the processor.

Step 12: The Cisco IOS software acknowledges the transmit interrupt, de−links the packet buffer from the

transmit ring, and returns the buffer to the pool of buffers from which it originated. The Cisco IOS software

then checks the output hold queue for the interface. If any packets await in the output hold queue, the Cisco

IOS software removes the next one from the queue and links it to the transmit ring. Finally, the transmit

interrupt is dismissed.

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