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, switching the packet and rewriting the MAC header as needed. Note that the packet still has not moved from the buffer it was originally copied into. After the packet is switched, the Cisco IOS software continues to the packet transmit stage.

3 − Transmitting the Packet

Step 8: If the packet was CEF or fast switched, then while still in receive interrupt context, the Cisco IOS software checks to see if there are packets on the output queue of the outbound interface.

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 by linking 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 is incremented, 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 is incremented.

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, unlinks 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 there are any packets waiting 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.

1

"CCIE Professional Development: Inside Cisco IOS Software Architecture" by Vijay Bollapragada, Curtis Murphy, Russ White (ISBN 1−57870−181−3).

Related Information

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Cisco 1600 Series Router Architecture

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Cisco Systems 1600 manual Related Information, Transmitting the Packet
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1600 specifications

Cisco Systems 1600 series routers are well-regarded in the networking industry for their reliability, versatility, and performance. Designed to meet the needs of small to medium-sized businesses, the Cisco 1600 series is primarily oriented towards providing robust connectivity for branch offices and remote sites.

One of the most significant features of the Cisco 1600 series is its modular architecture. This allows users to customize their routers with various interface cards, enabling support for different types of WAN technologies, including serial, ISDN, and Frame Relay. The modular design not only enhances flexibility but also extends the lifespan of the equipment, as businesses can upgrade their capacity and functionalities according to evolving demands.

Another key characteristic of the Cisco 1600 routers is their advanced routing capabilities. These devices support a wide range of routing protocols, including RIP, OSPF, EIGRP, and BGP. This versatility ensures seamless integration into existing network infrastructures, facilitating the scalability and adaptability that modern networks require.

Security is also a crucial aspect of the Cisco 1600 series. Incorporating features such as access control lists (ACLs) and support for Virtual Private Networks (VPNs), these routers help businesses safeguard their sensitive data during transmission. Enhanced security features allow companies to implement secure remote access for employees, thereby maintaining productivity without compromising safety.

In addition to routing and security, the Cisco 1600 series boasts Quality of Service (QoS) capabilities. This technology enables better management of network traffic, ensuring that critical applications receive the bandwidth they need for optimal performance. By prioritizing important data flows, businesses can enhance user experience and minimize disruptions across their network.

The Cisco 1600 series routers are also equipped with user-friendly interfaces and management tools. Cisco's IOS software platform provides a command-line interface (CLI) and web-based graphical interface for straightforward configuration and monitoring. This simplifies network management tasks and reduces the learning curve for IT personnel.

In summary, the Cisco Systems 1600 series routers offer a significant combination of modularity, robust routing protocols, enhanced security measures, Quality of Service features, and user-friendly management options. These characteristics make them an excellent choice for small to medium-sized enterprises looking to establish a reliable and secure networking infrastructure. With the continued evolution of networking technologies, the Cisco 1600 series stands as a dependable solution for businesses navigating an increasingly complex digital landscape.
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