Step 1: The interface media controller detects a packet on the network media and copies it into a buffer pointed to by the first free element in the receive ring. Media controllers use the Direct Memory Access (DMA) method to copy packet data into memory.

Step 2: The media controller changes ownership of the packet buffer back to the processor and issues a receive interrupt to the processor. The media controller does not have to wait for a response from the CPU and continues to receive incoming packets into the receive ring.

It's possible for the media controller to fill the receive ring before the processor processes all the new buffers in the ring. This condition is called an overrun. When this occurs, all incoming packets are dropped until the processor recovers.

Step 3: The CPU responds to the receive interrupt, and attempts to remove the newly−filled buffer from the receive ring and replenish the ring from the interface's private pool. Notice that packets are not physically moved within the I/O memory; only the pointers are changed. If the interface's input hold queue is full, the packet is dropped; otherwise, three outcomes are possible:

3.1: A free buffer is available in the interface's private pool to replenish the receive ring: the free buffer is linked to the receive ring and the

packet now belongs to the interface's private buffers pool.

3.2: A free buffer is not available in the interface's private pool, so the receive ring is replenished by falling back to the global pool that matches

the interface's MTU. The fallback counter is incremented for the private pool.

3.3: If a free buffer is not available in the public pool as well, the incoming packet is dropped and the ignore counter is incremented. In addition,

the interface is throttled and all incoming traffic is ignored on this interface for a short period of time.

2 − Switching the Packet

Step 4: After the receive ring is replenished, the CPU begins switching the packet. Cisco IOS software attempts to switch the packet using the fastest method configured on the interface. On shared memory routers, it first tries Cisco Express Forwarding (CEF) switching (if configured), then fast switching (unless "no ip route−cache" is configured on the interface), and finally, process switching if none of the others work.

Step 5: While still in the receive interrupt context, the Cisco IOS software attempts to use the CEF table or the fast switching cache to make a switching decision.

5.1: CEF switching − If there are valid CEF and adjacency table entries, the Cisco IOS software rewrites the Media Access Control (MAC) header on

the packet and begins transmitting it (Step 8). If there is no CEF entry for the destination, the packet is dropped.

5.2: Fast switching − If 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 transmitting the

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

6).

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

Cisco 1600 Series Router Architecture

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Cisco Systems 1600 manual Switching 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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