The routing process assesses the source and destination of traffic based on knowledge of network conditions. Routing functions identify the best path to use for moving the traffic to the destination out one or more of the router interfaces. The routing decision is based on various criteria such as link speed, topological distance, and protocol. Each protocol maintains its own routing information.

Routing is more processing intensive and has higher latency than switching as it determines path and next hop considerations. The first packet routed requires a lookup in the routing table to determine the route. The route cache is populated after the first packet is routed by the route-table lookup. Subsequent traffic for the same destination is switched using the routing information stored in the route cache.

The figure below illustrates the basic routing process.

A router sends routing updates out each of its interfaces that are configured for a particular protocol. It also receives routing updates from other attached routers. From these received updates and its knowledge of attached networks, it builds a map of the network topology.

When in severe overload conditions, routers that cannot keep up with the incoming packet stream must drop packets. If no intelligence is applied to choosing which packets to discard, the stability of routing protocols is impacted. The Selective Packet Discard (SPD) feature applies some simple choices to selectively discard packets likely to be unimportant for routing and interface stability. SPD is enabled by default; there are no commands or configuration tasks required.

Through the switching process, the router determines the next hop toward the destination address. Switching moves traffic from an input interface to one or more output interfaces. Switching is optimized and has lower latency than routing because it can move packets, frames, or cells from buffer to buffer with simpler determination of the source and destination of the traffic. It saves resources because it does not involve extra lookups. The figure below illustrates the basic switching process.

Figure 3

The Switching Process

In the figure below, packets are received on the Fast Ethernet interface and destined for the FDDI interface. Based on information in the packet header and destination information stored in the routing table, the router determines the destination interface. It looks in the routing table of the protocol to discover the destination interface that services the destination address of the packet.

The destination address is stored in tables such as ARP tables for IP or AARP tables for AppleTalk. If there is no entry for the destination, the router will either drop the packet (and inform the user if the protocol provides that feature) or discover the destination address by some other address resolution process, such as through ARP. Layer 3 IP addressing information is mapped to the Layer 2 MAC address for the next hop. The figure below illustrates the mapping that occurs to determine the next hop.

Figure 4

Layer 3-to-Layer 2 Mapping

In process switching the first packet is copied to the system buffer. The router looks up the Layer 3 network address in the routing table and initializes the fast-switch cache. The frame is rewritten with the destination address and sent to the outgoing interface that services that destination. Subsequent packets for that destination are sent by the same switching path. The route processor computes the cyclical redundancy check (CRC).

When packets are fast switched, the first packet is copied to packet memory and the destination network or host is found in the fast-switching cache. The frame is rewritten and sent to the outgoing interface that services the destination. Subsequent packets for the same destination use the same switching path. The interface processor computes the CRC. Fast switching is described in the Configuring Fast Switching module.

Note	IP unicast fast switching is not supported is not supported on Cisco IOS 12.2S releases.

When Cisco Express Forwarding mode is enabled, the Cisco Express Forwarding Forwarding Information Base (FIB) and adjacency tables reside on the Route Processor (RP), and the RP performs the express forwarding. You can use Cisco Express Forwarding mode when line cards are not available for Cisco Express Forwarding switching or when you need to use features not compatible with distributed Cisco Express Forwarding switching. For information on configuring Cisco Express Forwarding, see the Cisco Express Forwarding Overview module.

Note	Beginning with Cisco IOS Release 12.0, Cisco Express Forwarding is the preferred and default switching path. NetFlow switching has been integrated into Cisco Express Forwarding switching. For more information, see the Cisco Express Forwarding Overview module.

In distributed switching, the switching process occurs on VIP and other interface cards that support switching. When distributed Cisco Express Forwarding is enabled, line cards, such as VIP line cards or GSR line cards, maintain an identical copy of the FIB and adjacency tables. The line cards perform the express forwarding between port adapters, relieving the RSP of involvement in the switching operation. distributed Cisco Express Forwarding uses an Inter Process Communication (IPC) mechanism to ensure synchronization of FIBs and adjacency tables on the RP and line cards.

For model numbers and hardware compatibility information, refer to the Cisco Product Catalog. For information on configuring distributed Cisco Express Forwarding, see the Cisco Express Forwarding Overview module.

For information on configuring Multicast Distributed Switching (MDS), see the Configuring Multicast Distributed Switching module.

The figure below illustrates the distributed switching process on the Cisco 7500 series.

Figure 5

Distributed Switching on Cisco 7500 Series Routers

The VIP card installed in this router maintains a copy of the routing cache information needed to forward packets. Because the VIP card has the routing information it needs, it performs the switching locally, making the packet forwarding much faster. Router throughput is increased linearly based on the number of VIP cards installed in the router.

Depending on the routing platform you are using, availability and default implementations of switching paths varies. The table below shows the correlation between Cisco IOS switching paths and routing platforms.

Queueing occurs when network congestion occurs. When traffic is moving well within the network, packets are sent as they arrive at the interface. Cisco IOS software implements four different queueing algorithms as follows:

• FIFO queueing--Packets are forwarded in the same order in which they arrive at the interface.
• Priority queueing (PQ)--Packets are forwarded based on an assigned priority. You can create priority lists and groups to define rules for assigning packets to priority queues.
• Custom queueing (CQ)--You can control a percentage of interface bandwidth for specified traffic by creating protocol queue lists and custom queue lists.
• Weighted fair queueing (WFQ)--WFQ provides automatic traffic priority management. Low-bandwidth sessions have priority over high-bandwidth sessions. High-bandwidth sessions are assigned weights. WFQ is the default for interfaces slower than 2.048 Mbps.

Random Early Detection (RED) is designed for congestion avoidance. Traffic is prioritized based on type of service (ToS), or precedence. This feature is available on T3, OC-3, and ATM interfaces.

Depending on the protocol you are using, various compression options are available in Cisco IOS software. Refer to the Cisco IOS configuration guide for the protocol you are using to learn compression options available.

You can define access lists to control access to or from a router for a number of services. You could, for example, define an access list to prevent packets with a certain IP address from leaving a particular interface on a router. How access lists are used depends on the protocol. For information on access lists, refer to the appropriate Cisco IOS configuration guide for the protocol you are using.

Encryption algorithms are applied to data to alter its appearance, making it incomprehensible to those not authorized to see the data. For information about encryption features available with the Cisco IOS software, refer to the Cisco IOS Security Configuration Guide.

You can configure accounting features to collect network data related to resource usage. The information you collect (in the form of statistics) can be used for billing, chargeback, and planning resource usage. Refer to the appropriate Cisco IOS configuration guide for the protocol you are using for information regarding accounting features you can use.