Switching Paths Overview
Cisco IOS 12.0 Switching Services
Author: Technologies Riva; Systems Cisco
Publisher: Cisco Press (53)
This chapter describes switching paths that can be configured on Cisco
IOS devices. It provides an overview of switching methods. For specific configuration
information, refer to Chapter 2, “Configuring Switching
To understand how switching
works, it helps to first understand the basic router architecture and where
various processes occur in the router.
Fast switching is enabled by default on all interfaces that support fast switching. If you have
a situation where you need to disable fast switching and fall back to the
process-switching path, understanding how various processes affect the router
and where they occur will help you determine your alternatives. This understanding
is especially helpful when you are troubleshooting traffic problems or need
to process packets that require special handling. Some diagnostic or control
resources are not compatible with fast switching or come at the expense of
processing and switching efficiency. Understanding those resources can help
you minimize their effect on network performance.
Figure 1-1 illustrates
a possible internal configuration of a Cisco 7500 series router. In this configuration,
the Cisco 7500 series router has an integrated Route/Switch
Processor (RSP) and uses route caching to forward packets. The Cisco
7500 series router also uses Versatile
Interface Processors (VIPs), a RISC-based interface processor that receives
and caches routing information from the RSP. The VIP card uses the route cache
to make switching decisions locally, which relieves the RSP of involvement
and speeds overall throughput. This type of switching is called distributed
switching. Multiple VIP cards can be installed in one router.
The routing, or forwarding, function comprises two interrelated processes to move
information in the network:
Cisco IOS platforms perform both routing and switching, and there are
several types of each.
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 from one or more of the router
interfaces. The routing decision is based on a variation of criteria such
as link speed, topological distance, and protocol. Each separate protocol
maintains its own routing information.
Routing is more processing intensive and has a 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. Figure
1-2 illustrates the basic routing process.
A router sends routing
updates out to 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.
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 a lower latency than routing because it can move
packets, frames, or cells from buffer to buffer with a simpler determination
of the source and destination of the traffic. It saves resources because
it does not involve extra lookups.
Figure 1-3 illustrates the basic switching process.
In Figure 1-3,
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 protocol's routing table 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
and 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 it must discover the destination address by
some other address resolution process, such as through the ARP protocol.
Layer 3 IP addressing information is mapped to the Layer 2 MAC address
for the next hop. Figure
1-4 illustrates the mapping that occurs to determine the
Basic switching paths are
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
exit interface that services that destination. Subsequent packets for that
destination are sent by the same switching path. The route processor computes
the cyclic 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 exit interface that services the destination. Subsequent packets
for the same destination use the same switching path. The interface processor
computes the CRC.
Switching becomes more efficient the closer to the interface the function
occurs. In distributed
switching, the switching process occurs on VIP and other interface cards
that support switching. Figure
1-5 illustrates the distributed switching process on the Cisco 7500
Figure 1-5. 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.
switching enables you to collect the data required for flexible and
detailed accounting, billing, and chargeback for network and application
resource utilization. Accounting data can be collected for both dedicated
line and dial-access accounting. NetFlow switching over a foundation
of VLAN technologies provides the benefits of switching and routing
on the same platforms. NetFlow switching is supported over switched
LAN or ATM backbones, allowing scalable inter-VLAN forwarding. NetFlow
switching can be deployed at any location in the network as an extension
to existing routing infrastructures. NetFlow switching is described
in Chapter 8, Configuring NetFlow Switching.”
Depending on the routing platform you are
using, availability and default implementations of switching paths varies. Table 1-1 shows the correlation between Cisco
IOS switching paths and routing platforms.
Table 1-1. Switching Paths on RSP-Based Routers
Initializes switching caches
no protocol route-cache
Default (except for IP)
Using second-generation VIP line cards
protocol route-cache distributed
Configurable per interface
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