Cisco Switching Methods

cisco-switches

Cisco Switching Methods

Cisco switching methods refers to the behavior of Cisco IOS routers’ route processors. Multilayer switches can route and contain a routing process. This is why it is important to review these concepts.

One of three ways to forward packets from a Cisco IOS-based router is process switching, fast switch, or Cisco Express Forwarding (CEF). Remember from your analysis of routers that the process switch is the slowest type of routing, as the router processor must route and then rewrite using software. 

This method is not scalable due to the limitations of the route processor’s speed and core count. Fast switching is the second method. This involves routing the first packet of a flow and then rewriting it using software. Each subsequent packet is handled by hardware. CEF uses hardware forwarding tables to handle most traffic flows. 

There are a few exceptions. CEF means that the route processor is able to spend its cycles mostly on other tasks.

Both the architecture of cisco 1900 router and Nexus switches focuses primarily on CEF equivalents in Cisco routers. Process switching is the last-resort method of switching for Cisco Catalyst and Nexus switches. These switches’ route processors were not designed to route packets or switch routes. 

This will adversely affect performance. These switches default to using fast switching or CEF. Process switching is only performed when absolutely necessary.

Cisco Catalyst

Terminology refers to fast switching as route caching and topology-based switch as the application of CEF with shared hardware forwarding.

The following table summarizes topology-based forwarding and route caching on Cisco Catalyst switches.

Route caching: 

Also called flow-based or demand based switching, route caching refers to a Layer 3 route cache. It is created within the hardware functions of the switch as it detects traffic flow. This is equivalent to Cisco IOS Software’s fast switching.

Topology-based switching 

is when information from the routing table is used for populating the route cache regardless of traffic flow. CEF is responsible for building the FIB. This is functionally identical to CEF in 

Cisco IOS Software.

Next subsections provide more information about route caching as well as topology-based switching.

Route Caching

Route caching is a fast switching alternative in Cisco Catalyst switches. Route caching must have a destination MAC address that is compatible with Layer 3 switches in order to function. 

Because there is no cache entry for the new flow, the route processor switches the first packet in a stream in software. The route processor then makes the forwarding decision and programs the cache table (the hardware for forwarding table). All subsequent packets are switched in hardware. 

This is commonly known as using application-specific interconnect circuits (ASICs). The hardware forwarding table is where entries are created when the switch detects new traffic flows. 

Once they have been inactive for a certain time, the entries will be deleted.

Route caching is a way to forward at least one packet per flow that uses software, since entries are only created in the hardware cache when flows are detected by switches.

Route caching is also known by many other names such as NetfFow Ethernet switching, flow-based, demand-based switching and route once, turn many.

Topology-Based Switching

Topology-based switch is the CEF equivalent feature in Cisco Catalyst switches. Topology-based switching offers Layer 3 switching that is superior to route caching. It also provides the highest performance and scalability. 

All Cisco Catalyst switches that can perform Layer 3 routing use topology-based switching / CEF. Focus on topology-based switching’s benefits and operation for the purposes of CCNP Switch.

CEF uses information from the routing table to populate route caches (also known as FIBs). Traffic flows are not required to start the caching process. This hardware FIB is independent of traffic flow. Buy desktop huntkey smps online in India.

Assuming that a destination address has a route listed in the routing table then all flow packets will be forward by the hardware. Even the first packet in a flow is handled by the FIB. This behavior is illustrated in 

Topology-Based switching

CEF also supports parallel paths, optimizing load balancing at IP layer. CEF is available in most Catalyst switches of the current generation, including the Catalyst 6800 and Catalyst 4500. It supports load balancing depending on source IP address, destination IP address combination, source and destination IP, plus TCP/UDP port numbers.

Different Catalyst switch models have different load-balancing options. Cisco.com can help you find the Catalyst switch that is right for you.

Layer 3 load-balancing schemes in CEF allow Layer 3 switches to use multiple paths for load sharing. Even if there are multiple paths, packets for a given source/destination pair will always follow the same path. This ensures packets for a given source-destination host pair arrive in the correct order. In some cases, this may be desirable behavior with legacy applications.

Load balancing that is based on destination and source IP addresses has some flaws. This load-balancing method selects one path for each host pair. A heavily used source-destination pair such as a firewall to the web server might not be able to leverage all links because it uses the same path. 

This means that the load-balancing scheme could “polarize” traffic by choosing only one path for a host pair. It effectively negates the load-balancing benefits of multiple paths for that host pair.

The statistical distribution of traffic is crucial to optimal load-balancing schemes. Source and destination IP load sharing become more efficient the greater the number of pair of source-destination IP addresses. Polarization is not a problem if there is a wide distribution of traffic between host pairs. 

Polarization can be a problem in environments where data flows between small numbers of host pairs, which could result in a large percentage of packets traversing the network.

Load balancing, which is based on source IP and destination IP with TCP/UDP port numbers, is a popular alternative.

Polarization is less likely the more factors that are added to the load-balancing system.

Cisco Catalyst also supports load-balancing features and methods that can be used to adjust load balancing according to hardware model and software version. Cisco.com can help you with configuration optimizations.

Hardware Forward Details

Two possible locations for Layer 3 packet switching on Catalyst switches are available. The switches can be accessed in two ways. One, they are centralized, like on a supervisor module. 

Two, they are distributed, which allows for switching to occur on individual line modules. These two methods are known as distributed switching and centralized switching.

Catalyst 6500 was a great example of distributed switching. 

It allows you to place line modules in specific hardware versions or centralize all switches on the supervisor.

Centralized switching has lower complexity and hardware costs. Distributed switching is ideal for large enterprise core networks and scaling. Most small-form factor switches use centralized switching.

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