load balancing in eigrp

[vishalpatil86]

hi,

in load balancing section, there was following statement -

when packet is process switched, load balancing over equal cost paths occurs on a per-packet basis. when packets are fast switched, load balancing over equal cost path occurs on a per destination basis.

what is process switched and fast switched?

[Jon Marshall]

Vishal

See this link which explains process/fast and CEF switching -

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml

Jon

[cadet alain]

Hi,

The packets when forwarded can follow 3 different switching methods: CEF, Process switching or fast switching.

Taken from routing tcp/ip vol 1 second edition:

Table 3-1. IOS switching determination is based on  configuration of inbound and outbound interfaces.

Inbound Configuration	Outbound Configuration	Switching Method Used
CEF	Process	CEF
CEF	Fast	CEF
Process	CEF	Fast (or process if IPv6)
Process	Fast	Fast
Fast	CEF	Fast (or process if IPv6)
Fast	Process	Process

You can verify the interface switching method with the sh ip interface output.

Regards.

Alain.

[TarasKotov · Accepted solution]

Process Switched - when each packet is individually processed by CPU.

Fast Switched - when first packet of a flow processed by CPU and an entry about flow added to a cache, after all packets of that flow a processed more quickly (in terms of CPU cycles).

[vishalpatil86]

thanks taras, this explanation made ,my doubt clear. Thanks to others also.

[Joseph W. Doherty]

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Posting

Just to add to the information the other posters have already provided.

"Fast" switching, can be much, much faster than "process switching".  I.e. you want to very much avoid the latter.

Per-packet load balancing will generally provide much better multi-link utilization, especially short term, but can impact application traffic, as some might be especially sensitive to packet reordering.

EIGRP can also support unequal cost load balancing; using this capability might also increase the load on the device.