The difference between CEF Original and Universal(default) algorithm

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Apr 29th, 2017
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I have read the following URL and there are some CEF load-balancing algorithms. 

Especially I would like to know more precise information of Per-destination Load Balancing Algorithms between Original and Universal. Here is the summary I have understood the CEF load-balancing below.


- Per-Destination Load Balancing: 

 Packets for a given source-destination host pair are guaranteed to take the same path.

 It is little misunderstanding to take only destination host because the name is Per-destination.


-Load Balancing Algorithms

--Universal algorithm : The router is set by default, source-destination address pair. 

--Original algorithm :   I think this also source-address address pair but I do not understand what the difference between Universal and Original is.


Best Regards,


Masanobu Hiyoshi


[IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS Release 15M&T]

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configurat...

(Quotation from URL)


Per-Destination Load Balancing
Per-destination load balancing allows the router to use multiple paths to achieve load sharing across multiple source-destination host pairs.
Packets for a given source-destination host pair are guaranteed to take the same path, even if multiple paths are available.
Traffic streams destined for different pairs tend to take different paths.


Load-Balancing Algorithms
The following load-balancing algorithms are provided for use with Cisco Express Forwarding traffic.
You select a load-balancing algorithm with the ip cef load-sharing algorithm command.

Original algorithm--The original Cisco Express Forwarding load-balancing algorithm produces distortions in load sharing across multiple routers
because the same algorithm was used on every router. Depending on your network environment, you should select either the universal algorithm (default) or
the tunnel algorithm instead.


Universal algorithm--The universal load-balancing algorithm allows each router on the network to make a different load sharing decision
for each source-destination address pair, which resolves load-sharing imbalances. The router is set to perform universal load sharing by default.




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Jon Marshall Sun, 04/30/2017 - 03:12
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Put simply the universal algorithm uses a unique 32 bit number per router (the universal ID) which is generated when the router boots up. This number is used with the src/dst IPs to ensure that each router can select a different path in a redundant network.

The idea is to avoid something called CEF polarization, see this document for details -

http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/116376-technote-cef-00.html

there is a paragraph at the end of the document covering the universal algorithm in a bit more detail.

Jon

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