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Cisco Express Forwarding Redundancy

Posted on 2014-10-20
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Last Modified: 2014-10-21
Hi,

I have a quick question.  Does CEF support redundancy being that it is designed for load balancing.  The design that we are going to use is CEF in distribution mode (our l3 switch only support that).  If so, how does it work

Thank you for your help in advance
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Question by:thomasm1948
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by:Predrag Jovic
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It is supported. Link with details

Load Balancing with CEF
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by:thomasm1948
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From the document, it seems that it does it automatically once I enable CEF.  Is that correct
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by:Predrag Jovic
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Cisco Express Forwarding (CEF) is an advanced Layer 3 switching technology which can be used for load balancing in routers. By default, CEF uses per-destination load balancing. If it is enabled on an interface, per-destination load balancing forwards packets based on the path to reach the destination. If two or more parallel paths exist for a destination, CEF takes the same path (single path) and avoids the parallel paths. This is a result of the default behavior of CEF. CEF takes the single path in cases when load sharing is done simultaneously on interfaces of different physical types, such as serial and tunnel. The hash algorithm determines the path to be chosen. In order to utilize all the parallel paths in CEF and load balance the traffic, you must enable per-packet load balancing when you have different physical interfaces like serial and tunnel. So, on the basis of the configuration and topology (serial or tunnel), load sharing can fail to work correctly with the default CEF load balancing mode.

Enable these commands for load sharing on a per-packet basis:

    configure terminal
    interface serial 0
    ip load-sharing per-packet
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by:thomasm1948
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our switch doesn't support per packet
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by:Predrag Jovic
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In that case
If you have a single source sending to a single destination, then the default load balancing of CEF will not produce much load balancing. But if you have multiple sources sending to multiple destinations, then the default load balancing of CEF is likely to produce pretty good load balancing without doing anything fancy.
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by:thomasm1948
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That does make sense.  The main concern is to ensure if one line goes down then those computers on that route will go out from the line that is still up.

Also would they fail back to their original route once the line comes backup
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by:thomasm1948
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hmm, I am wondering if I should use PBR with SLA Monitor.  Granted that is static routing
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Predrag Jovic earned 500 total points
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You can use PBR with SLA Monitor, or you can use static routing with tracking, whatever is convenient for you.
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