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EIGRP- Equal Cost Paths Selection

Hello,

This might be a simple question; using EIGRP and having 2 equal cost paths to the destination; how does the router/L3 switch route select the path from those two equal cost paths? Is there a way to force one path over anthoer?

Thanks in advance.

Best, ~sK       

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

EIGRP- Equal Cost Paths Selection

To answer your question we need to be clear that there are 2 parts to the decision about which path will be used to forward traffic. The first part is the identification of paths and their insertion into the routing table. This is the part where EIGRP plays a role. If EIGRP determines that there are two equal cost viable paths then both of the paths will be put into the routing table. This ends EIGRP involvement in the process. The part of the process that takes a particular packet and determines which path to use is handled by CEF. By default CEF looks at the number of available paths toward a destination and when there are multiple paths CEF will use a process that calculates based on source and destination address to choose a particular path. For the same source address and same destination address CEF should choose the same path every time.

HTH

Rick

Hall of Fame Super Blue

EIGRP- Equal Cost Paths Selection

As Rick has said CEF does the actual forwarding and it will alternate between the two paths on a per destination basis.

As to how to prefer one path over the other, once they are both in the routing table you can't really. You could use PBR but then you are simply overriding the routing table rather than preferring one route over the other. The only way to prefer one path would be to make sure only one route is inserted into the routing table and how easy that would be depends on whether the two paths are received from the same neighbor or not. If they are then it would not be possible to distinguish between the two routes (as they are the same obviously) .

Jon

2 REPLIES
Hall of Fame Super Silver

EIGRP- Equal Cost Paths Selection

To answer your question we need to be clear that there are 2 parts to the decision about which path will be used to forward traffic. The first part is the identification of paths and their insertion into the routing table. This is the part where EIGRP plays a role. If EIGRP determines that there are two equal cost viable paths then both of the paths will be put into the routing table. This ends EIGRP involvement in the process. The part of the process that takes a particular packet and determines which path to use is handled by CEF. By default CEF looks at the number of available paths toward a destination and when there are multiple paths CEF will use a process that calculates based on source and destination address to choose a particular path. For the same source address and same destination address CEF should choose the same path every time.

HTH

Rick

Hall of Fame Super Blue

EIGRP- Equal Cost Paths Selection

As Rick has said CEF does the actual forwarding and it will alternate between the two paths on a per destination basis.

As to how to prefer one path over the other, once they are both in the routing table you can't really. You could use PBR but then you are simply overriding the routing table rather than preferring one route over the other. The only way to prefer one path would be to make sure only one route is inserted into the routing table and how easy that would be depends on whether the two paths are received from the same neighbor or not. If they are then it would not be possible to distinguish between the two routes (as they are the same obviously) .

Jon

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