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Muhammad Thanveer

Load balance using Static route question

Dear All,

I have a lab set up in GNS3 as shown below.

R1 has loopback,

R2 has loopback and

R3 has loopback

R1 and R2 are connected with IP add and on gig 0/0 interface on both the routers

R2 and R3 are connected with IP address and on Gig 1/0 and and on Gig 2/0

I have done all the necessary routing, but still I am unable to load balance why?

traceroute from R1



Type escape sequence to abort.
Tracing the route to

1 96 msec 68 msec 56 msec
2 56 msec 108 msec 120 msec


Mohammad Thanveer

Mark Malone

Hi If you want to do basic LB on R3 set 2 default routes pointing to same next-hop and the enable ip cef and under each outgoing interface ip cef per packet load sharing, for LB to happen the routes need to be identical

Hello Mark,

Thank you so much for your reply. 

I am able to understand that we need to enable IP CEF,

Should we write default routes pointing towards both the interface IP addresses?

I am unable to understand the statement,  "on R3 set 2 default routes pointing to same next-hop"


Mohammad Thanveer

sorry too early that was mistake diff ips , here is what im trying to explain to you

Hi Mark,

Hope you are doing good today.

Thank you so much for your help, I have enabled the IP CEF on all the routers, but still I am unable to load balance it, I would like to know the reason why?

I am going mad out of curiosity, hope you understand that.

Thanks in advance :)


Mohammad Thanveer

Thanks Mark,

I have understood that this has something to do with per packet load balancing.

CEF by default does per destination load balancing and it uses a hash algorithm to determine which link to use.

The inputs to the hash algorithm are source and destination IP (plus port numbers on some platforms) and a unique value that does not change while the router is up to help avoid CEF polarisation.

So if the source IP and destination IP are the same then it will use the same link and they are in your case.

With per packet load balancing CEF will simply alternate between the two links or should do.