Hello Nur,
first of all you need to install 8 equal cost and AD static routes in the routing table.
Something like
ip route 0.0.0.0 0.0.0.0 ser0/0
ip route 0.0.0.0 0.0.0.0 ser0/1
ip route 0.0.0.0 0.0.0.0 ser0/2
ip route 0.0.0.0 0.0.0.0 ser0/3
ip route 0.0.0.0 0.0.0.0 ser0/4
ip route 0.0.0.0 0.0.0.0 ser0/5
ip route 0.0.0.0 0.0.0.0 ser0/6
ip route 0.0.0.0 0.0.0.0 ser0/7
Notice: that depending on the IOS version of device the maximum number of parallel static routes installed in routing table can be less then 8 see later about MLPPP that can solve this issue.
The limit for parallel static routes can be less then that for dynamic protocols like OSPF or EIGRP.
on all serial intefaces and on the lan interfaces you need to enable per packet load sharing.
ip load-sharing per-packet
However, the better results can be achieved using MLPPP that build a single L3 link and then has its own load balancing per packet procedures
Hope to help
Giuseppe
