Simply put, when a router has more than one path to the same destination and these paths are equal cost paths then it can use both paths to send the traffic. ( Note that with EIGRP you have the capability to load balance traffic over unequal cost paths but most routing protocols only allow equal cost paths).
So within the routing table there will be 2 or more next hops for the same destination network. The router can then use per-packet or per-destination load-balancing. Per-destination load-balancing is often preferred because per-packet can result in packets arriving out of order and some applications are very sensitive to this.
If you have 2 connections from your ISP you could use one as the active connection and one as the backup or you could use both to send packets out. Without knowing your topology and routing details it is not possible to say how you can implement this.
Also note that load balancing is one way in effect ie. you could use both connections to send traffic out but this does not necessarily mean you will use both connections to receive packets.
When a router has 2 or more paths to a destination with equal metric then you can use them all at sametime to forward pkts to destination. This method of using multiple links at sametime to forward pkts is called load-balancing.
The metrics can be defined by use of either static routes or routing protocols. With some protocols you can use links with same metric & some use links with unequal metrics for load-balancing traffic.
In your case of links from 2 ISP's, you can either use a default route pointing to each ISP's device for load-balancing. Or can use BGP. Remember that you can also use these links as active-backup.
Based on your requirements & topology experts on forum may help.
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