Hi @apache_le ,
To add to @Sergiu.Daniluk 's excellent answer:
Let's start with a picture. Leaves 203 and 204 have only two connections to the spines, so are the focus of this discussion.
H3 and H4 are connected to leaf 203 and 204 respectively. When these hosts send traffic to hosts H1 an H2, it will be encapsulated by Leaf203 or 204 and sent to the appropriate leaf - so leaves 201 and 202 will ONLY ever send traffic to H3 or H4 via Leaf 203 or 204.
The point of this is to move on to the next part of the leaf's decision making process.
Both Leaf 201 and 202 will have an IS-IS routing table that tells them that the BEST path to either leaf 203 or 204 is via Spine 202 or 203 - NEVER via leaf 201 - so here we have yet another example of how Cisco has capitalised on the simplicity of running IS-IS on the underlay to make this work as desired right out of the box!
Similarly, leaves 203 and 204 will only ever send traffic via spines 102 and 103 - obvious when you look at the diagram.
Although I'd trust Sergiu with my life, I'm not so sure he's correct when he says traffic will be slightly sub-optimal for the multicast trees for which Spine 101 is the root, because a tree is a tree, and there is only one path. And I'm too lazy to look up how IS-IS determines which spines are multicast roots. But for now I'll accept his statement.
One more thing though, I'd recommend that you do NOT use a combination of leaves that have different numbers of spine connections as VPC pairs - in my diagram above that would mean that you DON'T make leaves 202 and 203 a VPC pair. Leaves 201 and 202 are OK because they both have three spine connections, and leaves 203 and 204 are also fine with their two spine uplinks.
AND FINALLY - just to throw a real curve-ball - there is the option of using Congestion awareness (CONGA - learn the theory from this excellent deck of slides - make you you PLAY the slide deck, don't just flick through. The animations are important). This is enabled via System > System Settings >> Load Balancer under Dynamic Load Balancing Mode and Load Balancing Mode options. But I don't know of anyone who uses these options.
