Inter Vlan Routing - Concept question

[s.ram]

Hi folks-

Let's say I have L2 networks (multiple VLANs) under a leaf in a leaf-spine DC setup. The leaf is the L2-L3 boundary.

So, I have vlan interfaces (SVI/IRB) created in the leaf for each of the VLANs. 

Question1: I need to have an IP address for each vlan interface in the subnet of the vlan and the hosts' default gateway has to be set to this IP address. Is this correct?

Now, if let's say a machine in VLAN 40 needs to talk to a machine in VLAN 50. So, I'm assuming it would send the packet with MAC address of the VLAN 40 interface. This packet would get routed in the leaf, and the next hop would point to the VLAN 50 interface. Is my understanding right?

Question 2: What would be the SMAC of the packet going out of the leaf?

Question 3: How is the DMAC of the outgoing packet determined?

Thanks all for your time.

[Hanfeng Cai]

Q1: Your understanding is correct

Q2: Outer SMAC is interface MAC which is connecting to SPINE

Q3: DMAC is SPINE interface MAC

[s.ram]

Hi Hanfeng-

In this case, both the VLAN networks are under the same leaf. So, these packets will not go to the spine. This is more of a router on stick model. My question for the second part is, if the route points to an IRB interface as the nexthop, there could be multiple hosts with different MAC addresses in that subnet. How does the router pick up the exact host's DMAC?

thanks!

[Hanfeng Cai]

This is local L3 forwarding, you rely on ARP/MAC table to fwd pkt.