The main difficulty of supporting L2 UDN is not having a node-specific pod network subnet to advertise with that node as next hop: L2 UDNs subnet is cluster wide.

One of the ideas was to advertise /32 pod specific routes but there are concerns on the scalability of that.

The other idea is to advertise the whole L2 UDN subnet with the selected nodes as next hop and let multi-path take care of the rest. With this alternative there is acceptance that this might not always route the traffic on the most optimum path.

This effort entails adding support for it in cluster manager route advertisement controller, and mimic the existing support of L3 zone and node network controllers for L2 as well.

This includes upstream testcases that should basically mimic L3 existing test cases which add some level of dependency with SDN-5712.