1. A networking device in a first data center fabric connected to a second data center fabric over a backbone network to form interconnected data center fabrics configured with improved scalability in managing states, each data center fabric having a respectively plurality of endpoints of which a subset is designated as one or more border leaves of the respective data center fabric, the networking device comprising: a computer processor; and a memory storing an application which, when executed on the computer processor, performs an operation comprising: receiving a packet with a header encapsulating a media access control (MAC) frame, wherein the packet has a group address as a destination address; applying aggregated source replacement such that a total count of distinct source addresses stored in unicast states maintained in the second data center fabric scales with border leaves rather than endpoints, thereby improving scalability of maintaining unicast states in the second data center fabric, wherein applying aggregated source replacement includes replacing, in a source address field in the header, an address of an endpoint in the first data center fabric with an address of a border leaf of the first data center fabric and comprising the networking device; and forwarding the packet towards the destination identified by the destination address; wherein the backbone network includes a primary rendezvous point for multicast traffic spanning the first and second data center fabrics, wherein the networking device serves as a secondary rendezvous point such that a total count of multicast states maintained in the backbone network scales with border leaves rather than with endpoints functioning as multicast sources, thereby improving scalability of maintaining multicast states in the backbone network.