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The GW and the WAN Edge functions can be decoupled into two separate systems (decoupled interconnect solution) or integrated into the same system (integrated interconnect solution).
This document focuses on an integrated solution, which is introduced in Dell Enterprise SONiC 4.0 release. Multi-Fabric or Multisite architecture terminates VxLAN tunnels originating from within the site/PoD at the Gateway or Border Leaf (BL) node. In a decoupled solution, VxLAN tunnels towards remote site are not originated at the same BL node. Instead, the data plane and control plane undergo handoff to another BL (WAN Edge) node from where the VxLAN tunnels to remote sites/PoDs originated. Even though this architecture provides a scalable solution for interconnecting multiple PoDs/sites, it imposes the limitation of provisioning another pair of nodes in each PoD/site. In the integrated GW architecture, the BL not only imports and re-originates BGP updates, but it also can also decapsulate and re-encapsulate VxLAN data traffic, crossing the BL. The goal here is to split the VxLAN network into two split horizons—Site Internal and Site External, as shown in Figure 3. Split horizon is a method of avoiding network loops.
The site-internal network consists of the VxLAN tunnels to VTEPs within the site. The site-external network consists of the VxLAN tunnels to BL of the remote sites. The site-internal and site-external networks separate split-horizon groups allowing BL to terminate and re-originate routes, and the data traffic, crossing the BL.
BL in Site-1 terminates VxLAN traffic originating from the Leaf nodes within the PoD/DC. The L2 traffic gets re-encapsulated and switched to VxLAN tunnels towards remote sites. Due to the split horizon, the L2 traffic is not switched back to the VxLAN tunnels towards VTEP within the PoD/DC. The same holds for the traffic arriving from remote BLs. Within a PoD/DC, VTEPs have direct tunnels to other VTEPs, and therefore BL is not required to switch back the traffic. The BGP control plane is also required to be aware of the site-internal and site-external neighbors. BGP on the BL will perform the following tasks: