EIGRP: Over the Top

Also referred to as OTP, this feature allows EIGRP to create overlay multi-point VPNs between edge routers without any coordination with the service provider connecting the edge routers, removing the need for a service provider to assist in distributing the routing protocol messages and exchanges.

OTP functions using the Locator/Identifier Separation Protocol or LISP.  LISP separates a host machines address and the location of the host itself, allowing the host to retain its identity regardless of its location in a network.  The location and identity are considered two separate independent entities, both represented by a complete address, and provides a way to define and map so that the address representing the identity of a host can be resolved into the address that represents the location.  Then a tunneling mechanism is used to encapsulate the packets between end hosts addressed using the end host identities into new packets that are destined to the addresses representing the end host locations.  This allows a host to change its location while retaining its identity without losing connectivity.

Host machines with LISP have an Endpoint ID or EID, that identifies its identity which never changes.   This EID can be an IPv4 address an IPv6 address or any other address format as needed.  The outside address of  the edge router represents the location of the EID of the hosts, this is referred to as  the Routing Locator or RLOC.  Multiple EIDs can exist behind a single RLOC.

LISP.PNG

LISP also has a control and data plane, where the control plane comprises of the registration protocol and procedures by which the tunnel routers register the EIDs they are responsible for along with their RLOCs, then use those registrations they map EIDs into the RLOCs.  The data plane defines the actual tunnel encapsulation used between RLOC routers.

Configuration of OTP involves the use of statically configured EIGRP neighbors to exchange EIDs and RLOCs, however this does not scale if there are multiple and growing number of sites.  OTP uses a route-reflector feature very similar to BGP to exchange routes in a hub and spoke model fashion.  The only neighbor routers are configured with in this scenario is the hub router which exchanges all OTP EID and RLOC information to the spoke sites.

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