A proxy egress tunnel router is a router that is connected to a non-LISP site, such as a data centre or the internet. It used when a LISP site needs to communicate with a non-LISP site.
As a proxy egress tunnel router is connected to non-LISP sites, it does not register any end point prefix addresses with the database system.
If an ingress tunnel router sends a map request and the endpoint identifier is not registered in the mapping database system, a negative response is sent from the mapping server. When that ingress tunnel router receives the reply, it forwards the LISP encapsulated traffic to the proxy egress tunnel router instead. The ingress tunnel router must be configured to send traffic to the proxy egress tunnel router when a negative response is received.
When the mapping database system receives a map request for a non-LISP destination, it calculates the shortest prefix that matches the destination but does not match any LISP end point identifiers. The calculated non-LISP prefix is included in the negative reply so the ingress tunnel router can add the prefix to its map cache and forwarding information base. The ITR is able to quickly lookup this mapping in its cache and forward the traffic to the proxy egress tunnel router