How LISP Operates

Map Registration and Notification

When a LISP is set up, the ETR routers will need to be configured with the endpoint identifier (EID) prefixes within that LISP site that will be registered with the map server (MS).

Any subnets that are attached to the egress tunnel router (ETR) that were not configured as endpoint identifier (EID) prefixes will be forwarded using traditional routing.

Map Registration Process

The egress tunnel router sends a map register message to the map server (MS) to register it’s associated prefix (e.g. 192.168.1.0). The registration message will also include the Routing Locator (RLOC) IP address (10.0.0.1) to be used by the map server (MS) when forwarding map requests receive through the database system.

The egress tunnel router (ETR) can respond to map request messages, but in a map register message it can request that the map server (MS) answers these map requests on the map server (MS) on the egress tunnel routers (ETR) behalf by setting the proxy map reply flag (P-bit).

The map server will respond with a map notify message to to the egress tunnel router to confirm that the map register message has been processed. The map notify message uses port 4342 on UDP for both its source and destination on this message.

Map Request and Reply

When an endpoint in a LISP site is trying to communicate with a host outside of the LISP site, the ingress tunnel router needs to perform some steps to route the traffic.

The first host will send IP packets of the destination IP address to its default gateway which is the ingress tunnel router (ITR).

The ingress tunnel router performs a forwarding information base lookup to check if the packet matches a default route or a specific route. If a specific route is found, the packet is forwarded using that. If the packet only matches a default route, a LISP check is performed if the source IP address part of a registered endpoint identifier prefix in the local map cache.

If the source IP address is part of a endpoint identifier prefix, a ingress tunnel router will send an encapsulate map request to the map resolver for the destination address in the packet with a destination UDP port of 4342 and a specifically chosen source port.

If the map resolver and map server are on the same device, the database system will forward the request to the authoritative egress tunnel router.

The egress tunnel router (ETR) will respond to the ingress tunnel router (ITR) a map reply message that includes an endpoint identifier to routing locator (EID-to-RLOC) mapping, unless the egress tunnel router requested the map server responds to messages on its behalf using the proxy map reply flag (P-bit). The map reply message uses the UDP source port of 4342 and the destination port is the one that was specifically selected by the ITR in the map request message.

The ingress tunnel router will install the EID-to-RLOC mapping in its local map cache and forwarding information base, ready to forward traffic.

The LISP Data Path

After the ingress tunnel router receives the endpoint identifier to routing locator mapping from the egress tunnel router or map server, it is ready to forward traffic.

The ITR will continue to receive packets from Host A and encapsulate them with an outer header containing the routing locator IP address from the ingress tunnel router as the source, and egress tunnel routers routing locator IP address as the destination.