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+.. _arpentries:
+
+ARP Entries
+^^^^^^^^^^^
+
+.. figure:: /_images/fib20fig1.png
+
+Figure 1: ARP data model
+
+Figure 1 shows the data model for an ARP entry. An ARP entry contains the mapping
+between a peer, identified by an IPv4 address, and its MAC address on a given
+interface.  The VRF the interface is bound to, is not part of the data. VRFs are
+an ingress function not egress. The ARP entry describes how to send traffic to a
+peer, which is an egress function.
+
+The *arp_entry_t* represents the control-plane addition of the ARP entry. The
+*ip_adjacency_t* contains the data derived from the *arp_entry_t* that is need to
+forward packets to the peer. The additional data in the adjacency are the *rewrite*
+and the *link_type*. The *link_type* is a description of the protocol of the packets
+that will be forwarded with this adjacency; this can be IPv4 or MPLS. The *link_type*
+maps directly to the ether-type in an Ethernet header, or the protocol filed in a
+GRE header. The rewrite is a byte string representation of the header that will be
+prepended to the packet when it is sent to that peer. For Ethernet interfaces this
+would be the src,dst MAC and the ether-type. For LISP tunnels, the IP src,dst pair
+and the LISP header.
+
+The *arp_entry_t* will install a *link_type=IPv4* when the entry is created and a
+link_type=MPLS when the interface is MPLS enabled. Interfaces must be explicitly
+MPLS enabled for security reasons.
+
+So that adjacencies can be shared between route, adjacencies are stored in a single
+data-base, the key for which is {interface, next-hop, link-type}.