ARP Entries

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}.