Understanding Linux Network Internals

Creating a neighbour Entry

Like most cached items, the creation of neighbour entries is event driven: an instance is created when the system needs a neighbor and there is a cache miss. Specifically, a new instance is created when one of the following takes place:

Transmission request

When there is a transmission request toward a host whose L2 address is not known, the address needs to be resolved. This is the most common case and is depicted in Figure 27-13(a). When the target host is not directly connected to the sender, the L2 address to resolve will be that of the next hop gateway, not that of the target host.

Reception of a solicitation request

Because the host sending the request identifies itself in that request, the recipient automatically creates a cache entry on the assumption that communication between the two systems is imminent. (For details involving ARP, see Figure 28-2 in Chapter 28). However, information learned in this way (passively) is not considered as authoritative as information learned with an explicit solicitation request and reply (see the section "Transitions Between NUD States" in Chapter 26 for more details).

Manual coding

An administrator can create a cache entry through an ip neigh add command, as described in the section "System Administration of Neighbors" in Chapter 29.

When one of these events happens, and a query to the neighboring subsystem cache returns a miss, the neighboring protocol tries to resolve the association (normally by sending a solicitation ...