Dynamic ARP

Definition

Dynamic ARP entries are automatically generated and maintained when ARP packets are sent and received. They can be aged, updated, or overwritten by Static ARP entries.

Dynamic ARP applies to complex networks that transmit delay-sensitive services.

Address Resolution Process

Dynamic ARP broadcasts ARP Request packets and unicasts ARP Reply packets to complete address resolution.

Figure 1 Address resolution process

In Figure 1, Host_1 and Host_3 are on the same network segment, and Host_1 needs to communicate with Host_3.

1. Host_1 searches its local ARP table for the ARP entry of Host_3. If the MAC address of Host_3 is found, Host_1 uses this MAC address to encapsulate the data packet into a frame and sends it to Host_3. If Host_1 does not find the MAC address of Host_3, Host_1 caches the data packet and broadcasts an ARP Request packet. (For details on the format of the ARP Request packet, see ARP Packet Format.)
2. Switch_1 receives the ARP Request packet and forwards it across the appropriate broadcast domain.
3. Host_2 and Host_3 in that broadcast domain both receive the ARP Request packet. Because Host_3 finds that the destination IP address of the ARP Request packet is its own IP address, it adds the source IP and MAC addresses of the ARP Request packet to its own ARP table. It then unicasts an ARP Reply packet to Host_1. (For details on the format of the ARP Reply packet, see Figure 1.)
4. Switch_1 receives the ARP Reply packet and forwards it to Host_1. Upon receipt, Host_1 adds the MAC address of Host_3 to its ARP table. It then encapsulates the data packet into a frame and forwards it to Host_3.

In Figure 1, Host_1 and Host_4 are on different network segments, and a default gateway address (the Router's IP address) has been set on Host_1. If Host_1 and Host_4 need to communicate, Host_1 must first discover the Router's MAC address. The process for Host_1 to learn the Router's MAC address and for the Router to learn Host_4's MAC address is similar to that between Host_1 and Host_3. When the data packet from Host_1 arrives at the Router, the Router forwards the packet to Host_4.

ARP Aging Mechanism

In Figure 1, if Host_1 broadcasts an ARP Request packet every time it sends data to Host_3, network communication traffic will greatly increase. Furthermore, all other hosts on the network have to receive and process the ARP Request packets, which lowers network efficiency. To resolve this problem, each host or device maintains a high-speed cache that stores the recently learned IP-to-MAC mappings (namely, dynamic ARP entries).

A host or device first queries the local high-speed cache for the IP-to-MAC mappings every time it sends a packet. If the MAC address is found, the host or device no longer sends an ARP Request packet but directly sends the data packet to the MAC address. If the MAC address is not found, the host or device broadcasts an ARP Request packet for address resolution.

Devices age and update dynamic ARP entries to ensure that the capacity limit of the ARP tables is not exceeded and that the ARP entries in the table are correct.

The aging parameters of a dynamic ARP entry include the aging time, number of aging probe attempts, and aging probe mode. After the aging time of a dynamic ARP entry expires, the device sends an aging probe packet (ARP Request packet) to detect whether the peer device is present. If the device receives an ARP Reply packet, it updates this dynamic ARP entry. If the device does not receive an ARP Reply packet, it deletes the dynamic ARP entry.

The aging probe packet can be a unicast or broadcast packet. By default, the device only broadcasts the last ARP aging probe packet to the peer host or device, and unicasts other ARP aging probe packets. When the MAC address of the peer device is fixed, an interface can be configured to unicast ARP aging probe packets.

If the interface goes Down, the device immediately deletes the interface's dynamic ARP entries.

ARP Packet Format

The length of an ARP packet is 42 bytes. The first 14 bytes represent an Ethernet frame header, and the last 28 bytes contain the ARP packet information.

Figure 2 shows the format of an ARP packet.

Figure 2 Format of an ARP Request or Reply packet

Table 1 describes the fields within an ARP packet.