ARP Foundation
Role:
Resolves the destination MAC address based on the destination IP address. An ARP table is maintained on the host or three-tier network device to store the relationship between IP address and MAC address.
Trigger condition:
When the destination IP address needs to be accessed, there is no corresponding table entry in the ARP cache table;ARP message:
ARP request (usually broadcast):
a) MAC address corresponding to the requested destination IP address when the destination IP address of the access is the same network segment;
b) Request the MAC address of the gateway IP when the destination IP address of the access is not in the same network segment
ARP reply (unicast reply)
a) replies to ARP reply when receiving a request message and the destination IP address is the same as the IP address of the receiving interface, including the corresponding relationship between its own interface IP address and MAC address;
b) When a request message is received and the destination IP address is inconsistent with the IP address of the receiving interface, no message will be returned if the ARP proxy function is not turned on;
c) If the ARP proxy function is turned on, reply to ARP reply after certain conditions are met, including the relationship between the requested destination IP address and the MAC address of its own interface;
Effect:
Hosts implemented on the same network segment but not on the same physical network can access each other
Implementation:
Routed Proxy ARP:
Scenarios where the hosts that need to be interconnected (no default gateway is configured on the host) are in the same network segment but not in the same physical network (that is, not in the same broadcast domain).
When PC1 needs to communicate with PC2, since the destination IP address is the same network segment as the local IP address, PC1 broadcasts ARP request messages. When Router enables Routing Proxy ARP, Router looks up the routing table when it receives an ARP request message. Because PC2 is directly connected to Router, there are routing table entries to PC2 on Router. Router uses its MAC address to send an ARP reply to PC1. PC1 will forward data to Router's MAC address.
Proxy ARP within VLAN:
Hosts that need to be interoperable are in the same network segment and belong to the same VLAN, but ports isolated scenarios are configured within the VLAN.
Because different interfaces within the VLAN are configured on Router to be isolated from each other, PC1 and PC2 cannot communicate directly on the second layer. If Router's interface enables Proxy ARP functionality within VLAN, PC1 and PC2 can be interconnected in three layers. When Router's interface receives an ARP request message whose destination address is not its own, Router does not immediately discard the message but looks up the ARP table entry for the interface. If there are ARP table entries for PC2, send your MAC address to PC1 via ARP reply message and forward PC1 to PC2 instead.
