موضوع:Configuring a Static ARP Entry
In order to ensure layer three to layer two communication, a switch or router needs to have an ARP table, this table is a local table that contains ip addresses and their matching associated MAC addresses.
Lets say for example R1 needs to communicate with R2. R1 will first send an ARP request thus requesting the MAC address of R2′s IP address (10.1.1.2/24) and once it receives a reply the information will be placed into R1′s ARP table. Now R1 knows the destination MAC address to use in the frame(s) when sending traffic destined to 10.1.1.2
R1 sends a packet with the destination IP and MAC address of R2, when the local switch receives the frame, it looks at the destination MAC address and looks up the MAC address in the cam table (also known as mac-address table). When it finds the matching MAC address and associated switch port and then forwards the frame out the correct port to get to the destination MAC address.
However, sometimes ARP is not always reliable and in some special scenarios routers and/or switches cannot learn the MAC address of a directly or indirectly connected host. Such as a virtual machine or a host that does not reply to ARP request for whatever reason. In this case a static ARP entry is necessary to ensure efficient layer two communications.
Configure a static ARP entry on R1 with the IP address 10.1.1.25 and the mac address 00ac.a1f3.01ab
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#arp 10.1.1.25 00ac.a1f3.01ab arpa
ARPA stands for Advanced Research Projects Agency for those who are curious as to what arpa stands for at the end of the static ARP statement. ARPA developed the Ethernet II frame which is currently used by the Internet Protocol.
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.1 – ca00.0d78.0008 ARPA FastEthernet0/0
Internet 10.1.1.25 – 00ac.a1f3.01ab ARPA