ARP Timers and Flag Types

ARP is quite an essential protocol in LAN Networking. Computer systems will be unable to communicate with each other in LAN if ARP is not present. Just to refresh our understanding of ARP, when an end host or computer in LAN environment wants to transfer data to another host (like computer etc.) it must first know the MAC Address of the destination. If the destination host MAC address entry is not there, in this case, ARP protocol sends a broadcast out to the LAN network asking for the MAC address of the destination host IP address.  The machine with the IP address will respond with its MAC address. There other scenarios also where ARP is used, however we’ll now move our attention further to ARP timeouts and related flags.

ARP Timeout values

ARP timeout means how much time the system will keep the dynamic leaned MAC address on its memory before flushing it if it’s not reused. Many of the networking systems have a timeout of 20 minutes for a completed entry and 3 minutes for an incomplete entry. However, there are array of vendors in networking markets who have created their own standard timeout values for Address Resolution protocol. Below are the default ARP cache timeout of few vendors:

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ARP Flag Types (Linux):

Next let’s look at the ARP flags types especially in Linux system. ARP flags in Linux are the ones which will help us understand what type of entry is being placed in the memory as seen in the ARP table. Below is the output from “ARP –n” command statement where Flags are highlighted in RED circle –

Based on the above output, 3 of the entries are showing Flag Type as “C”. Further, lets understand other flag types. There are 3 type of Flags in Linux

  1. C = This type of entry is seen when entries are dynamically learned by arp protocol.
  2. M = This flag tells that entries have been manually entered/added in the memory instead of dynamically learned from arp protocol.
  3. P = P here means Publish. It tells the host to respond to packets which are ARP request and ARP response.

Hope this article helped our viewers with concepts of –

  • ARP Timeouts with different vendor devices
  • ARP Flag types in Linus Operating System
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