Dell Force10 E1200i FTOS Version 8.4.1.5 Configuration Guide for ExaScale - Page 467

Beginning with FTOS version 8.3.1.0, when ARP Learning via Gratuitous ARP is enabled, the system installs a new ARP entry, or updates an existing entry for all received ARP requests. Figure 24-2. Learning via Gratuitous ARP VLAN ID: 1.1.1.1 ARP Learning via Gratuitous ARP enabled ARP Request Target IP: 1.1.1.3 X Host 1 IP: 1.1.1.2 MAC: AA Target IP is not the VLAN interface IP. Install new entry for Host 1, or update existing Host 1 entry. Drop packet. Host 2 IP: 1.1.1.3 MAC: BB Whether ARP Learning via Gratuitous ARP is is enabled or disabled, the system does not look up the Target IP. It only updates the ARP entry for the Layer 3 interface with the source IP of the request. Configurable ARP Retries In FTOS versions prior to 8.3.1.0 the number of ARP retries is set to 5 and is not configurable. After 5 retries, FTOS backs off for 20 seconds before it sends a new request. Beginning with FTOS version 8.3.1.0, the number of ARP retries is configurable. The backoff interval remains at 20 seconds. Task Set the number of ARP retries. Display all ARP entries learned via gratuitous ARP. Command Syntax arp retries number Default: 5 Range: 5-20 show arp retries Command Mode CONFIGURATION EXEC Privilege FTOS Behavior: Due to ARP Pruning, the total number of ARP requests sent might exceed, but is never less than, the configured number of ARP retries. This occurs when the ARP Pruning timer expires while ARP retry is in progress. ARP Pruning is a mechanism that clears stale entries every 1 minute. A stale entry is an IP address for which either the ARP expiry timer-which by default is set to 4 hours-expires, or ARP cannot resolve an IP address. When there is a coincidence between an ARP Pruning timer expiry and the ARP retry mechanism FTOS sends more than the configured number of ARP requests. To illustrate why this occurs, take a time T1=0 seconds, at which point the pruning timer starts. At time T2=55 seconds, when the pruning timer is 55 seconds, suppose the ARP retry for an unresolved address begins, with ARP retry configured for 20 retries. By time T3=60 seconds, the total number of ARP requests sent is 5. However, at T3, the pruning timer expires and clears all stale entries, including the entry for which ARP retry is in progress. In this case, ARP retry starts over and sends another 20 ARP request over 20 seconds. As a result, the total number of ARP requests sent is 25, not the configured 20. IPv4 Addressing | 467