Dell Force10 E300, Configuration Manual

Page: 475 / 1262

IPv4 Addressing | 475
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.
ICMP
For diagnostics, Internet Control Message Protocol (ICMP) provide routing information to end stations by
choosing the best route (ICMP redirect messages) or determining if a router is reachable (ICMP Echo or
Echo Reply). ICMP Error messages inform the router of problems in a particular packet. These messages
are sent only on unicast traffic
Configuration Task List for ICMP
Use the following steps to configure ICMP:
• Enable ICMP unreachable messages on page 476
• Enable ICMP redirects on page 476
See the
FTOS Command Line Reference Guide
for a complete listing of all commands related to ICMP.
Task Command Syntax Command Mode
Set the number of ARP retries.
arp retries
number
Default: 5
Range: 5-20
CONFIGURATION
Display all ARP entries learned via gratuitous ARP.
show arp retries 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.