Dell Networking 2048, Reference Manual

Page: 384 / 1823

384
Ethernet Configuration Commands
User Guidelines
Unknown unicast and multicast packets are copied to the CPU on the lowest
priority QoS queue. Unknown packets are those that do not have hardware
forwarding entries. Known unicast/multicast packets are hardware forwarded
and are not queued to the CPU. Control plane packets (e.g. spanning tree
BPDUs) are copied or forwarded to the CPU on higher priority queues. The
rate limiting for unknown packets occurs on the internal CPU port and does
not affect hardware based traffic routing/forwarding in any way. Typically, the
switch examines the received packets in software to check if there is a
forwarding entry, create a forwarding entry (e.g., add a L2 MAC address or
ARP response), and then either discard the packet or software forward the
packet (only occurs during the brief transitional period when the system is
actively adding a hardware forwarding entry but the hardware is not yet
updated). Processing delays for higher priority packets may occur when the
internal CPU queue is continually kept busy handling low priority packets.
This command does not affect the rate limits for control plane packets. It is
almost never necessary to use this command to change from the default
value. The use of this command should be restricted to situations in which
moderate to high rates of unknown unicast/multicast are continually sent to
the switch CPU as evidenced by the show proc cpu command and where the
ipMapForwardingTask is showing high CPU usage. This occurs most
frequently in networks where a high number of ARPs are continually received
on untrusted ports, high numbers of L2 stations are timing out and
reappearing or multicast flooding is occurring in the network. If problems
with L2, L3 or multicast learning occur after changing this value, set the rate
limit back to the default value and take other steps to correct or mitigate the
underlying network issue directly.
Use the
show system internal pktmgr
command to show the configured
value.
Example
The following example shows output with higher than normal CPU usage due
to packets copied to the software forwarding task.
console#show process cpu
Memory Utilization Report
status bytes
2CSNXXX_SWUM200.book Page 384 Tuesday, December 10, 2013 1:22 PM