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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-07
Chapter 9 Configuring Interfaces
Configuring the System MTU
Frames sizes that can be received by the switch CPU are limited to 1998 bytes, no matter what value was
entered with the
system mtu
or
system mtu jumbo
commands. Although frames that are forwarded or
routed are typically not received by the CPU, in some cases packets are sent to the CPU, such as traffic
sent to control traffic, SNMP, Telnet, or routing protocols.
Because the switch does not fragment packets, it drops:
•
switched packets larger than the packet size supported on the
egress
interface
•
routed packets larger than the routing MTU value
For example, if the
system mtu
value is 1998 bytes and the
system mtu jumbo
value is 5000 bytes,
packets up to 5000 bytes can be received on interfaces operating at 1000 Mbps. However, although a
packet larger than 1998 bytes can be received on an interface operating at 1000 Mbps, if its destination
interface is operating at 10 or 100 Mbps, the packet is dropped.
Routed packets are subjected to MTU checks on the sending ports. The MTU value used for routed ports
is derived from the configured
system mtu
value (not the
system mtu jumbo
value). That is, the routed
MTU is never greater than the system MTU for any VLAN. The routing protocols use the system MTU
value when negotiating adjacencies and the MTU of the link. For example, the Open Shortest Path First
(OSPF) protocol uses this MTU value before setting up an adjacency with a peer router. To view the
MTU value for routed packets for a specific VLAN, use the
show platform port-asic mvid
privileged
EXEC command.
Note
If Layer 2 Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces,
jumbo frames received on a Layer 2 Gigabit Ethernet interface and sent on a Layer 2 10/100 interface
are dropped.
Beginning in privileged EXEC mode, follow these steps to change the MTU size for all 10/100 or Gigabit
Ethernet interfaces:
If you enter a value that is outside the allowed range for the specific type of interface, the value is not
accepted.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
system mtu
bytes
(Optional) Change the MTU size for all interfaces on
the switch that are operating at 10 or 100 Mbps. The
range is 1500 to 1998 bytes; the default is 1500 bytes.
Step 3
system mtu
jumbo
bytes
(Optional) Change the MTU size for all Gigabit
Ethernet interfaces on the switch. The range is 1500 to
9000 bytes; the default is 1500 bytes.
Step 4
system mtu routing
bytes
(Optional) Change the system MTU for routed ports.
The range is 1500 to the system MTU value, the
maximum MTU that can be routed for all ports.
Although larger packets can be accepted, they cannot be
routed.
Step 5
end
Return to privileged EXEC mode.
Step 6
copy running-config startup-config
Save your entries in the configuration file.
Step 7
reload
Reload the operating system.
Содержание ME 3400 Series
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