Reference Guide

Interfaces | 445
Configure MTU Size on an Interface
If a packet includes a Layer 2 header, the difference in bytes between the link MTU and IP MTU must be
large enough to include the Layer 2 header. For example, for VLAN packets, if the IP MTU is 1400 bytes,
the Link MTU must 1422 bytes or greater to accommodate the 22-byte VLAN header:
1400-byte IP MTU + 22-byte VLAN Tag = 1422-byte link MTU
On the E-Series and C-Series, you configure the Link MTU size on an interface by entering the mtu
command.
On the E-Series, you must manually configure the IP MTU size on an interface by entering the
ip mtu
command. On the C-Series and S-Series, the IP MTU size is automatically configured on an interface by
taking into account the Layer 2 overheads and difference in the number of bytes as shown in Table 23-7,
"Difference between Link MTU and IP MTU," in Interfaces.
Command Syntax Command Mode Purpose
flowcontrol rx [off | on] tx [off | on]
[threshold {<1-2047> <1-2013>
<1-2013>}]
INTERFACE Control how the system responds to and generates 802.3x
pause frames on 1 and 10Gig line cards.
Defaults:
C-Series: rx off tx off
E-Series: rx on tx on
S-Series: rx off tx off
Parameters:
rx on: Enter the keywords rx on to process the received flow control frames on this
port.
rx off: Enter the keywords rx off to ignore the received flow control frames on this
port.
tx on: Enter the keywords tx on to send control frames from this port to the
connected device when a higher rate of traffic is received.
tx off: Enter the keywords tx off so that flow control frames are not sent from this
port to the connected device when a higher rate of traffic is received.
threshold (C-Series and S-Series only): When tx on is configured, the user can set
the threshold values for:
Number of flow-control packet pointers: 1-2047 (default = 75)
Flow-control buffer threshold in KB: 1-2013 (default = 49KB)
Flow-control discard threshold in KB: 1-2013 (default= 75KB)
Pause control is triggered when either the flow control buffer threshold or flow
control packet pointer threshold is reached.
Table 23-7. Difference between Link MTU and IP MTU
Layer 2 Overhead Difference between link MTU and IP MTU
Ethernet (untagged) 18 bytes
VLAN Tag 22 bytes
Untagged Packet with VLAN-Stack Header 22 bytes
Tagged Packet with VLAN-Stack Header 26 bytes