Interface Mtu Configuration Guidelines - Cisco ASR 1000 Series Software Configuration Manual

Modifying the Interface MTU Size
size allowable (in bytes) for an interface before drops occur. If the frame is smaller than the interface
MTU size, but is not smaller than the minimum frame size for the interface type (such as 64 bytes for
Ethernet), then the frame continues to process.
• IP MTU—Can be configured on an interface or subinterface. If an IP packet exceeds the IP MTU size,
then the packet is fragmented.
• Tag or Multiprotocol Label Switching (MPLS) MTU—Can be configured on an interface or subinterface
and allows up to six different labels, or tag headers, to be attached to a packet. The maximum number
of labels is dependent on your Cisco IOS software release.
Different encapsulation methods and the number of MPLS MTU labels add additional overhead to a packet.
For example, Subnetwork Access Protocol (SNAP) encapsulation adds an 8-byte header, dot1q encapsulation
adds a 2-byte header, and each MPLS label adds a 4-byte header (n labels x 4 bytes).
For Gigabit Ethernet SPAs on the Cisco ASR 1000 Series Aggregation Services Routers, the default MTU
size is 1500 bytes. The maximum configurable MTU is 9216 bytes. The SPA automatically adds an additional
22 bytes to the configured MTU size to accommodate some of the additional overhead.
In the Cisco ASR 1000 Series Route Processor 1 (RP1), 2RU and 2RU-Fixed chassis, the MTU size for
Note
the Management Ethernet interface (interface gigabitethernet 0) is limited to 2370 bytes.

Interface MTU Configuration Guidelines

When configuring the interface MTU size on a Gigabit Ethernet SPA on a Cisco ASR 1000 Series Routers,
consider the following guidelines:
• The default interface MTU size accommodates a 1500-byte packet, plus 22 additional bytes to cover
the following additional overhead:
• If you are using MPLS, be sure that the mpls mtu command is configured for a value less than or equal
to the interface MTU.
• If you are using MPLS labels, then you should increase the default interface MTU size to accommodate
the number of MPLS labels. Each MPLS label adds 4 bytes of overhead to a packet.
Interface MTU Configuration Task
To modify the MTU size on an interface, use the following command in interface configuration mode:
Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide, Cisco IOS
XE Everest 16.5
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◦Layer 2 header—14 bytes
◦Dot1q header—4 bytes
◦CRC—4 bytes
Configuring the Ethernet SPAs
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