Cisco ASR 1000 Series Software Configuration Guide

This command shows a serial SPA as a representative example, however the same slot/subslot/port format is
similarly used for other SPAs (such as ATM and POS) and other non-channelized SPAs.
For more information about identifying slots and subslots, see the Configuring the Encapsulation Type, on
page 202.
Modifying the Interface MTU Size
The Cisco IOS software supports three different types of configurable maximum transmission unit (MTU)
options at different levels of the protocol stack:
• Interface MTU—Checked by the SPA on traffic coming in from the network. Different interface types
support different interface MTU sizes and defaults. The interface MTU defines the maximum packet
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 three bytes of payload size, then the frame continues to process.
• IP MTU—Can be configured on a subinterface and is used by the Cisco IOS software to determine
whether fragmentation of a packet takes place. 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 a 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, for an Ethernet packet, 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).
In the Cisco ASR 1000 Series Route Processor 1 (RP1), 2RU and 2RU-Fixed chassis, the MTU size for
the Management Ethernet interface (interface gigabitethernet 0) is limited to 2370 bytes.
Note
Interface MTU Configuration Guidelines
When configuring the interface MTU size on the POS SPAs, consider the following guidelines:
•
If you are also using MPLS, be sure that the mpls mtu command is configured for a value less than or
equal to the interface MTU.
•
If you change the interface MTU size, the giant counter increments when the interface receives a packet
that exceeds the MTU size that you configured, plus an additional 24 bytes for overhead, and an additional
2 or 4 bytes for the configured cyclic redundancy check (CRC).
For example, with a maximum MTU size of 9216 bytes, the giant counter increments:
•
For a 16-bit CRC (or FCS), when receiving packets larger than 9242 bytes (9216 + 24 + 2).
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For a 32-bit CRC, when receiving packets larger than 9244 bytes (9216 + 24 + 4).
•
The Frame Relay Local Management Interface (LMI) protocol requires that all permanent virtual circuit
(PVC) status reports fit into a single packet. Using the default MTU of 4470 bytes, this limits the number
Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide, Cisco IOS
XE Everest 16.5
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Configuring the POS SPAs
Modifying the Interface MTU Size

Cisco ASR 1000 Series Specifications

General

Brand	Cisco
Model	ASR 1000 Series
Category	Network Router
Language	English