Cisco ASR 1000 Configuration Guide

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Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide
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Chapter 10 Configuring the Ethernet SPAs
Configuration Tasks
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 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.
Interface MTU Configuration Guidelines
When configuring the interface MTU size on a Gigabit Ethernet SPA on a Cisco ASR 1000 Series
Router, consider the following guidelines:
• The default interface MTU size accommodates a 1500-byte packet, plus 22 additional bytes to cover
the following additional overhead:
– Layer 2 header—14 bytes
– Dot1q header—4 bytes
– CRC—4 bytes
• 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.

Cisco ASR 1000 Specifications

General

Operating System	Cisco IOS XE
Interfaces	10 Gigabit Ethernet
Power Supply	AC or DC
Number of Total Expansion Slots	Varies by model
Management Port	Console
Redundant Power Supply Supported	Yes
Width	17.5 inches
Memory	Varies by model
Storage	Varies by model
Dimensions (H x W x D)	Varies by model
Weight	Varies by model
Operating Temperature	32°F to 104°F (0°C to 40°C)
Humidity	5% to 95% non-condensing
MTBF	Varies by model
Redundancy	Redundant power supplies, fans, and route processors (depending on model)