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

Configuring the ATM SPAs
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 an ATM SPA, consider the following guidelines.
The Cisco IOS software supports several types of configurable MTU options at different levels of the protocol
stack. You should ensure that all MTU values are consistent to avoid unnecessary fragmentation of packets.
These MTU values are the following:
• Interface MTU—Configured on a per-interface basis and defines the maximum packet size (in bytes)
that is allowed for traffic received on the network. The ATM SPA checks traffic coming in from the
network and drops packets that are larger than this maximum value. Because different types of Layer 2
interfaces support different MTU values, choose a value that supports the maximum possible packet
size that is possible in your particular network topology.
• IP MTU—Configured on a per-interface or per-subinterface basis and determines the largest maximum
IP packet size (in bytes) that is allowed on the IP network without being fragmented. If an IP packet is
larger than the IP MTU value, the ATM SPA fragments it into smaller IP packets before forwarding it
on to the next hop.
• Multiprotocol Label Switching (MPLS) MTU—Configured on a per-interface or per-subinterface basis
and defines the MTU value for packets that are tagged with MPLS labels or tag headers. When an IP
packet that contains MPLS labels is larger than the MPLS MTU value, the ATM SPA fragments it into
smaller IP packets. When a non-IP packet that contains MPLS labels is larger than the MPLS MTU
value, the ATM SPA drops it.
All devices on a particular physical medium must have the same MPLS MTU value to allow proper MPLS
operation. Because MPLS labels are added on to the existing packet and increase the packet's size, choose
appropriate MTU values so as to avoid unnecessarily fragmenting MPLS-labeled packets.
If the IP MTU or MPLS MTU values are currently the same size as the interface MTU, changing the interface
MTU size also automatically sets the IP MTU or MPLS MTU values to the new value. Changing the interface
MTU value does not affect the IP MTU or MPLS MTU values if they are not currently set to the same size
as the interface MTU.
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, and each MPLS
label adds a 4-byte header. Consider the maximum possible encapsulations and labels that are to be used in
your network when choosing the MTU values.
The MTU values on the local ATM SPA interfaces must match the values being used in the ATM network
Tip
and remote ATM interface. Changing the MTU values on an ATM SPA does not reset the local interface,
but be aware that other platforms and ATM SPAs do reset the link when the MTU value changes. This
could cause a momentary interruption in service, so we recommend changing the MTU value only when
the interface is not being used.
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Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide, Cisco IOS XE
Modifying the Interface MTU Size
Everest 16.5
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