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Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide
OL-14127-08
Chapter 7 Configuring the ATM SPAs
Configuration Tasks
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.
Tip The MTU values on the local ATM SPA interfaces must match the values being used in the ATM network
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.
Note The interface MTU value on the ATM SPA also determines which packets are recorded as “giants” in the
show interfaces atm command. The interface considers a packet to be a giant packet when it is more
than 24 bytes larger than the interface MTU size. For example, if using an MTU size of 1500 bytes, the
interface increments the giants counter when it receives a packet larger than 1524 bytes.
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