Path Mtu Discovery; Licensing Requirements For Ip Tunnels - Cisco 7206 - VXR Router Configuration Manual

Chapter 8 Configuring IP Tunnels
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m

Path MTU Discovery

Path maximum transmission unit (MTU) discovery (PMTUD) prevents fragmentation in the path
between two endpoints by dynamically determining the lowest MTU along the path from the packet's
source to its destination. PMTUD reduces the send MTU value for the connection if the interface
receives information that the packet would require fragmentation.
When you enable PMTUD, the interface sets the Don't Fragment (DF) bit on all packets that traverse the
tunnel. If a packet that enters the tunnel encounters a link with a smaller MTU than the MTU value for
the packet, the remote link drops the packet and sends an ICMP message back to the sender of the packet.
This message indicates that fragmentation was required (but not permitted) and provides the MTU of the
link that dropped the packet.
Note PMTUD on a tunnel interface requires that the tunnel endpoint can receive ICMP messages generated
by devices in the path of the tunnel. Check that ICMP messages can be received before using PMTUD
over firewall connections.
Virtualization Support
You can configure IP tunnels only in the default virtual device context (VDC) and the default Virtual
Routing and Forwarding (VRF) instance.
Beginning with Cisco NX-OS Release 4.2, you can configure a tunnel interface as a member of a Virtual
Routing and Forwarding (VRF) instance and as a member of any VDC. By default, Cisco NX-OS places
you in the default VDC and default VRF unless you specifically configure another VDC and VRF. A
tunnel configured in one VDC is isolated from a tunnel with the same number configured in another
VDC. For example, Tunnel 0 in VDC 1 is independent of tunnel 0 in VDC 2.
Your tunnel source IP address and destination IP address should be in the same VRF. You can also
configure what VRF to use to look up the tunnel destination. This VRF should match the VRF of the
tunnel source IP address.
See the Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 5.x, for
information about VDCs and see the Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration
Guide, Release 5.x, for information about VRFs.
High Availability
IP tunnels support stateful restarts. A stateful restart occurs on a supervisor switchover. After the
switchover, Cisco NX-OS applies the runtime configuration after the switchover.

Licensing Requirements for IP Tunnels

The following table shows the licensing requirements for this feature:
Product License Requirement
Cisco NX-OS IP tunnels require an Enterprise Services license. For a complete explanation of the Cisco NX-OS licensing
scheme and how to obtain and apply licenses, see the Cisco NX-OS Licensing Guide.
OL-23435-03
Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x
Licensing Requirements for IP Tunnels
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