Table Of Contents
Distributed GRE Tunneling Support
This feature module describes the Distributed GRE Tunneling Support feature in Cisco IOS Release 12.2(14)S.
History for the Distributed GRE Tunneling Support Feature
Finding Support Information for Platforms and Cisco IOS Software Images
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Prerequisites for Distributed GRE Tunneling Support
dCEF must be explicitly enabled on the router.
Restrictions for Distributed GRE Tunneling Support
•dCEF must be enabled before generic routing encapsulation (GRE) tunneling.
•dCEF and CEF GRE tunnels do not support packet sequencing or check summing as defined in RFC 1721.
•At the tunnel exit point, dCEF and CEF GRE tunnels do not support reassembly of fragmented packets.
Information About Distributed GRE Tunneling Support
To implement distributed GRE tunneling support, you must understand the following concept:
Distributed GRE Tunnels
Distributed GRE Tunneling allows Cisco IOS software to switch packets into and out of the GRE tunnels using distributed Cisco Express Forwarding (dCEF). The tunneling is performed using recursive or "double" switching techniques that are currently deployed on existing nondistributed platforms. The relevant bits are ported into this development.
Double switching is performed by the handling of the received IP packet in the existing code path until it is determined that the packet needs encapsulation or de-encapsulation. Recursively forwarding the IP packet through the IP switching path again explains the "double" aspect of the switching.
The GRE tunneling allows service providers to support a large number of tunnels by forwarding distributed tunneled packets. This feature is an extension of the nondistributed forwarding information base (FIB) forwarding paths.
How to Configure Distributed GRE Tunneling Support
There are no tasks for the Distributed GRE Tunneling Support feature.
Note See the "Configuring a GRE Tunnel" section of the "Implementing Tunnels" chapter in the Cisco IOS Interface and Hardware Component Configuration Guide, Release 12.4.
Configuration Examples for Distributed GRE Tunneling Support
There are no configuration examples for the Distributed GRE Tunneling Support feature.
Note See the "Configuration Examples for Implementing Tunnels" section of the "Implementing Tunnels" chapter in the Cisco IOS Interface and Hardware Component Configuration Guide, Release 12.4.
The following sections provide references related to Distributed GRE Tunneling Support.
Related Topic Document Title
dCEF configuration tasks
"Configuring Cisco Express Forwarding" chapter in the Cisco IOS Switching Services Configuration Guide, Release 12.2
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
MIBs MIBs Link
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs 1 Title
Generic Routing Encapsulation (GRE)
Generic Routing Encapsulation over IPv4 Networks
1 Not all supported RFCs are listed.
CEF—A mode of packet switching in Cisco IOS Software. Also known as FIB-switching.
Note See the Internetworking Terms and Acronyms for terms not included in this glossary.
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