Home > Technical > Cisco > Cisco – GRE Tunnel Keepalives

Cisco – GRE Tunnel Keepalives

The GRE Tunnel Keepalives implemnetation is quite ingenious. The router sending the keepalive packet sends a GRE packet that would be sent back to it from the remote router. It’s effectively building its own GRE Reply. It sets the GRE protocol type to zero to indicate it’s a keepalive packet and sends the packet through the GRE tunnel. The receiving router strips the GRE envelope and routes the inside packet, which is already a properly formatted GRE keepalive reply.

Excerpt from Cisco: The GRE tunnel keepalive mechanism is slightly different than for Ethernet or serial interfaces. It gives the ability for one side to originate and receive keepalive packets to and from a remote router even if the remote router does not support GRE keepalives. Since GRE is a packet tunneling mechanism for tunneling IP inside IP, a GRE IP tunnel packet can be built inside another GRE IP tunnel packet. For GRE keepalives, the sender pre-builds the keepalive response packet inside the original keepalive request packet so that the remote end only needs to do standard GRE decapsulation of the outer GRE IP header and then forward the inner IP GRE packet. This mechanism causes the keepalive response to forward out the physical interface rather than the tunnel interface. This means that the GRE keepalive response packet is not affected by any output features on the tunnel interface, such as ‘tunnel protection …’, QoS, and so forth. ).

Here is the official Cisco GRE Tunnel Keepalives on Cisco the website: http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a008048cffc.shtml#backinfo

Copyright © 2012-2017 Yared Consulting Inc. All Rights Reserved. | Privacy Policy | Terms & Conditions