From: Paul Korkowski (korkc130@xxxxxxxxxxx)
Date: Fri Jun 01 2001 - 11:05:03 GMT-3
The problem of a recursive routing loop occurs when you have a
dynamic routing protocol (RIP, EIGRP, OSPF, etc) running through the tunnel.
The tunnel then sees it's destination through itself.
The way it was explained to me by a Cisco CCIE was that the tunnel
engine sees IP destined for the other end ("interesting" traffic for the
tunnel), then encapsulates it into an IP packet for the other end of the
tunnel, with the tunnel source/destination on the header of the packet.
After the "tunnel engine" of the router finishes the encapsulation of the
payload, it sends it back to the dynamic routing protocol if there is one
(in this case EIGRP). The routing protocol then determines that the best
route to the tunnel endpoint is through the tunnel, then sends the packet
back to the tunnel engine to encapsulate to send through the tunnel. This
loop only happens if the transport protocol (IP) and the tunnel protocol
(IP) are the same with a dynamic routing protocol.
There are 2 ways that I know of to solve this. First, you can do a
static route with a lower administrative distance for the tunnel's endpoint
so that the tunnel does not "feed on itself". Or, you can do policy
routing, changing the administrative distance or source interface that
traffic is meant to go out of. Hope this helps.
Paul Korkowski, CCDP, CCNP
Up-n-Running Consulting, Inc.
paulk@up-n-running.com
(507) 454-7786 Winona / (608) 786-1555 La Crosse
(888) 735-7486 Toll Free / (507) 457-9715 FAX
www.up-n-running.com
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