Recursive Routing and Tunnels

From: ALL From_NJ (all.from.nj@gmail.com)
Date: Sun Feb 22 2009 - 02:35:05 ARST

Interesting lab testing tonight, was playing w/ tunnel interfaces and OSPF.

I had a scenario I was practicing, basically it was:

NBMA w/ network type of non-broadcast. Can not use neighbor command or make
any config changes to the physical interface on either sides.

So I created a tunnel and added it to OSPF.

As you would expect, recursive routing began to occur.

Here is a nifty debug output which quickly showed why the neighbor bounces:

R2#debu ip ospf adj
OSPF adjacency events debugging is on

*Mar 1 04:34:36.531: %TUN-5-RECURDOWN: Tunnel1 temporarily disabled due to
recursive routing
*Mar 1 04:34:37.531: %LINEPROTO-5-UPDOWN: Line protocol on Interface
Tunnel1, changed state to down
*Mar 1 04:34:37.531: OSPF: Interface Tunnel1 going Down

I know one answer to solve this is to simply remove the physical link's
network address from OSPF. Odd learning though, as I tried to configure
other method of filters.

OSPF would not take a distribute list OUT for the tunnel interface ...
figured I would simply keep the local physical link's network from being
advertised out via the tunnel. No-go on this.

OSPF did take a Distribute list IN via the tunnel. I denied the local
link's network ... and all was well in la-la land (where I live).

R1(config-router)#distribute-list 12 in tunnel 1

Would have been easier and quicker for me to simply remove the physical
link's network from OSPF, but I was / am learning ...

Also, ... I actually prefer a distribute list IN, since if I am able to keep
the physical link's network in OSPF on my hub router, other peers will still
learn about it. I am only filtering via these two routers, and all other
routers still learn about the local link's network.

Question for the group:

For recursive learning, any additional thoughts on ways for me to learn this
/ explore this / solve this?

I suppose in all of life, the kiss method is the best ... Just figured I
would ping yall to see what insights I might learn.

Many TIA,

Andrew Lee Lissitz 

-- 
Andrew Lee Lissitz
all.from.nj@gmail.com

Blogs and organic groups at http://www.ccie.net

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