From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Fri Jul 09 2004 - 16:53:42 GMT-3
Tim,
The problem occurs when the tunnel destination is learned via a
physical interface, the tunnel comes up, and then the tunnel destination
is learned via the tunnel with a preferred metric or distance. The
following configuration demonstrates the problem between two connected
routers:
R1:
Interface ethernet0/0
Ip address 12.0.0.1 255.0.0.0
!
Interface loopback0
Ip address 1.1.1.1 255.255.255.255
!
Interface tunnel 0
Ip address 21.0.0.1 255.0.0.0
Tunnel source loopback0
Tunnel destination 2.2.2.2
!
Router rip
Version 2
No auto-summary
Network 12.0.0.0
Network 1.0.0.0
!
Router eigrp 1
No auto-summary
Network 21.0.0.0
Network 1.0.0.0
R2:
Interface ethernet0/0
Ip address 12.0.0.2 255.0.0.0
!
Interface loopback0
Ip address 2.2.2.2 255.255.255.255
!
Interface tunnel 0
Ip address 21.0.0.2 255.0.0.0
Tunnel source loopback0
Tunnel destination 2.2.2.2
!
Router rip
Version 2
No auto-summary
Network 12.0.0.0
Network 1.0.0.0
!
Router eigrp 1
No auto-summary
Network 21.0.0.0
Network 1.0.0.0
Try that out and see what happens.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
> -----Original Message-----
> From: ccie2be [mailto:ccie2be@nyc.rr.com]
> Sent: Friday, July 09, 2004 9:15 AM
> To: Brian McGahan
> Subject: Re: How Tunnels and Recursive Routing Occur
>
> Thanks Brian,
>
> But, I'm still wondering under what circumstances this recursive
problem
> occur. For example, if the tunnel is running one routing protocol and
the
> physical interface is running another - is this when this problem
occurs
> potentially?
>
> Tim
> ----- Original Message -----
> From: "Brian McGahan" <bmcgahan@internetworkexpert.com>
> To: "ccie2be" <ccie2be@nyc.rr.com>; "Kenneth Wygand"
> <KWygand@customonline.com>; "Ty" <tycampbell@comcast.net>;
> <ccielab@groupstudy.com>
> Sent: Friday, July 09, 2004 9:53 AM
> Subject: RE: How Tunnels and Recursive Routing Occur
>
>
> Tim,
>
> You will know if there is a recursive error because you get the
> %TUN-5-RECURDOWN message logged. If there is a recursive routing
error
> that is not directly related to the tunnel you will see it in the
"debug
> ip routing" output.
>
> HTH,
>
> Brian McGahan, CCIE #8593
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987 x 705
> Outside US: 775-826-4344 x 705
>
>
> > -----Original Message-----
> > From: ccie2be [mailto:ccie2be@nyc.rr.com]
> > Sent: Friday, July 09, 2004 4:57 AM
> > To: Kenneth Wygand; Ty; Scott Morris; Brian McGahan;
> > ccielab@groupstudy.com
> > Subject: Re: How Tunnels and Recursive Routing Occur
> >
> > Hi Ken,
> >
> > Thanks for taking the time to do that write up. It was excellent.
> >
> > As I said, I understand the theory, but I'm still not 100% positive
> how I
> > would recognize a potential problem.
> >
> > Would it be possible to elaborate on your example to highlight how
> your
> > example creates a recursive routing problem?
> >
> > Thanks, Tim
> >
> > ----- Original Message -----
> > From: "Kenneth Wygand" <KWygand@customonline.com>
> > To: "Ty" <tycampbell@comcast.net>; "Scott Morris" <swm@emanon.com>;
> "Brian
> > McGahan" <bmcgahan@internetworkexpert.com>;
<ccielab@groupstudy.com>;
> > <ccie2be@nyc.rr.com>
> > Sent: Thursday, July 08, 2004 8:14 PM
> > Subject: How Tunnels and Recursive Routing Occur
> >
> >
> > Tim,
> >
> > In reply to:
> >
> > <snip>
> > PS: Have you been following the thread re: tunnels & recursive
> routing?
> > If
> > so, maybe you could give me an example of this recursive problem so
I
> can
> > recongize it and know what to look out for. I understand it
> conceptually,
> > but can't figure out how the tunnel & ip addressing would be
> configured to
> > cause this problem, so an example would be really helpful.
> > </snip>
> >
> > The theory, which I think you already understand, is as follows:
> >
> > 1) You live next to a park
> > 2) I tell you to get to the supermarket, you must go through the
park.
> > 3) Your friend tells you he lives a block past the store, so you
know
> how
> > to
> > get there.
> > 4) If anyone tells you where they are relative to the store, you can
> > figure
> > out where they are by a recursive lookup (If John lives next to the
> store,
> > I
> > know I first need to go to the store and then I take the last hop to
> > John's
> > house. Hmm... but how do I get to the store, let me look that up -
oh,
> > yes,
> > to get to the store, I must go through the park, so now I know I
must
> go
> > through the park to get to the store and then I take the last hop to
> > John's
> > house)
> > 5) The only place I can't get to relative to the store is the store
> > itself.
> > If I ask the store how to get to itself, it will say "just go to the
> store
> > and the store is right there". Now I will say "Oh, ok, well it's
> shorter
> > to
> > go to the store through my direct knowledge of where the store is -
my
> > other
> > option was to go through the park to get to the store, but its
shorter
> for
> > me to just take the single-hop route to the store via the store".
> > 6) If I make this change, I know I get to the store through the
store,
> and
> > I
> > lost the information about having to pass through the park because I
> > interpreted that path as a suboptimal path and flushed it from my
> routing
> > table. So now I say "Ok, I want to get to John's house, so first I
> have to
> > go to the store. OK, well to get to the store, I must go to the
store
> > first.
> > OK, well to get to the store, I must go to the store first. OK, well
> to
> > get
> > to the store, I must go to the store first. OK, well to..... you
get
> the
> > idea!
> >
> > Although this sounds complicated and confusing, it's really a simple
> > concept. You cannot get to the destination of the tunnel through the
> > tunnel
> > itself. This is why you have certain things when it comes to a
> tunnel.
> >
> > 1) You have IP addresses on each end of the tunnel
> > 2) You have source/destination IP addresses (inverted on each side)
to
> > create the tunnel.
> >
> > You simply must make sure you do not LEARN the destination IP
address
> of
> > the
> > tunnel THROUGH the tunnel. This can be achieve by either filtering
the
> > advertisement of the destination interface from any routing
protocols
> > running over the tunnel. Remember, the tunnel interface (tunnel IP
> address
> > now), looks like a directly connected interface. If you learn routes
> over
> > this interface, most likely they will natively be more preferred
than
> the
> > physical path required to get to the same spot.
> >
> > Here is an example:
> >
> > (4th octet of IP addresses is router number)
> > Loopback of 1.1.1.1/24 on R1, 2.2.2.2/24 on R2 and 3.3.3.3/24 on R3.
> > R1----12.12.12.x----R2----23.23.23.x----R3
> >
> > I set up a tunnel from R1 (12.12.12.1) to R3 (23.23.23.3). I create
> > tunnel0
> > on R1 with IP address 13.13.13.1 and tunnel0 on R3 with IP address
> > 13.13.13.3.
> >
> > Requirement 1: Ensure R1 does not learn of 23.23.23.3 via Tunnel0 !
> > Requirement 2: Ensure R3 does not learn of 12.12.12.1 via Tunnel0 !
> >
> > That's all there is to it!
> >
> > Make a little more sense now? I hope I didn't confuse anyone any
> more!
> > Ken
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