From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Wed Nov 08 2006 - 11:14:08 ART
The stipulation is that the tunnel destination cannot be learned
through the tunnel. Also for the subnet mismatch problem why not just
put a new IP subnet on the tunnel instead of unnumbering it to another
interface?
Brian McGahan, CCIE #8593 (R&S/SP)
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
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> -----Original Message-----
> From: Michael Zuo [mailto:mzuo@ixiacom.com]
> Sent: Tuesday, November 07, 2006 11:28 PM
> To: Brian McGahan; ccielab@groupstudy.com
> Subject: RE: confused about tunneling
>
> Thanks Brian, that was very helpful.
>
> For item 1, there are two problems with establishing tunnels between
two
> OSPF routers in different areas (say loopback 0 on each) that I ran
> into:
>
> The first one is that both loopbacks have to be in area 0. if the
area
> numbers are different, I would get this periodically message saying
area
> ID mismatch (make sense). If they are both in the same non-zero areas
> and the tunnel goes thru area 0, the tunnel would be shut down
> automatically because of recursive routing. Is this expected? (of
> course passive interface is also a workaround to advertise the
loopbacks
> without the area mismatch error message)
>
> The second issue is that loopback IPs from the two routers are most
> likely not on the same subnet. So I would have a tunnel with IPs from
> different subnet on both ends. I tried this setup and the policy
> routing does not seem to mind and works just fine. Is having IPs from
> different subnets on two ends of a point-to-point link such as a
tunnel
> an acceptable practice (in lab and real world environment)?
>
> I need some expert advice :)
>
>
> Thanks a lot...
>
> -----Original Message-----
> From: Brian McGahan [mailto:bmcgahan@internetworkexpert.com]
> Sent: Tuesday, November 07, 2006 5:15 PM
> To: Michael Zuo; ccielab@groupstudy.com
> Subject: RE: confused about tunneling
>
> > 1. can unnumbered tunnel interfaces be advertised into OSPF (does
> > this happen automatically if the original interface is in OSPF
> already)?
>
> Look at the "show ip ospf interface" output. Unnumbered links
> share the same OSPF area as the numbered link.
>
> > 2. what happens when packet comes out of a tunnel, how are they
> > routed? Exactly like any other packets?
>
> They are routed just like any normal IP packet. Route recursion
> out the tunnel interface recurses back to the outgoing interface for
the
> tunnel destination. I.e if we have the configuration:
>
> interface tunnel0
> ip address 10.0.0.1 255.0.0.0
> tunnel source loopback0
> tunnel destination 20.0.0.2
> !
> ip route 30.0.0.0 255.0.0.0 tunnel0
> ip route 20.0.0.2 255.255.255.255 serial0
>
> Traffic to the 30.0.0.0 network will be sent out serial0 with a
> GRE destination of 20.0.0.2.
>
> > 3. what happens when tunnel configuration is done only on one side?
> > The interface comes up, but does it do anything?
>
> No it doesn't do anything with just a one sided tunnel
> configuration. A tunnel will always be up unless keepalives are
enabled
> and the other end is down.
>
> > 4. if I use policy routing, the set statement would be "set
> > interface tunnel 0" or do I need to set next hop as well?
>
> GRE tunnels are treated like any other point-to-point link so
> you don't necessarily need to set the next hop. All traffic sent to
the
> tunnel is always encapsulated towards the tunnel destination so routes
> can point directly to the tunnel without a need for additional layer 3
> to layer 2 resolution.
>
> HTH,
>
> Brian McGahan, CCIE #8593 (R&S/SP)
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987 x 705
> Outside US: 775-826-4344 x 705
> 24/7 Support: http://forum.internetworkexpert.com
> Live Chat: http://www.internetworkexpert.com/chat/
>
>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > Michael Zuo
> > Sent: Tuesday, November 07, 2006 6:55 PM
> > To: ccielab@groupstudy.com
> > Subject: confused about tunneling
> >
> > Hi Group,
> >
> >
> >
> > Anyone knows a good link from doc CD that explains the in and outs
of
> > GRE tunneling?
> >
> >
> >
> > I like to become more knowledgeable on how exactly tunneling work
with
> > IP routing. Specifically:
> >
> >
> >
> > 1. can unnumbered tunnel interfaces be advertised into OSPF (does
> > this happen automatically if the original interface is in OSPF
> already)?
> > 2. what happens when packet comes out of a tunnel, how are they
> > routed? Exactly like any other packets?
> > 3. what happens when tunnel configuration is done only on one side?
> > The interface comes up, but does it do anything?
> > 4. if I use policy routing, the set statement would be "set
> > interface tunnel 0" or do I need to set next hop as well?
> >
> >
> >
> >
> >
> > Thanks a bunch....!
> >
> >
>
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