From: Michael Zuo (mzuo@ixiacom.com)
Date: Wed Nov 08 2006 - 02:27:47 ART
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
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> -----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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