I'm hoping to give a good answer to this to end the debate.
The answer is B.
Some have brought up the fact that the default route will have the preferred AD of 1.
While this is true, it is forgetting the path selection order:
1: Prefer the most specific route - Longest Match
2: Prefer the lowest Administrative Distance - Most Trusted Routing Protocol
3: Prefer the lowest Metric - Shortest Calculated Distance/Cost
In other words, while the default route is going to have an AD of 1, the more specific destination address is going to be learned from OSPF through the Tunnel. The Source Router is going to learn the more specific route and prefer it over the default route, causing the recursive routing error.
As long as the GRE tunnel is valid it does not matter what internet is running as the GRE tunnel will be transporting the multicast traffic of OSPF if configured correctly. (which I believe is the case?)
Correct answer is B. Tested in CML.
Tunnel destination will be known via tunnel interface.
Look at network command in router ospf configuration and their wildcard. It include also default route.
*Dec 16 15:19:48.624: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel100, changed state to up
*Dec 16 15:19:49.576: %OSPF-5-ADJCHG: Process 100, Nbr 209.165.201.2 on Tunnel100 from LOADING to FULL, Loading Done
Router(config)#
Router(config)#exit
Router#
*Dec 16 15:19:54.146: %ADJ-5-PARENT: Midchain parent maintenance for IP midchain out of Tunnel100 - looped chain attempting to stacksh i
*Dec 16 15:19:55.527: %SYS-5-CONFIG_I: Configured from console by console
*Dec 16 15:19:58.624: %TUN-5-RECURDOWN: Tunnel100 temporarily disabled due to recursive routing
*Dec 16 15:19:58.624: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel100, changed state to down
*Dec 16 15:19:58.624: %OSPF-5-ADJCHG: Process 100, Nbr 209.165.201.2 on Tunnel100 from FULL to DOWN, Neighbor Down: Interface down or detached
this makes sense. Initially the tunnel is up. After the tunnel is up, the tunnel interface is included into OSPF. The router learns all interface networks from the other router, which also include the tunnel destination address. Then causes a loop, and then bring down the tunnel
OSPF is enabled on all active interfaces (with the “network 0.0.0.0 255.255.255.255 area 0” command).
So on SiteB the tunnel interface (192.168.100.2) is also running OSPF. When SiteA receives the OSPF advertisement from SiteB, it realizes it can reach the other side of the tunnel via OSPF (and it would not use the default route any more).
In other words, it reaches the tunnel destination through the tunnel itself.
Another attribute of GRE tunnel keepalives is that the keepalive timers on each side are independent and do not have to match, similar to PPP keepalives. (ref: https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/118370-technote-gre-00.html#:~:text=Another%20attribute%20of%20GRE%20tunnel,match%2C%20similar%20to%20PPP%20keepalives.) A is not true. when you configure in ospf net 0.0.0.0 255.255.255.255 means all interfaces (incluides ligic interfaces) so the network of interface tunnel will be pushib via the same tunnel. ANS: B
Another attribute of GRE tunnel keepalives is that the keepalive timers on each side are independent and do not have to match, similar to PPP keepalives. (ref: https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/118370-technote-gre-00.html#:~:text=Another%20attribute%20of%20GRE%20tunnel,match%2C%20similar%20to%20PPP%20keepalives.) A is not true. ANS: B
Labed it up
*Jun 7 16:19:04.544: %TUN-5-RECURDOWN: Tunnel100 temporarily disabled due to recursive routing
*Jun 7 16:19:04.545: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel100, changed state to down
The network 0.0.0.0 255.255.255.255 area in the section ospf section is going to introduce a recursive looping.
I did a simulation with the given configuration. The interface tunnel ip address 192,168.100.2 was automatically disable to prevent a looping. Hence, it route from 10.1.1.0 to 10,2,2,0 does not work.
as soon as tunnel will be up, the ospf process will run between the two router. This is per configuration all network participate in OSPF. Since they will exchange the route, the internet subnet will be exchanged and learned through the tunnel.
Answer is B. To stop the flapping, we can use route filtering, increasing the ospf cost on the tunnel interface, or make the tunnel an ospf passive interface.
But, they are default routes, any other routes learned by OSPF as 209.165.200.226/30 and 209.155.201.2/30 are more specific than:
ip route 0.0.0.0 0.0.0.0 209.165.200.225
ip route 0.0.0.0 0.0.0.0 209.165.201.1
About the first comment, I understand the OSPF is propagating encapsulated inside the tunnel
It's complicated to sure but I select B
Since when does OSPF run freely on the internet? You know the answer to that I pressume.. But from a purely networking perspective, all of this is valid.
OSPF has precedence over static routes in case of a more-specific match (or longer match), or in case the distance of the static route was adjusted - you need to think outside of the box..
It will work temporarily until the tunnel destination is known via the tunnel itself as it will be advertised via OSPF once the tunnel is up, and you end up with recursive routing = bad. Correct answer = B
C. Verifying the scenario The correct answer is the provided
A. No matter if the tunnnel keep aliver are differente the tunnel will be up (Wrong)
B. (MOS TRICKY ONE) even the ospf process are active for all interface including the tunnel, remember that the outgoing interface is on the internet (not running OSPF or maybe BGP) and also a static route pointing to the public is configured to take precedence in case of you are configured internet with BGP. So the tunnel destination wont be learned by the tunnel thus(WRONG)
C. is correct beacuse any configuration show will cause problem to the tunnel (CORRECT)
D. The default MTU (1476) was not changed on the configuration so (WRONG)
You are presented with a full configuration that is relevant to the question. If you see OSPF enabled on an interface with public IP address, that is correct and still the same principles apply. Just because it probably wouldn't be configured this way ever in real life, it does test your understanding of the technology. B is correct - play it through in your head or test it in a lab.
You are very wrong.
The best part here is that the 'internet' doesn't even need OSPF. The learning of the networks representing the ends of the GRE tunnel via the tunnel interfaces themselves, is not done by OSPF between the sites and the internet; it is done by OSPF between the sites directly via the GRE tunnel.
To verify this, just make a lab and configure static routing all across the 'internet'. Then, on the sites routers, just add all interfaces to ospf and see what happens
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