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Beginner

Default Originate Command Question

My instructor explained the attached question, but I'm still having difficulty understanding the question. I'm not understanding why R2 has trouble reaching the DNS server on the Internet if, when it forwards the packet to R1, R1 has a route to the Internet.

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Hi @ajc0509 

 

Without the default static route, routers will not have a route to send packets that have an unknown destination IP(internet).
All the routers in your LAN, which have devices that must have internet access, must have a static default route.

This static default route can be configured manually on each router, but it is more efficient to configure it on the edge router and allow the dynamic protocol to propagate this route to the other routers.

 

In your case, without the default static route, no device on your LAN could go out to the internet, since the routers will discard those packets as they do not have an outgoing route to unknown destination IPs.

 

For example, if a packet originating from SiteB wants to go to the internet (unknown destination IP), it will first reach R2, but this router will discard the packet, since it does not have a route to the destination IP.

 

Regards

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VIP Advisor

Hi @ajc0509 

 

The correct answer is the first.
If you look at the output of the show ip route command on R1, you will notice that the default route is not configured on that router.
Once the default static route is configured on R1, this router will propagate that route to R2, using OSPF updates.

 

Regards

 

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Thanks for replying, Luis. I understand how the default originate command works, just not why it's needed if R1 can send R2's packet to the Internet.

Highlighted

Hi @ajc0509 

 

Without the default static route, routers will not have a route to send packets that have an unknown destination IP(internet).
All the routers in your LAN, which have devices that must have internet access, must have a static default route.

This static default route can be configured manually on each router, but it is more efficient to configure it on the edge router and allow the dynamic protocol to propagate this route to the other routers.

 

In your case, without the default static route, no device on your LAN could go out to the internet, since the routers will discard those packets as they do not have an outgoing route to unknown destination IPs.

 

For example, if a packet originating from SiteB wants to go to the internet (unknown destination IP), it will first reach R2, but this router will discard the packet, since it does not have a route to the destination IP.

 

Regards

View solution in original post

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Thanks so much, @luis_cordova! Reading your explanation finally helped me understand where I went wrong. The packet arrives at the first router, which has no static or default route to the Internet. Therefore, it can’t even send the packet to the next router. Routing 101; how could I forget that?! I think my brain is so full with Cisco topics that I made it much harder than it was. I appreciate you, @luis_cordova!

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