Default Route misconfiguration question

ramrunner800

Hey all! I am completely stumped with this scenario. Imagine that I am running a network of several routers with OSPF as my routing protocol. Router1 has two serial interfaces, s0/0 goes out to the ISP, while s0/1 is running OSPF and connects to the rest of my network. I need to configure a default route for my network to access the internet. If I misconfigure only that route, and enable it on S0/1 instead of S0/0 (ip route 0.0.0.0 0.0.0.0 s0/1) , what will the effect be on my network?

As R1 is advertising to the rest of my network that it knows how to get to any unknown IP address, they will forward the traffic to R1, correct? And it will then send it back out s0/1, to a router that's only known route to the unknown IP address is through R1. So I will have created a loop? Or does OSPF prevent this from happening? I’ve been trying to lab this out in Boson NetSim, but apparently default-information originate is a command that is only supported in the CCNP version of the software, not the CCNA.

TechGuru80

The packet should be dropped. You are saying send anything unknown 0.0.0.0 0.0.0.0 to that location however it will not have that in its routing table.

ramrunner800

Why will that route not be in the routing table? I thought that if I used default-information to inject a default route all the routers in the area would have the default route as part of their routing tables. And that R1 automatically forwards anything unknown out s0/1 without thinking.

networker050184

I was curious about this so I labbed it up real quick. What happens is the router directly connected to the one with the incorrect default route will not install it into it's routing table because the forwarding address is itself. So, the rest of the routers will install the default route, point it to the penultimate router but the traffic will be dropped there.

My topology:

R1 <10.12.12.0/24> R2 <10.23.23.0/24> R3 (Each router has it's own router number as the last octet. So on the R2<->R3 link R2 is 10.23.23.2 etc.)

R3 generates the default route. If the default route is set correctly everyone enters it in their routing table and all is good.

If I configure the default route on R3 pointing back to R2 only R1 will put the default route in it's routing table due to the below.

R2#show ip o d e


            OSPF Router with ID (2.2.2.2) (Process ID 1)


        Type-5 AS External Link States


  LS age: 665
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0xACBF
  Length: 36
  Network Mask: /0
    Metric Type: 2 (Larger than any link state path)
    MTID: 0 
    Metric: 1 
    Forward Address: 10.23.23.2
    External Route Tag: 1

So as you can see R2 is looking at this LSA and sees the forwarding address set to it's own interface and then does not install it into the routing table.

Debug output from R2.

*Nov  8 12:37:07.719: OSPF: Start partial processing Type 5 External LSA 0.0.0.0, mask 0.0.0.0, adv 3.3.3.3, age 1, seq 0x80000003, metric 1, metric-type 2, fw-addr 10.23.23.2
*Nov  8 12:37:07.723: OSPF:   more specific non-OSPF route to forwarding address

ramrunner800

Okay, so even though we configured the default route as using an exit interface instead of a next hop address, the router at the next hop can tell that it is the intended destination for this route?

networker050184

One thing I just noticed. If you set it to the outgoing interface instead of the directly connected next-hop IP the forwarding address is set to 0.0.0.0 and it will then create the loop. So it really comes down to how you configure it!


Edit: Was just posting the answer to your question. Great minds think alike as they say.

ramrunner800

Thank you so much for helping me figure this one out. I don't know why Boson doesn't allow the use of default-information originate at the ccna level, it seems that's a necessary part of configuring OSPF.

TechGuru80

Interesting with the interface.

Earlier I was referring to the intended route so google.com or whatever ip.