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New Member

What is behavior of OSPF "default-information" ?

I want to clarify the behavior of OSPF “default-information”. I have 2 routes and 1 Firewall to exit to outside. They on the same segment.

.

R1

Interface e0

Ip add 1.1.1.1 255.255.255.0

!

Ospf process 100

Network 1.1.1.0 0.0.0.255 area 0

Default-information originator

!

ip route 0.0.0.0 0.0.0.0 1.1.1.100 ; 1.1.1.100=Firewall.

.

R2

Interface e0

Ip add 1.1.1.2 255.255.255.0

!

Ospf process 100

Network 1.1.1.0 0.0.0.255 area 0

.

the routing table on R1 will showing ,

S* 0.0.0.0/0 [1/0] via 1.1.1.100

.

what about in the R2 ?

O*E2 0.0.0.0/0 [110/1] via 1.1.1.1

or

O*E2 0.0.0.0/0 [110/1] via 1.1.1.100

.

Thanks,

10 REPLIES
Silver

Re: What is behavior of OSPF "default-information" ?

Looks to be working fine to me. Are there specific questions you had about default-info in OSPF. Basically as long as you have a 0/0 route your R1 will advertise this out. If your interface to your firewall goes down this route would not be advertised unless the "always" key word was added to the default-information cmd.

Hope this helps,

Don

New Member

Re: What is behavior of OSPF "default-information" ?

The history is I upgraded R1 and R2 from 12.1(5) to 12.2(12b). I don’t have previous routing information but currently R2’s routing table shows,

O*E2 0.0.0.0/0 [110/1] via 1.1.1.100

.

I think R2’s table should,

O*E2 0.0.0.0/0 [110/1] via 1.1.1.1

.

Thanks,

New Member

Re: What is behavior of OSPF "default-information" ?

It depends on how R2 is configured. If there is a default route already set to 1.1.1.1, then you should see:

S*E2 0.0.0.0/0 [1/1] via 1.1.1.1

If there is not default route defined on R2, then you should see since the default route was redistributed through OSPF:

O*E2 0.0.0.0/0 [110/1] via 1.1.1.100

New Member

Re: What is behavior of OSPF "default-information" ?

The default gateway information advertising by R1 so I think R2 will have next-hop 1.1.1.1 instead of 1.1.1.100. please explain me in detail if next-hop is 1.1.1.100

.

Thanks,

New Member

Re: What is behavior of OSPF "default-information" ?

From you configuration, R1 has a default route set "ip route 0.0.0.0 0.0.0.0 1.1.1.100" and in the OSPF AS, you have the "Default-information originate" command. That means that R1's OSPF process will send the route out with it's LSA's.

On R2, if there is a default route set (ip route 0.0.0.0 0.0.0.0 1.1.1.1), then this route will show up when you do a "show ip route" command. If there is no default route configured on R2, then it will enter the default route it learned from OSPF. The static default route will be entered into the route table (if there is one) because it has a lower administrative distance than an OSPF route.

I hope this helps you out.

New Member

Re: What is behavior of OSPF "default-information" ?

Okay, I just want to discuss based on my configuration(no assumption). R1, R2 and Firewall on the same segment.

- R1 e0=1.1.1.1

- R2 e0=1.1.1.2

- Firewall interface=1.1.1.100

.

R1 configured static and default-information.

R2 no default gateway configured so learning the gateway information from R1’s Ethernet interface(1.1.1.1).

.

R2 learning the gateway information from 1.1.1.1. Here I want to know the mechanism. Is R2 pointing the gateway 1.1.1.100 even it learned from 1.1.1.1 ?. is it cause of on the same segment ?

.

Thanks,

New Member

Re: What is behavior of OSPF "default-information" ?

What is showing up when you do a show ip route on R2. I have never tried to redistribute a default route that was on the same segment as the other OSPF Routers. Usually with OSPF, when a router redistributes a default route, it will advertise itself as the next hop, not the gateway configured in the configuration file. I would think that the gateway would be set to 1.1.1.1. I know I said 1.1.1.100, but I miss read the post, sorry. This is based on how OSPF works in general. Like I said, ususally each OSPF Router is on a different subnet, so I don't know how the route will show up since all three devices are in the same subnet.

New Member

Re: What is behavior of OSPF "default-information" ?

I could test the situation in the LAB.

R1=12.1(12).

R2=12.2(13)T

.

R1, configured static and default-information.

R2, learning from R1.

--> R2’s routing table.

O*E2 0.0.0.0/0 [110/1] via 1.1.1.1

.

R1, learning from R2.

R2, configured static and default-information.

--> R1’s routing table.

O*E2 0.0.0.0/0 [110/1] via 1.1.1.100

.

It looks IOS related. I cannot find any related information from Release Note. Please guide me my testing result. If this is true, where can I find the information.

New Member

Re: What is behavior of OSPF "default-information" ?

I don't really understand what you are asking. There are a few OSPF documents on default-information originate, depending on the senario. They can be found by doing a search on that command on the Cisco TAC website.

http://www.cisco.com/tac

This command is designed to be used on a Autonomous System Boundary Router. The ASBR is the path out of the AS, and they point to themselves as the path out of the AS. With the firewall being on the same subnet as both routers, I am not sure how it will show up in the route table.

New Member

Re: What is behavior of OSPF "default-information" ?

The command tells the OSPF process to redistribute the Default Route on that router into the OSPF Routing tables. This will send that route to other OSPF Routers.

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