OSPF defualt information originate

Bosefus · October 2009

If I have multiple routers in the same area configured with default information originate, how is it choose which default route will be installed into everyones routing table?

networker050184 · October 2009

How will the routers choose any route to put in the routing table?

Bosefus · October 2009

If the route is to the same place.

Longest Mask
AD
Metric

What will happen if all of this information is the same?

Can you load balance across a default route?

ColbyG · October 2009

You passed BSCI without knowing this stuff?

You could have multiple default routes advertised in a network, but OSPF will install the route with the lowest cost into the routing table.

Bosefus · October 2009

ColbyNA wrote: »

You passed BSCI without knowing this stuff?

You could have multiple default routes advertised in a network, but OSPF will install the route with the lowest cost into the routing table.

I scored an 85% on the ospf stuff, not 100%, no need to be rude.

I realize with regular routes OSPF will load balance across equal cost paths (by default up to four I believe max 16) but I am unsure if it would load balance across equal cost default routes that are advertised out.

ColbyG · October 2009

Wasn't trying to be rude, it was a serious question.

Yes, OSPF will install equal routes, but your question is more than that. Having multiple routers advertising defaults could have different costs from each router. But the original question is pretty basic. OSPF will choose the default route(s) to put in the table based on its criteria (cost).

Bosefus · October 2009

I labbed it out, check out my network diagram.

Below is the the output when the R0 and R1 have default routes pointing to their Ethernet interface going to the switch. R2 has no default route configured.

Below is the routing table of R2 when R0 has default information originate:

Gateway of last resort is 50.50.50.1 to network 0.0.0.0

     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/11] via 50.50.50.2, 00:11:35, Ethernet1/2
     50.0.0.0/24 is subnetted, 1 subnets
C       50.50.50.0 is directly connected, Ethernet1/2
     32.0.0.0/30 is subnetted, 1 subnets
C       32.32.32.0 is directly connected, Serial0/2
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback3
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/11] via 50.50.50.1, 00:11:35, Ethernet1/2
     25.0.0.0/30 is subnetted, 1 subnets
C       25.25.25.0 is directly connected, Serial0/1
     10.0.0.0/30 is subnetted, 1 subnets
O       10.10.10.0 [110/74] via 50.50.50.2, 00:11:36, Ethernet1/2
                   [110/74] via 50.50.50.1, 00:11:36, Ethernet1/2
O*E2 0.0.0.0/0 [110/1] via 50.50.50.1, 00:00:03, Ethernet1/2

Here is R2s routing table after R1 and R0 are both using default information originate.

Gateway of last resort is 50.50.50.2 to network 0.0.0.0

     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/11] via 50.50.50.2, 00:00:00, Ethernet1/2
     50.0.0.0/24 is subnetted, 1 subnets
C       50.50.50.0 is directly connected, Ethernet1/2
     32.0.0.0/30 is subnetted, 1 subnets
C       32.32.32.0 is directly connected, Serial0/2
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback3
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/11] via 50.50.50.1, 00:00:00, Ethernet1/2
     25.0.0.0/30 is subnetted, 1 subnets
C       25.25.25.0 is directly connected, Serial0/1
     10.0.0.0/30 is subnetted, 1 subnets
O       10.10.10.0 [110/74] via 50.50.50.2, 00:00:00, Ethernet1/2
                   [110/74] via 50.50.50.1, 00:00:00, Ethernet1/2
O*E2 0.0.0.0/0 [110/1] via 50.50.50.2, 00:00:00, Ethernet1/2
               [110/1] via 50.50.50.1, 00:00:00, Ethernet1/2

This is why I am confused.

In the routing table, it shows that there is load balancing going on, however the default route does not reflect this.

As for my original question, I should have given more detail to indicate that I was referencing the installation of default routes being advertised and not regular routes as I understood how that process worked.

networker050184 · October 2009

So what are you not understanding? R2 gets two equal cost routes so it puts them both in the routing table. Not sure what you are confused about.

Bosefus · October 2009

For some reason I thought that a router was not able to load balance to a default gateway and that advertised default routes were treated differently than regular advertised routes, obviously I was incorrect.

jason_lunde · October 2009

I hear what your saying, but routes are routes. Here is a decent cisco page, and there is a note at the very bottom, right before the summary that might be of some interest to you.

Configuring a Gateway of Last Resort Using IP Commands - Cisco Systems

Hopefully in a situation like this an admin or engineer has thought out the design, and has come to reasonable explanation as to why they would advertise multiple default routes into their area(s) or domain.

Bosefus · October 2009

Thank you, that little note at the bottom made it crystal.

OSPF defualt information originate

Comments