When you configure OSPF network statements, IOS orders them most-specific to least-specific then does a top-to-bottom match of the interfaces. It doesn’t matter which order you put them in, the configuration will always be ordered with the longest prefix matches first.  Lab time!

I have router R1 with these interfaces.

R1#sh ip int brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            10.0.0.1        YES manual up                    up
FastEthernet0/1            unassigned      YES unset  administratively down down
Loopback100                10.0.101.1      YES manual up                    up
Loopback200                10.2.101.1      YES manual up                    up

Let’s add the OSPF configuration where 10.0.0.0/8 is in area 2 then check what OSPF thinks is happening.

Here’s a story from last week with little of no teaching value.

I got a call from one of our business units looking for some routing help.  We don’t usually care about their production networks, but they were seeing some funky traceroutes, so I agreed to try and help them out.

They sent over two fresh traceroutes from a host on a 7600.  In one of them, the trace went to the 7600 and then on down the line as expected.  In the other, the trace showed the 7600, another router’s far interface IP (that is, an interface not facing the 7600), then the 7600’s interface facing that router.  Every few minutes, the path was switch between the two.  The dude told me that they were an OSPF shop, so I asked him to send me the standard show ip route and show ip ospf database commands so I could see what’s going on.  The word “unexpected” comes to mind when trying to describe what I found.  So do other words that aren’t very appropriate.

A few hours ago, the last of the IPv4 addresses were allocated by IANA.  Now’s the time to learn more about IPv6!  Yesterday, I posted about EIGRP for IPv6, so I think I’ll continue the trend by introducing OSPFv3, which is the IPv6 implementation of OSPF.  As always, I’m using Cisco routers here.  Just as yesterday, this is just a guide to the absolutely basics; if you want to do some funky OSPF magic, you won’t find it here - perhaps in time, though.

Feel free to correct all this stuff.  Additions are also welcome.

Study Questions

• How do I keep an area route from reaching a router in that area?

You don’t.  That defeats the whole purpose of having the topology database on every router.  If you filtered one route from a router, there’s no way that SPF could calculate routes correctly.

• Fine, then.  Where do I filter routes?

You filter routes on an ABR or ASBR.  Since routers only have the whole topology for their area, it’s safe to filter routes from another area or from a redistributed routing protocol.  On a more technical note, you’re filtering type-3 LSAs on an ABR and type-5 LSAs on an ASBR.