Some Questions on OSPF 5

[Tony414]

Hello,
I am going to implement OSPF on our network. Not a big network, a total of only 5 1700 routers. Can anywone give me some common mistakes that people might make when implementing it? If there is evey any. Can someone give me a quick example? Lets just say I have 2 routers. Router A and Router B. One thing I'm not sure of is the area setting. Thanks in advance for the help.

Tony

 

[Tony414]

If I telnet into 10.111.0.1 and do a traceroute to 10.81.0.1 it stops right after it hits the serial port on 10.111.0.1 then I get a bunch of * and then it times out.

 

[brianinms]

Yes, its because you only have routes for your lan interfaces and not your Serial interfaces. When you ping from the 10.111.0.1 router to 10.81.0.1 it uses the address of the serial interface. Thus 10.81.0.1 doesn't know how to get to 172.16.51.14 so it drops the packet.

 

[jneiberger]

Oh, geez...Brian is right. I can't believe I missed that. I'm going to go hang my head in shame.

Some of your ping tests work because you just happen to have default routing configured in such a way that it works. In the cases where your pings are not working, it's because the destination does not have a route pointing back to the WAN interface.

That's yet another reason to be more consistent in your addressing scheme and to drop static routing in favor of simple dynamic routing.

Turn on RIPv2 or EIGRP, make sure it's working, then get rid of your static routes, except for perhaps your default routes.

OSPF might be a good choice for one simple reason: it is super easy to advertise a default route throughout your network. This is good if you have one site providing Internet access, for example. You want your internal routes to be advertised by your routing protocol, but then have a single default route available for external destinations.

 

[Tony414]

Ok. Now the default route is something like this "ip route 0.0.0.0 0.0.0.0 10.0.0.1" correct? Or should it be something else? OSPF is what I originally was thinking of doing but someone mentioned it's only worth it in a large network.

I hope I'm not asking for too much, but would you give me an OSPF example of the 3 routers I specified? That would lead me in the right direction. Or maybe some common mistakes I might make. Thanks!

 

[jneiberger]

To put it simply, the default route is where you want to send traffic that doesn't have a more specific route. If you want that to be 10.0.0.1 then that's what you should make it.

OSPF is typically used in larger networks, but I happen to prefer it if you want to dynamically advertise a default route. It makes that very easy. It would probably be best if you were to read a bit about configuring OSPF on Cisco's site. The configuration would be pretty simply in your network, but I'd prefer that you know how it works. We can give you configs that work for now, but troubleshooting will be far more difficult if you don't know how OSPF operates.

 

[Tony414]

Yes I have read up on it somewhat before. And I actally tried to configure it between two of the networks. But something went wrong and the two networks did not communicate. So obviously I didn't read good enough So that's why I asked for an example between two of my routers. Or I could put up here what I did and someone can critique it...

 

[Tony414]

Ok. I was playing around with ospf. But I'm already stumped on something. If you look at my TownHall2. You can see I have 2 serial interfaces. But when I do a sh ip ospf it only show 1 serial interface. Is there something I'm missing? Here is what I put in.

router ospf 1
network 172.16.51.0 0.0.0.255 area 0
--------------------------------------------------

nterface FastEthernet0
description connected to TownHall LAN
ip address 10.81.0.5 255.255.0.0
speed 100
full-duplex
!
interface Serial0
description connected to PublicWorks
ip address 172.16.51.9 255.255.255.252
encapsulation ppp
no fair-queue
service-module t1 clock source internal
service-module t1 remote-alarm-enable
service-module t1 fdl ansi
!
interface Serial1
description connected to ParkRec
ip address 172.16.51.13 255.255.255.252
encapsulation ppp
service-module t1 clock source internal
service-module t1 remote-alarm-enable
service-module t1 fdl ansi

 

[jneiberger]

I would recommend explicitly stating each interface you want to run OSPF. In other words use the interface IP addresses plus a 0.0.0.0 mask instead, one for each interface. It's a bit more work, but I think it's a good practice to get into.

 

[Tony414]

Here is what I just entered in:

router ospf 1
log-adjacency-changes
network 172.16.51.9 0.0.0.0 area 0
network 172.16.51.13 0.0.0.0 area 0

But when I do a sh ip ospf I am seeing this at the top "Routing Process "ospf 1" with ID 172.16.51.13"

That's the only 1 serial int....

I really appreciate your help with this !!

 

[jneiberger]

It's been a while since I used OSPF. I wonder if your neighbor has to be running OSPF before it will show up. Is the router on the other end of your .9 connection running OSPF yet? How about the one on the .13 connection?

It's completely slipping my mind at the moment, but I suppose it's possible that an adjacency must form before the process will show as running on any given link.

 

[Tony414]

No neither one. I'm being really cautious I didn't change anything with the static routes. So I should be ok, correct?

 

[jneiberger]

Yes, you'll be fine. The static routes take precedence over any identical routes learned dynamically.

Oh, and it just occurred to me that the ID you're seeing is just the router ID. When you enable OSPF, the router has a way of selecting one of its IP addresses as a router ID for the OSPF process. That doesn't mean that that's the only interface running OSPF.

 

[jneiberger]

Continue configuring OSPF on your routers. On the router that you want to be the default router for any unknown or unspecified routes, also add the command "default-information originate always" under the OSPF config. That will advertise a default route throughout your area.

Once you have all routers configured, go to each of them and type "show ip route ospf" and make sure all the routes you want to see are there: all LAN subnets, all serial subnets, and your default route. If everything is correct, save your config and then remove your static routes. If something breaks and you lose access to a router, you can just reload it to regain access.

In fact, you might want to try the "reload in" command prior to removing the static routes just in case.

 

[Tony414]

That's weird. I was just getting ready to ask some of these questions. I will make the changes you mentioned and report back. Thanks!

 

[Tony414]

Now if I have 2 routers on the same lan, is there anything special I need to do for them?

 

[jneiberger]

No. OSPF does behave a bit differently where LAN interfaces are involved, but that won't matter unless you have a third router connected to that same LAN.

 

[Tony414]

When I run a "sh ip route ospf" I am coming up empty...
I am running it fromt he TownHall router which is the one I added the "default-information originate always" command.

 

[jneiberger]

Ah. That's probably because the static routes are overriding the OSPF routes. What I should have actually told you to look at is the OSPF database. I can't remember the command, but it's one of the "show ip ospf" commands. Look around to see if you can see the database. If the routes are in the database then you should be okay.

When you do a "show ip route", you're only seeing the routes that have actually been installed into the routing table, not all available routes. The static routes are more preferred than the OSPF routes, so the OSPF routes will not show up in the actual routing table until you remove the static routes.

 

[Tony414]

Here is what I get with a sh ip ospf database

TownHall>sh ip ospf database

OSPF Router with ID (172.16.51.5) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
172.16.51.2 172.16.51.2 806 0x80000002 0x008368 2
172.16.51.5 172.16.51.5 374 0x80000004 0x00F0B3 3

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
0.0.0.0 172.16.51.5 374 0x80000002 0x00229A 1

 

[jneiberger]

Interesting. Do you have OSPF configured on all of your routers already? If so, your database should be bigger and you should also have your 10.x.x.x links in there.