Wednesday, November 3, 2010

OSPF on 3 routers

The next step for my 3 router (and 1 switch) Cisco lab is to set up a routing protocol so that all of the subnets on the network can communicate.  Here is the current setup (see my last post for setup details):
Right now, if I try to ping TOP's E0/0 interface (172.16.10.1) from SWITCH, the ping will fail, even after setting the IP DEFAULT GATEWAY 172.16.30.1 global configuration command on SWITCH in order to set BOTTOM as it's default gateway.  The ping fails when the packet reaches BOTTOM and BOTTOM and none of BOTTOM's routes include 172.16.10.1.

Let's look at BOTTOM's routes as they stand:

BOTTOM>SHOW IP ROUTE


Gateway of last resort is not set


   172.16.0.0/24 is subnetted, 1 subnets
C     172.16.30.0 is directly connected, G0/0
   192.168.1.0/24 is variably subnetted, 4 subnets, 2 masks
C     192.168.1.9/32 is directly connected, S0/0/1:0
C     192.168.1.8/30 is directly connected, S0/0/1:0
C     192.168.1.0/30 is directly connected, S0/0/0:0
C     192.168.1.2/32 is directly connected, S0/0/0:0

So, you can see from the routing table that there is no way a ping going through BOTTOM is going to reach the 172.16.10.1, because BOTTOM doesn't know where to send it!

Now, let's enable the a very basic instance of the OSPF routing protocol on BOTTOM and TOP so they will know about each other's connected subnets.

BOTTOM>enable
BOTTOM#configure terminal
BOTTOM(config)#router ospf 1
----- This command tells the router that you want to configure instance #1 of OSPF

BOTTOM(config-router)#network 0.0.0.0 255.255.255.255 area 0
----- This tells the router to include ALL attached interfaces in the OSPF topology.  I'm not going to go into the details here, but you can use the network command to specify only interfaces with specific subnets attached to be included in the OSPF topology.


BOTTOM(config-router)#area 0 authentication message-digest
----- This sets all interfaces encompassed by the previous command to use MD5 authentication for OSPF communication.


BOTTOM(config-router)#do show ip ospf interface
----- This will list all interfaces encompassed by the OSPF network command.  We will need to set the MD5 password on each of these interfaces:


BOTTOM(config-router)#interface s0/0/1:0
BOTTOM(config-if)#ip ospf message-digest-key 1 md5 c3s76k
----- This bizarre command sets the first OSPF MD5 key to "c3s76k".  You'll need to run the same command on any interface that connects to another router running OSPF in this environment:


BOTTOM(config-if)#interface s0/0/0:0
BOTTOM(config-if)#ip ospf message-digest-key 1 md5 c3s76k

That completes the OSPF configuration for BOTTOM.  Notice, nothing has changed in the routing table.  You'll have to perform the same steps on TOP and SMALL before your OSPF environment will be properly configured!  Make sure to use the exact same area number (0) and MD5 key, or your router's won't communicate OSPF information.

After running these commands on TOP and SMALL, here is BOTTOM's new routing table:
BOTTOM>SHOW IP ROUTE

Gateway of last resort is not set

   172.16.0.0/24 is subnetted, 3 subnets
C     172.16.30.0 is directly connected, G0/0
O     172.16.20.0 [110/66] via 192.168.1.9 ... S0/0/1:0
O     172.16.10.0 [110/75] via 192.168.1.2 ... S0/0/0:0
   192.168.1.0/24 is variably subnetted, 4 subnets, 2 masks
C     192.168.1.9/32 is directly connected, S0/0/1:0
C     192.168.1.8/30 is directly connected, S0/0/1:0
C     192.168.1.0/30 is directly connected, S0/0/0:0
C     192.168.1.2/32 is directly connected, S0/0/0:0
O     192.168.1.4/30 [110/130] via 192.168.1.9 ... S0/0/1:0
                     [110/130] via 192.168.1.2 ... S0/0/0:0

I have bolded the routes added by OSPF.  You'll notice that there are two equal-cost routes to 192.168.1.4/30, because the calculated distance to the serial link between TOP and SMALL is exactly the same regardless of whether you go through TOP or SMALL to get there.

You should now be able to successfully ping 172.16.10.1 from SWITCH, because your routers have routes there and back!

Best of all, even if you lose one of the serial links, all of your subnets will be able to talk to each other once OSPF converges (convergence takes less than 5 seconds in this network).

Up next, EIGRP!

2 comments:

  1. One problem with your topology the bottom router has s0/0/0 and s0/0/0 on the same network/subnet which is not possible to do.

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