Injecting a default route into OSPF

This post will build off the BGP Basics post that I did earlier this month. Here is what we are looking to achieve this time:

• The ISP router will advertise a default route to his neighbor R1 (acme.com's internet gateway)
• R1 will then advertise to R2 and R3 via OSPF the default route
• R2 and R3 will be able to access the 172.16.1.0/21 network even though they have no specific route to this network

Here we go:



• Starting with the ISP router, we are going to remove the default route that we added in on the last post

ISP#
!
no ip route 0.0.0.0 0.0.0.0 Null0

We still have the default-originate statement under the router bgp 64004 statement so we should be good. Over on R1 we should see that he has an entry that looks like this:

R1#
!
show ip route
!
B* 0.0.0.0/0 [20/0] via 10.10.1.2, 00:16:27

• Next we are going to look at R2 and R3 to see what routes they have. They should look something like this (taken from R2)



• On R1 we are going to add a statement to the OSPF process that will advertise the default route.

R1#
!
router ospf 1
!
default-information originate

On R2 and R3 we see this entry for the default route and we also see a gateway of last resort set (from R2):



• Now if we were to go back to the ISP router and pull the default-originate statement from BGP (or down the serial 0/0 interface) what do you think we will see on R1?

- Answer: R1 will no longer have a route to 0.0.0.0 from the ISP router. Because of this R2 and R3 have lost their default route and gateway of last resort. The reason for this is because the route will not be advertised to other OSPF neighbors unless it's in R1's table.

Now that we have established this we can restore the default route and test the theory that we can reach things from R2 and R3 that are not in their route table.

• The network 172.16.3.0 exists off the ISP router and specifically 172.16.3.1. If we ask R2 about this network is tells us it has no idea what we are talking about:

R2#show ip route 172.16.3.1
% Network not in table

When we ping it here is what we see:

R2#ping 172.16.3.1
!
Sending 5, 100-byte ICMP Echos to 172.16.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/30/44 ms

A traceroute shows that it gets to the ISP router just fine:

R2#traceroute 172.16.3.1
!
Tracing the route to 172.16.3.1

1 192.168.2.1 24 msec 20 msec 20 msec
2 10.10.1.2 24 msec * 20 msec
[stop]

BGP Basics

I have long admitted that I am not a BGP expert and so I will try my best get better with it over the course of the next few blogs related to configuration and redistribution.

• The example I am working with today is a very basic setup with an ISP router and three routers inside what we will call the acme.com network. The ISP router is in AS 64004 and has a default route which it will be advertising into R1 at acme. The R1 router will be running BGP AS 64001 and the two will be defined as neighbors.




Here is what is needed to make this work:

ISP#
!
router bgp 64004
neighbor 10.10.1.1 remote-as 64001

R1#
!
router bgp 64001
neighbor 10.10.1.2 remote-as 64004

• With that basic information entered in we should see the neighbors form and a "show ip bgp summary" on the ISP router will show you this:



• The next part of this is to get the networks that connect from R1 to R2 and R3 (192.168.2.0/24 and 192.168.3.0/24) advertised to the ISP router. First we need to get them into OSPF and this is what needs to be placed into R1, R2, and R3:

Rx#
!
router ospf 1
log-adjacency-changes
network [IP of the loopback interface of the router] area 0
network 192.168.0.0 0.0.3.255 area 0

This will get you adjacency between all three routers. Something like this (shown from R3's perspective):



• Now on the R1 router we add in the network statements under the BGP process for the 192.168.2.0 and 192.168.3.0 networks

R1#
!
router bgp 64001
network 192.168.2.0
network 192.168.3.0
!

The ISP router should now see the two networks:

ISP# show ip route
!
B 192.168.2.0/24 [20/0] via 10.10.1.1, 00:00:21
B 192.168.3.0/24 [20/0] via 10.10.1.1, 00:00:22

• Now lets create a default route on the ISP router (in this case to null 0)

ISP#
!
ip route 0.0.0.0 0.0.0.0 Null0

• Now lets advertise that route to our neighbor R1 at 10.10.1.1

ISP#
!
router bgp 64004
neighbor 10.10.1.1 default-originate

On the R1 router we now see the a default route learned via BGP

R1# show ip route
!
B* 0.0.0.0/0 [20/0] via 10.10.1.2, 03:07:04

• This is the final result. A very simple network design that we will continue to modify over the next few blogs: