Whats the big deal about a defaultroute? The defaultroute is the last resort path your IP traffic takes to get out of your local network if it doesn’t have a more specific route in it’s routing table.

Well you know, the current internet routing table is hitting around 370,000 entries or so today. Having all of those routes sat on your PC is not going to be a good thing for your RAM or CPU when you have to store and lookup those routes. So what do we do, we have a default route pointing at a gateway device on our network. So, that gateway might be a layer 3 switch where the gateway address is a virtual switch interface (VSI). Do you think your switch was built to hold 370k+ routes? No sir you are correct. Most likely therefore your first hop gateway device will also have a default gateway configured pointing to an upstream device like your firewall. So, that firewall, is that likely to be fantastic enough to hold these routes to the internet? You guessed right, no way. So your firewall has a default route to another upstream gateway like you CPE (Customer/Provider Edge) router. Now finally this is the most likely place you’ll not find a default route. If you are taking a full BGP feed from your ISP then you don’t need a default route because your router learns all of the networks out there. So do you need a full routing table here? If you are connected to one ISP with one link...nope you do NOT need the full routing table and a default route from the next hop (the ISP neighbor router) is enough.

Now we see the defaultroute and it’s importance it’s important to understand why it’s a bad thing for network security. If you give your devices access to a network which contains a default route then it’s something which an attacker can use to ‘get around’. Without a route to a destination packets are dropped - just think about that - network security without a firewall - awesome.

So if we want a default route how can we have one?

We’re going to start our ramble into default routes using the humble and powerful but ultimately high maintenance static route.

  • Static route - easiest to configure but least scalable. In this example we’ll walk through a defaultroute to get traffic via from the source interface of Loopback0 on R1 to the destination interface of Loopback0 on R4.

  • Here is the topology:

  • Screen shot 2011-05-03 at 22.48.25


  • Some topology information:
  • R1 Fa1/0 10.1.13.1/24 <-> R3 Fa1/0 10.1.13.3/24
  • R1 Fa0/0 10.1.12.1/24 <-> R2 Fa0/0 10.1.12.2/24
  • R3 Fa0/0 10.1.34.3/24 <-> R4 Fa0/0 10.1.34.4/24
  • R2 Fa1/0 10.1.24.2/24 <-> R4 Fa1/0 10.1.24.4/24
  • R1 Loopback0 1.1.1.1/32
  • R2 Loopback0 2.2.2.2/32
  • R3 Loopback0 3.3.3.3/32
  • R4 Loopback0 4.4.4.4/32
Screen shot 2011-05-03 at 23.02.10

Screen shot 2011-05-03 at 23.02.22

Screen shot 2011-05-03 at 23.02.34

Screen shot 2011-05-03 at 23.02.45

I’ve configured R2, R3 and R4 to ‘know’ how to reach the loopback interface of R1. This is key to the topology and you should try to think of R2 and R3 as intermediate systems rather like those of the internet. On the internet there are man hops in the network between you and your destination...just look at this traceroute for an example. I’ve removed the IP information from the first 4 hops to anonymise the result but you can see that my packets hop across 9 nodes before they get to the www.google.com server. In our example we’re going to hop twice to get to R4.

Screen shot 2011-05-03 at 23.11.21

So lets start with showing the routing table on R1

Screen shot 2011-05-03 at 23.03.55

So we see that there are 3 networks listed 1.0.0.0/32 is the loopback0 interface and the other two are the Fast Ethernet ports. Each interface is listed as code ‘C’ which means connected (Juniper call this ‘direct’). If we try to ping the remote loopback interface on R4 we get a fail because R4 doesn’t know how to get to the R4 network of 4.4.4.4/32.

Screen shot 2011-05-03 at 23.18.29

Simple enough so far and hopefully no worries.

So we want to get to google.com and in our wisdom we’ve put into two links to the internet. We’re going to use the link between R1 and R2 as ‘way out’ and we’re going to trust that R2 knows how to get to google.com. Lets put in our default route now, point it at R2.

Screen shot 2011-05-03 at 23.20.49

The first command we put in adds the default route. The IP address of 0.0.0.0 and netmask of all zeros means any IP address basically. It can sometimes be shown as 0.0.0.0/0. The last IP address is the address of the next hop from us. R1 has a connected network link between it’s Fa0/0 interface and R2 using network 10.1.12.0/24. R2’s IP address on this link is 10.1.12.2 so we point our defaultroute at that.

The result of the ‘show ip route’ now has a new route at the bottom prefaced with the code of ’S*’ which means a static route. So now we have a defaultroute pointing at the next hop on R2. R2 knows how to get to the Lo0 interface of R4...lets try a ping now.

Screen shot 2011-05-03 at 23.28.35
  • Great stuff we did it - we can now ping - lets see the path we took.
Screen shot 2011-05-03 at 23.30.13

So, as we know our packets went via R2. But hey you’ve bought two uplinks to the internet, one via R2 and one via R3...can we have two default routes? Well if your have one ISP and two uplinks to that ISP then the answer is yes. If you have two uplinks but are using different ISP’s on each link then the answer is a vague maybe and we’re not getting into the reasons why here.

So lets add another default route into R1 via R3.
Screen shot 2011-05-03 at 23.32.23

This time the ‘show ip route’ has two lines under 0.0.0.0/0. This is IOS telling you there are two equal cost routes to the destination. This means your packets can take either way to get there. We do a traceroute...

Screen shot 2011-05-03 at 23.34.17

Check out how the ICMP packets are telling us that sometime the first hop is 10.1.13.3 then 10.1.12.2? This is your packets being ‘load balanced’ in a round robin way. The second hop is either the network between R3 and R4 10.1.34.4 or the network between R2 and R4 10.1.24.4. Perfect. Just what we wanted...right? So what happens no if the link between R1 and R3 fails...we’ll administratively shutdown the interface for the demo.

Screen shot 2011-05-03 at 23.37.12
Screen shot 2011-05-03 at 23.37.34

Now lets ping the R4 loopback0 interface. What has happened now is that every other packet is failing to get to the 4.4.4.4 address. Thats because we effectively load balanced the traffic and now one of those links is down...hence the loss.

Screen shot 2011-05-03 at 23.42.56

So what can we do? Well in most cases you wouldn’t use static routes you’d be using dynamic routing protocols, some sort of first hop redundancy protocol and certainly be tracking upstream connectivity to failover and load balance properly but we’ll leave that for another time. For our brief demo we’ll concentrate on weighted static routes sometimes called ‘floating static routes’.

Floating static routes allow you to put a ‘primary route’ into the table but in situations where that route is no longer available a second route with a higher metric can be brought into service. Here we’re testing everything is good...and we’ve brought the link between R1 and R3 back up for this.

Screen shot 2011-05-03 at 23.47.38

The second configuration command basically takes out with a ‘no’ the second load balanced default route to R4 via R3. So now lets see the routing table...

Screen shot 2011-05-03 at 23.48.57

Ok you can see we’ve got one route now via 10.1.12.2. Lets add in our floating static route. Now by default a static route to a next hop gets a metric of 1. You can see the metric in the above ‘show ip route’ as the [1/0] after the route 0.0.0.0/0. We’ll add a ‘floating’ route with a metric of 200 but it can be anything between 2 and 255.

Screen shot 2011-05-03 at 23.50.42

So the route is in but crucially you WON’T see this in the routing table. It will still look like it did before we added the floating route...see...

Screen shot 2011-05-03 at 23.51.12

The floating route is only used when the lower metric route is lost from the table. To demonstrate it we’ll shutdown the interface on R1 connected to the link between R1 and R2.

Screen shot 2011-05-03 at 23.52.47

Check out the ‘show ip route’ now. You see that the defaultroute now points to 10.1.13.3 which before was not shown. R1 has realised that the next hop of the previous route is no longer valid so it drops that route from the table. The new defaultroute has a metric of 200 which is what we configured earlier. job done.

Thanks for reading.
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