GRE Tunnels
Configuring GRE
GRE tunnels appear as directly connected logical interfaces to the router even though the traffic that goes into the tunnel will actually be carried over other physical interfaces to the destination. To configure a GRE tunnel you have to set the source and the destination of the tunnel on each end. The source can be an interface or the IPv4 or IPv6 address of an interface. The tunnel destination can only be an IPv4 or IPv6 address on the other router.
As you can see, the tunnel can run over IPv4 or IPv6 depending on the tunnel mode. By default a tunnel interface is set to gre ip. The multipoint option will be discussed in another article. GRE tunnels can also transport IPv4 and IPv6, depending on the Layer 3 addresses that are configured on the tunnel interface:
A tunnel interface will come up on one router regardless of the end-to-end connectivity. To have a more reliable status of the interface we should enable keepalives, which are disabled by default. The keepalive mechanism is pretty ingenious and can be enabled on only one side, or with different options on both sides. Read this article for more details.
A limited security mechanism can be implemented, in which the routers must attach a preshared numerical key to each packet. If the keys don’t match, the destination will drop the packets.
Normally, when bringing up a tunnel between 2 routers we create a loop because there are now 2 logical paths for the traffic: One over the physical links and one over the logical tunnel interface. One thing that is very probable to happen is to have a “recursive routing” loop. This will happen if the routing mechanism decides to use the logical tunnel interface to reach the defined tunnel destination. Cisco IOS detects this and will move the tunnel interface to a down state. This will probably disable the route through the tunnel interface which will re-enable the Tunnel interface and then the route through the interface will come up and IOS will again detect the “recursive routing” situation, and so on.
The example
Each router will be configured with a loopback address. We will create a tunnel between R1 and R3 and we will use EIGRP for route distribution. Here’s the starting configs that only include IP addressing information
Now let’s create the tunnel interfaces on both ends:
Let’s test connectivity:
OK – the tunnel seems to work – Let’s see how a traceroute looks:
As expected, R2 does not show up in the traceroute because the tunnel looks like a directly connected interface. Let’s see how can we reach R3’s loopback:
Hmm, it seams we can actually see R2 in the trace. How come? Let’s look at the routing table:
We see that in order to reach 3.3.3.3 we are using the link to R2, not through R3. Here’s why:
According to the EIGRP topology table, we have 2 routes towards 3.3.3.3, one over Fa0/0 through R2, and one over the Tunnel0 interface, through R3, but the metric via the Tunnel0 interface is a lot worse than via Fa0/0 and here’s the explanation:
The default values for bandwidth and delay on a tunnel interface are much worse than on the physical interface. Let’s change this values to see if we can force the use of the other link:
Now, it won’t take much until we see these errors:
We have hit the Recursive Routing scenario, in which the router uses the Tunnel interface to reach the Tunnel destination. This is, of course an illegal operation. The Tunnel interface will change state do down. When the routes over the Tunnel interface expire, the router will try to bring up the tunnel interface. The tunnel will come up, a new EIGRP adjacency will form over the Tunnel interface, it will have a better path towards the Tunnel destination and we entered the loop. So, remember that the low bandwidth and high delay values on the Tunnel interface are just a protection mechanism against those kind of loops. OSPF also uses the bandwidth in the default cost formula so we should be safe most of the time (but not always!), but what about RIP? RIP will definitely cause some problems since the default metric only uses hop count, and we will have just one hop between the 2 ends of a tunnel. So how can we send traffic over the tunnel interface? One option is to use static routes to force the traffic over the Tunnel interface, but using static routes is not a good idea in large environments. Another option is to Policy Route traffic towards 3.3.3.3 via 13.0.0.3 but is just as unscalable as the static route and it is also more difficult to read out in the config. A third option is to use another routing protocol with a lower Administrative Distance than the one running over the Tunnel interface, to reach the tunnel destination. Probably the best option is to limit the routes advertised or received over the Tunnel interface, so that we don’t use the Tunnel interface to reach the Tunnel destination. To do this, we should use a distribute-list:
I filtered on R1 both incoming and outgoing routes over Tunnel0 interface. Another method would have been to filter one way on each router, but the result is the same. Let’s see how that worked out:
It finally looks good.
Conclusions on Recursive Routing
You saw that EIGRP prevents recursive routing situations in small scenarios like this one by using a default metric that is worse through the tunnel than through the physical path. The same happens with OSPF as long as the physical path has a bandwidth higher than 9kbps (default for GRE tunnels). However with RIP, things won’t go as smoothly because it’s metric is based on hop count and the tunnel interface will always count as 1 hop, while the physical path will probably be more than that. So, pay attention when using RIP over GRE tunnels, as it will enter the recursive routing state by default.
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