Cisco – Stuck at GRE troubleshooting with recursive route

ciscogreroutingtunnel

As with my last question, I am learning for my CCNA exam and currently I am bashing my head against the wall, while trying to solve "7.2.2.4 Troubleshooting GRE"

I'll explain the topology in a second. First let me document, which steps I have taken so far:

  1. RA's Ip addresses on t0 and G0/0 were switched. I corrected them
  2. RA's tunnel 0 interface had the wrong source and destination addresses. I set them to s:172.31.0.1 d: 192.168.1.1

Now, on RB theres this message: 

 %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnel0, changed state to up
 %ADJ-5-PARENT: Midchain parent maintenance for IP midchain out of 0 65E900C0 - looped chain attempting to stack
 %TUN-5-RECURDOWN: 0 temporarily disabled due to recursive routing

Topology:

GRE topology

Every IP address/network address as shown on the image is correct.

This is show interface tunnel 0 on RA:

Tunnel0 is up, line protocol is up (connected)
  Hardware is Tunnel
  Internet address is 192.168.1.1/30
  MTU 17916 bytes, BW 100 Kbit/sec, DLY 50000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation TUNNEL, loopback not set
  Keepalive not set
  Tunnel source 172.31.0.1 (GigabitEthernet0/0), destination 192.168.1.2
  Tunnel protocol/transport GRE/IP

Show interface tunnel 0 on RB:

Tunnel0 is up, line protocol is up (connected)
  Hardware is Tunnel
  Internet address is 192.168.1.1/30
  MTU 17916 bytes, BW 100 Kbit/sec, DLY 50000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation TUNNEL, loopback not set
  Keepalive not set
  Tunnel source 172.31.0.1 (GigabitEthernet0/0), destination 192.168.1.2
  Tunnel protocol/transport GRE/IP
    Key disabled, sequencing disabled
    Checksumming of packets disabled

Show ip route on RA:

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
C       172.31.0.0/24 is directly connected, GigabitEthernet0/0
L       172.31.0.1/32 is directly connected, GigabitEthernet0/0
     192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.1.0/30 is directly connected, Tunnel0
L       192.168.1.1/32 is directly connected, Tunnel0
     209.165.122.0/24 is variably subnetted, 2 subnets, 2 masks
C       209.165.122.0/30 is directly connected, Serial0/0/0
L       209.165.122.2/32 is directly connected, Serial0/0/0
S*   0.0.0.0/0 is directly connected, Serial0/0/0

Show ip route on RB:

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

     64.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       64.103.211.0/30 is directly connected, Serial0/0/0
L       64.103.211.2/32 is directly connected, Serial0/0/0
     172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
C       172.31.1.0/24 is directly connected, GigabitEthernet0/0
L       172.31.1.1/32 is directly connected, GigabitEthernet0/0
     192.168.1.0/32 is subnetted, 1 subnets
L       192.168.1.2/32 is directly connected, Tunnel0
S*   0.0.0.0/0 is directly connected, Serial0/0/0

Please help me understand this error by explaining what I am missing here and how to resolve this.

Kind regards.

**//EDIT: ** The task which needs to be fullfilled is that traffic from the 172.31.x.x networks on both sides needs to be tunneled.

//EDIT2: As it turns out Iw as talking Bullsh**. Here are the steps taken so far:

  1. RA's Ip addresses on t0 and G0/0 were switched. I corrected them

  2. RA's tunnel 0 interface had the wrong source and destination addresses. I set them to s:172.31.0.1 d: 192.168.1.1

  3. Changed Source address of tunnel 0 on RA to 172.31.0.1

  4. Changed Destination address of tunnel 0 on RB to 209.165.122.2
  5. copy run start on RB and then reload
  6. Tunnel 0 on RB is now up and connected.

//EDIT3:

I finally worked it out. I went back in my curriculum to an earlier task, which did guide me through the whole configuration task.

The source on both routers for the tunnel is the S0/0/0 interface and the destination is the other routers S0/0/0 interface IP address.
This is highly counter intuitive, since in my head I was under the impression, the source is the traffic form the local LANs (the 172 interfaces), since the traffic from there needs to be tunneled.

Just for good measure I added 2 static routes for both routers, to make sure the packets get routed this way in case OSPF / EIGRP or RIP think otherwise.

I have upvoted both answers, but accepted Ron's, since he answered first with a correct set up.

Best Answer

Serv,

Your tunnel source and destination addresses are wrong. They should be 209.165.122.2 and 64.103.211.2 on RA and the opposite on RB.

There are many different ways to explain this, but essentially, the route to the tunnel endpoint cannot be through the tunnel. In your case, in order to send a packet (from RA) to 192.168.1.2, RA has to encapsulate it in a GRE packet and then send it to the tunnel destination. But the route to the tunnel destination is through the tunnel itself. that is not allowed because you would be infinitely encapsulating the packet.

Another explanation (metaphor):

Think of a bridge being built across a river. the bridge engineers need to know where on each side of the river to put the bridge. Once they have built the bridge, you can route people to use the bridge to get to the other side. But that is separate from where the bridge has to be built.

The bridge (tunnel) is built from RA's s0/0/0 interface to RB s0/0/0. Once the tunnel is up, you can then route traffic from 172.31.x.x through the tunnel. In other words, building the tunnel is separate from routing traffic through the tunnel.

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