GRE Multilink Sim Troubleshooting - GRE Tunnel CCNA 200-125 Lab

Refer to the topology below and answer the following questions.

GRE Multilink Sim Troubleshooting

Note: There may be different faults in this sim so please notice the following faults:

1. ACL blocking traffic on interface between R2 and R3
2. The network was not advertised in EIGRP
3. Two routers couldn’t establish adjacency because of ACL
4. Another two routers couldn’t establish adjacency because of wrong IP configuration on interface between them

Question 1:
Why is the Branch2 network 10.1 0.20.0/24 unable to communicate with the Server farm1 network 10.10.10.0/24 over the GRE tunnel?

A. The GRE tunnel destination is not configured on the R2 router.
B. The GRE tunnel destination is not configured on the Branch2 router.
C. The static route points to the tunnel0 interface that is misconfigured on the Branch2 router.
D. The static route points to the tunnel0 interface that is misconfigured on the R2 router.

Explanation/Reference:

The Branch2 network is communicating to the Server farm, which is connected to R2, via GRE Tunnel so we should check the GRE tunnel first to see if it is in “up/up” state with the show ip interface brief command on the two routers.

On Branch2:

On R2:

We see interfaces Tunnel0 at two ends are “up/up” which are good so we should check for the routing part on two routers with the show running-config command and pay attention to the static routing of each router. On Branch2 we see:

The destination IP address for this static route is not correct. It should be 192.168.24.1 (Tunnel0’s IP address of R2), not 192.168.24.10 -> Answer C is correct.

Note: You can use the show ip route command to check the routing configuration on each router but if the destination is not reachable (for example: we configure ip route 10.10.10.0 255.255.255.0 192.168.24.10 on Branch2, but if 192.168.24.10 is unknown then Branch2 router will not display this routing entry in its routing table.

Note: The IP address or configuration may be different in the exam.

Question 2:
Why has the Branch3 router lost connectivity with R1?
Use only show commands to troubleshoot because usage of the debug command is restricted on the Branch3 and R1 routers.

A. A PPP chap hostname mismatch is noticed between Branch3 and R1.
B. A PPP chap password mismatch is noticed between Branch3 and R1.
C. PPP encapsulation is not configured on Branch3.
D. The PPP chap hostname and PPP chap password commands are missing on the Branch3 router.

Explanation/Reference:

First we should check Branch3 (and R1) with the show ip interface brief command to find any Layer1/Layer 2 issue.

We see the interfaces connecting between them are in “up/down” states which indicates an Layer 2 issue so we should check the configuration of these interfaces carefully with the show running-config command and pay attention to these interfaces.

and on Branch3:

We learn from above config is R1 is using CHAP to authenticate Branch3 router (via the ppp authentication chap command on R1). Branch3 router is sending CHAP hostname “Branch_3” and CHAP password “Branch3_Secret!” to R1 to be authenticated. Therefore we should check if R1 has already been configured with such username and password or not with the “show running-config” command on R1:

On R1 we see the configured username is “Branch3”, not “Branch_3” so the usernames here are mismatched and this is the problem -> Answer A is correct.

Question 3:
Which statement about the router configurations is correct?

A. PPP PAP is authentication configured between Branch2 and R1.
B. Tunnel keepalives are not configured for the tunnel0 interface on Branch2 and R2.
C. The Branch2 LAN network 192.168.11 0/24 is not advertised into the EIGRP network.
D. The Branch3 LAW network 192.168.10.0/24 is not advertised into the EIGRP network.
E. PPP CHAP is authentication configured between Branch1 and R1.

Explanation/Reference:

In this question we have to check each option to see if it is correct. When we check Branch3 router we notice that network 192.168.10.0 command is missing under “router eigrp 100” -> Answer D is correct.

Note: In the exam you should check other config as well. There are some reports saying that the answer of this question is different in the exam (answer A).

Question 4:
Why did Branch1 router lose WAN connectivity with R1 router?

A. The IP address is misconfigured on PPP multilink interface on the Branch1 router.
B. The PPP multilink group is misconfigured on the ￡ ranch1 serial interfaces.
C. The PPP multilink group is misconfigured on the R1 serial interfaces.
D. The Branch1 serial interfaces are placed in a shutdown condition

Explanation/Reference:

This question clearly stated there is a WAN connectivity issue between R1 and Branch1 so we should check both of them with the show ip interface brief command. On R1:

On Branch1:

We can see that although the Multilink1 interfaces are in “up/up” state but they are not in the same subnet. According to the IP address scheme shown on the topology we can deduce the Multilink interface on Branch1 has been misconfigured, it should be 192.168.14.2 instead.