IPv6 Tunneling Techniques practice questions

A network engineer is troubleshooting an IPv6 connectivity issue between two sites connected via a 6to4 tunnel. The tunnel is configured on both routers and shows as up/up, but the engineer cannot ping the IPv6 address of the remote tunnel endpoint. The engineer checks the routing table and sees no route to the remote IPv6 prefix. What is the most likely cause of this problem?

Trap 1: The tunnel mode is incorrectly set to ipv6ip instead of 6to4.

Incorrect because 'tunnel mode ipv6ip' is used for manual tunnels, not 6to4. However, the tunnel would still show up/up but might not encapsulate correctly; the primary issue is the source address.

Trap 2: The tunnel destination is misconfigured with the remote router's…

Incorrect because 6to4 tunnels do not use a tunnel destination; they use automatic destination derivation from the 6to4 prefix. A misconfigured destination would not cause the tunnel to come up.

Trap 3: The IPv6 address on the tunnel interface is not in the 2002::/16…

Incorrect because the tunnel interface IPv6 address is not required to be in 2002::/16 for the tunnel to work; the tunnel endpoint addresses are derived from the source IPv4 address.

A
The tunnel source interface is configured with a private IPv4 address, causing the 6to4 prefix to be invalid.
Correct because 6to4 requires a global IPv4 address to form a valid 2002::/16 prefix. A private address leads to an invalid 6to4 address, preventing proper routing.
B
The tunnel mode is incorrectly set to ipv6ip instead of 6to4.
Why wrong: Incorrect because 'tunnel mode ipv6ip' is used for manual tunnels, not 6to4. However, the tunnel would still show up/up but might not encapsulate correctly; the primary issue is the source address.
C
The tunnel destination is misconfigured with the remote router's IPv6 address instead of its IPv4 address.
Why wrong: Incorrect because 6to4 tunnels do not use a tunnel destination; they use automatic destination derivation from the 6to4 prefix. A misconfigured destination would not cause the tunnel to come up.
D
The IPv6 address on the tunnel interface is not in the 2002::/16 range.
Why wrong: Incorrect because the tunnel interface IPv6 address is not required to be in 2002::/16 for the tunnel to work; the tunnel endpoint addresses are derived from the source IPv4 address.

Question 2mediummultiple choice

An engineer is troubleshooting an ISATAP tunnel between a Windows host and a Cisco router. The host can ping the router's IPv6 address configured on the tunnel interface, but cannot reach any other IPv6 networks beyond the router. The router has a default route pointing to an upstream IPv6 router. What is the most likely cause?

Trap 1: The host's ISATAP interface has an incorrect IPv4 address for the…

Incorrect because the host can ping the router's IPv6 address, indicating the tunnel is working for direct communication.

Trap 2: The router's tunnel interface is missing the 'ipv6 enable' command.

Incorrect because the tunnel interface is up and reachable, so IPv6 is enabled on it.

Trap 3: The upstream router does not have a route back to the ISATAP prefix.

Incorrect because the issue is that the host is not sending traffic to the router for off-link destinations, not that the upstream router lacks a return route.

A
The router is not configured as an ISATAP router; it only has the tunnel interface but lacks the 'ipv6 isatap' command.
Correct because without 'ipv6 isatap', the router does not respond to ISATAP router solicitations, and the host will not use it as a default gateway for off-link traffic.
B
The host's ISATAP interface has an incorrect IPv4 address for the router's tunnel source.
Why wrong: Incorrect because the host can ping the router's IPv6 address, indicating the tunnel is working for direct communication.
C
The router's tunnel interface is missing the 'ipv6 enable' command.
Why wrong: Incorrect because the tunnel interface is up and reachable, so IPv6 is enabled on it.
D
The upstream router does not have a route back to the ISATAP prefix.
Why wrong: Incorrect because the issue is that the host is not sending traffic to the router for off-link destinations, not that the upstream router lacks a return route.

Question 3mediummultiple choice

A network engineer is troubleshooting a manual IPv6-in-IPv4 tunnel between two Cisco routers. The tunnel is up, and both routers can ping each other's tunnel IPv6 addresses. However, traffic from a host behind Router A to a host behind Router B fails. The engineer notices that Router A has a route to the remote IPv6 prefix via the tunnel, but Router B does not have a route to the local IPv6 prefix. What is the most likely cause?

Trap 1: The tunnel mode is set to 'ipv6ip 6to4' instead of 'ipv6ip'.

Incorrect because 'tunnel mode ipv6ip' is correct for manual tunnels; 6to4 mode would change the encapsulation and addressing.

Trap 2: The tunnel source on Router B is misconfigured with the wrong IPv4…

Incorrect because if the tunnel is up and they can ping each other's tunnel IPv6 addresses, the tunnel source is correct.

Trap 3: The IPv6 access-list on Router B is blocking incoming traffic from…

Incorrect because the issue is routing, not filtering; the engineer should check the routing table first.

A
Router B is missing a static route pointing the local IPv6 prefix to the tunnel interface.
Correct because without a return route, Router B cannot forward packets destined to the local prefix, breaking bidirectional communication.
B
The tunnel mode is set to 'ipv6ip 6to4' instead of 'ipv6ip'.
Why wrong: Incorrect because 'tunnel mode ipv6ip' is correct for manual tunnels; 6to4 mode would change the encapsulation and addressing.
C
The tunnel source on Router B is misconfigured with the wrong IPv4 address.
Why wrong: Incorrect because if the tunnel is up and they can ping each other's tunnel IPv6 addresses, the tunnel source is correct.
D
The IPv6 access-list on Router B is blocking incoming traffic from the local prefix.
Why wrong: Incorrect because the issue is routing, not filtering; the engineer should check the routing table first.

Review the full OSPF breakdown →

Question 4hardmultiple choice

An engineer is troubleshooting a GRE IPv6 tunnel between two sites. The tunnel is up, and the engineer can ping the remote tunnel endpoint IPv6 address. However, OSPFv3 neighbors over the tunnel fail to form. The engineer verifies that OSPFv3 is configured on both tunnel interfaces with the same area and that the network type is broadcast. What is the most likely cause?

Trap 1: The OSPFv3 network type is set to point-to-point instead of…

Incorrect because both broadcast and point-to-point can form neighbors; the issue is MTU, not network type.

Trap 2: The tunnel interface is missing the 'ipv6 ospf 1 area 0' command.

Incorrect because the engineer verified OSPFv3 is configured on both interfaces.

Trap 3: The tunnel keepalive is misconfigured, causing the tunnel to flap.

Incorrect because the tunnel is up and pingable, so keepalive is not the issue.

A
The tunnel MTU is set to 1500, but the GRE encapsulation adds 24 bytes, causing OSPFv3 packets to be fragmented.
Correct because the default tunnel MTU of 1500 does not account for GRE overhead, leading to fragmentation that OSPFv3 may not handle properly, especially with authentication or large LSAs.
B
The OSPFv3 network type is set to point-to-point instead of broadcast.
Why wrong: Incorrect because both broadcast and point-to-point can form neighbors; the issue is MTU, not network type.
C
The tunnel interface is missing the 'ipv6 ospf 1 area 0' command.
Why wrong: Incorrect because the engineer verified OSPFv3 is configured on both interfaces.
D
The tunnel keepalive is misconfigured, causing the tunnel to flap.
Why wrong: Incorrect because the tunnel is up and pingable, so keepalive is not the issue.

Open the full BGP breakdown →

Question 5hardmultiple choice

A network engineer is troubleshooting an IPv6 connectivity problem across an IPv4 MPLS network using 6PE. The 6PE routers have MP-BGP sessions to exchange IPv6 prefixes, and the tunnel between them is up. However, a customer edge router behind one 6PE router cannot reach an IPv6 prefix behind the other 6PE router. The engineer checks the 6PE router's BGP table and sees the prefix, but the routing table shows the next-hop as unreachable. What is the most likely cause?

Trap 1: The 6PE router is missing the 'ipv6 unicast-routing' command.

Incorrect because the router has BGP routes, so IPv6 unicast routing is enabled.

Trap 2: The tunnel interface is not in the VRF of the customer.

Incorrect because 6PE does not use VRFs; it uses global IPv6 routing with MPLS labels.

Trap 3: The remote 6PE router is not advertising the IPv6 prefix via BGP.

Incorrect because the BGP table has the prefix, so it is advertised.

A
The MPLS LDP session between the 6PE routers or between the 6PE and P routers is down, so no label exists for the BGP next-hop.
Correct because 6PE relies on MPLS labels to reach the remote 6PE router; without a label, the next-hop is unreachable, and traffic is dropped.
B
The 6PE router is missing the 'ipv6 unicast-routing' command.
Why wrong: Incorrect because the router has BGP routes, so IPv6 unicast routing is enabled.
C
The tunnel interface is not in the VRF of the customer.
Why wrong: Incorrect because 6PE does not use VRFs; it uses global IPv6 routing with MPLS labels.
D
The remote 6PE router is not advertising the IPv6 prefix via BGP.
Why wrong: Incorrect because the BGP table has the prefix, so it is advertised.

Question 6hardmultiple choice

An engineer is troubleshooting a DMVPN phase 2 deployment with IPv6 over mGRE tunnels. The spoke routers can ping the hub's tunnel IPv6 address, but cannot reach IPv6 networks behind other spokes. The engineer verifies that NHRP is configured and that the hub has a route to the spoke's internal networks. What is the most likely cause?

Trap 1: The NHRP authentication key is mismatched between the spokes.

Incorrect because NHRP authentication is only between spoke and hub; spokes do not authenticate directly with each other in phase 2.

Trap 2: The tunnel key is not configured on the mGRE interface.

Incorrect because the tunnel key is used for identifying the tunnel, but the spokes can still communicate with the hub without it, and it would not prevent spoke-to-spoke traffic if routing is correct.

Trap 3: The hub is not configured with 'ip nhrp redirect' and the spokes…

Incorrect because these commands are used for phase 3 DMVPN, not phase 2. In phase 2, spoke-to-spoke tunnels are initiated by NHRP requests without redirect.

A
The spoke routers are missing a static route for the remote spoke's internal network pointing to the mGRE tunnel interface.
Correct because without a route to the remote spoke's network via the tunnel, the spoke will send traffic to the hub, which may not forward it correctly, or the spoke may use a default route that does not use the tunnel.
B
The NHRP authentication key is mismatched between the spokes.
Why wrong: Incorrect because NHRP authentication is only between spoke and hub; spokes do not authenticate directly with each other in phase 2.
C
The tunnel key is not configured on the mGRE interface.
Why wrong: Incorrect because the tunnel key is used for identifying the tunnel, but the spokes can still communicate with the hub without it, and it would not prevent spoke-to-spoke traffic if routing is correct.
D
The hub is not configured with 'ip nhrp redirect' and the spokes with 'ip nhrp shortcut'.
Why wrong: Incorrect because these commands are used for phase 3 DMVPN, not phase 2. In phase 2, spoke-to-spoke tunnels are initiated by NHRP requests without redirect.

Question 7mediummultiple choice

A network engineer is troubleshooting an IPv6 over IPv4 tunnel using Teredo on a Windows host. The host can access some IPv6 resources on the internet but cannot reach a specific internal IPv6 server. The engineer suspects the Teredo relay is misconfigured. What is the most likely issue?

Trap 1: The host's firewall is blocking Teredo traffic on UDP port 3544.

Incorrect because the host can access some IPv6 resources, so Teredo is working for those destinations; the issue is specific to the internal server.

Trap 2: The internal server is not configured with an IPv6 address.

Incorrect because the server is reachable via other means (e.g., IPv4), but the question states the host cannot reach it via IPv6.

Trap 3: The Teredo client on the host is using an incorrect server.

Incorrect because the Teredo server is used for address assignment, not for forwarding traffic to native IPv6 networks.

A
The Teredo relay is not in the same IPv4 network as the host, or the relay does not have a route to the internal IPv6 server.
Correct because the Teredo relay must be able to forward packets to the destination IPv6 network. If the relay lacks a route, traffic will be dropped.
B
The host's firewall is blocking Teredo traffic on UDP port 3544.
Why wrong: Incorrect because the host can access some IPv6 resources, so Teredo is working for those destinations; the issue is specific to the internal server.
C
The internal server is not configured with an IPv6 address.
Why wrong: Incorrect because the server is reachable via other means (e.g., IPv4), but the question states the host cannot reach it via IPv6.
D
The Teredo client on the host is using an incorrect server.
Why wrong: Incorrect because the Teredo server is used for address assignment, not for forwarding traffic to native IPv6 networks.

Question 8hardmultiple choice

An engineer is troubleshooting an IPv6 manual tunnel between two routers that are not directly connected (the tunnel endpoints are separated by an IPv4 network). The tunnel is up, but the engineer notices that the tunnel interface flaps every few minutes. The show interfaces tunnel output shows input errors and CRC errors. What is the most likely cause?

Trap 1: The tunnel keepalive is set too low, causing the tunnel to go down…

Incorrect because keepalive loss would cause the tunnel to go down, not just flap with CRC errors.

Trap 2: The tunnel source interface is a loopback that is not reachable…

Incorrect because the tunnel is up, so the source is reachable.

Trap 3: The tunnel is using GRE encapsulation, but the remote router is…

Incorrect because a mode mismatch would prevent the tunnel from coming up at all.

A
The IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss.
Correct because the tunnel MTU is typically 1500, but the IPv4 path may have an MTU of 1500 or less, leading to fragmentation when the 20-byte IP header is added. This causes errors and flapping.
B
The tunnel keepalive is set too low, causing the tunnel to go down when a keepalive is lost.
Why wrong: Incorrect because keepalive loss would cause the tunnel to go down, not just flap with CRC errors.
C
The tunnel source interface is a loopback that is not reachable from the remote router.
Why wrong: Incorrect because the tunnel is up, so the source is reachable.
D
The tunnel is using GRE encapsulation, but the remote router is configured for ipv6ip.
Why wrong: Incorrect because a mode mismatch would prevent the tunnel from coming up at all.

Question 9mediummultiple choice

A network engineer is troubleshooting an IPv6 over IPv4 tunnel that is used to connect two remote sites. The tunnel is configured with a tunnel source that is a loopback interface. The tunnel is up, but the engineer cannot ping the remote tunnel endpoint IPv6 address. The engineer checks the routing table and sees a route to the remote loopback's IPv4 address via a default route. What is the most likely cause?

Trap 1: The tunnel destination is configured with the loopback address of…

Incorrect because the tunnel destination should be the IPv4 address of the remote tunnel endpoint, which could be a loopback. The issue is reachability, not mismatch.

Trap 2: The tunnel interface is missing the 'tunnel mode ipv6ip' command.

Incorrect because the tunnel is up, so the mode is likely correct.

Trap 3: The IPv6 address on the tunnel interface is not in the same subnet…

Incorrect because the tunnel endpoints can be in different subnets; the tunnel is a point-to-point link.

A
The remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix.
Correct because the tunnel source loopback address must be reachable from the remote router. If the default route does not include that specific prefix (e.g., due to routing policy or subnet mismatch), the tunnel cannot encapsulate packets.
B
The tunnel destination is configured with the loopback address of the remote router, but the remote router's tunnel source is a different interface.
Why wrong: Incorrect because the tunnel destination should be the IPv4 address of the remote tunnel endpoint, which could be a loopback. The issue is reachability, not mismatch.
C
The tunnel interface is missing the 'tunnel mode ipv6ip' command.
Why wrong: Incorrect because the tunnel is up, so the mode is likely correct.
D
The IPv6 address on the tunnel interface is not in the same subnet as the remote tunnel IPv6 address.
Why wrong: Incorrect because the tunnel endpoints can be in different subnets; the tunnel is a point-to-point link.

Question 10mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 tunnel brief

Tunnel2: IPv6/IP, intf id 0/0/2, 6to4, mtu 1280 Source: 192.168.1.1 (GigabitEthernet0/0) Destination: 192.168.2.1 Tunnel transport: IPv4

Based on this output, which statement is correct?

Trap 1: This is a correctly configured 6to4 tunnel.

6to4 tunnels do not have a configured destination; the destination is derived from the 6to4 prefix.

Trap 2: The tunnel is an ISATAP tunnel.

ISATAP tunnels use a different interface type and addressing.

Trap 3: The tunnel is in an up/up state and passing traffic.

The output does not show interface status; only configuration parameters.

A
This is a correctly configured 6to4 tunnel.
Why wrong: 6to4 tunnels do not have a configured destination; the destination is derived from the 6to4 prefix.
B
This tunnel is actually a manually configured IPv6/IP tunnel, not a 6to4 tunnel.
The presence of a specific destination address indicates a manual tunnel; 6to4 tunnels have no configured destination.
C
The tunnel is an ISATAP tunnel.
Why wrong: ISATAP tunnels use a different interface type and addressing.
D
The tunnel is in an up/up state and passing traffic.
Why wrong: The output does not show interface status; only configuration parameters.

Question 11mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 interface tunnel 0

Tunnel0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::A8BB:CCFF:FE00:1

No global unicast address is configured

Joined group address(es): FF02::1 FF02::2 FF02::1:FF00:1 MTU is 1480 bytes ICMP error messages limited to one every 100 milliseconds ICMP redirects are enabled ICMP unreachables are sent ND DAD is enabled, number of DAD attempts: 1 ND reachable time is 30000 milliseconds ND advertised reachable time is 0 milliseconds ND advertised retransmit interval is 0 milliseconds ND router advertisements are sent every 200 seconds ND router advertisements live for 1800 seconds Hosts use stateless autoconfiguration for addresses.

Based on this output, what is a likely problem?

Trap 1: The tunnel interface is down.

The interface is up, line protocol is up.

Trap 2: The MTU is too small for IPv6.

1480 bytes is typical for tunnels; not a problem.

Trap 3: IPv6 is not enabled on the tunnel.

IPv6 is enabled as shown.

A
The tunnel interface is down.
Why wrong: The interface is up, line protocol is up.
B
The tunnel is not configured with a global IPv6 address, which may prevent routing of IPv6 traffic.
No global unicast address means the tunnel cannot be used for IPv6 routing beyond link-local.
C
The MTU is too small for IPv6.
Why wrong: 1480 bytes is typical for tunnels; not a problem.
D
IPv6 is not enabled on the tunnel.
Why wrong: IPv6 is enabled as shown.

Question 12mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 route | include Tunnel

O 2001:DB8:1::/48 [110/2] via FE80::1, Tunnel0 O 2001:DB8:2::/48 [110/3] via FE80::2, Tunnel1

Based on this output, which statement is correct?

Trap 1: The routes are learned via EIGRP.

The 'O' indicates OSPF, not EIGRP.

Trap 2: The tunnels are using IPv4 as the transport.

The output does not indicate the transport protocol; it only shows IPv6 routing.

Trap 3: Tunnel0 has a higher metric than Tunnel1.

Tunnel0 has metric 2, Tunnel1 has metric 3; Tunnel0 has lower metric.

A
The routes are learned via EIGRP.
Why wrong: The 'O' indicates OSPF, not EIGRP.
B
The tunnels are using IPv4 as the transport.
Why wrong: The output does not indicate the transport protocol; it only shows IPv6 routing.
C
OSPFv3 is running over the tunnel interfaces and these routes are reachable.
The OSPF routes are present with next-hop addresses via tunnel interfaces.
D
Tunnel0 has a higher metric than Tunnel1.
Why wrong: Tunnel0 has metric 2, Tunnel1 has metric 3; Tunnel0 has lower metric.

Question 13mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 mld interface tunnel 0

Tunnel0 is up, line protocol is up Internet address is FE80::1 MLD is enabled on interface Current MLD version is 2 MLD query interval is 125 seconds MLD querier timeout is 255 seconds MLD max query response time is 10 seconds Last member query response interval is 1 second MLD activity: 0 joins, 0 leaves MLD querying router is FE80::1 (this system)

Based on this output, what can be concluded?

Trap 1: MLD is disabled on this interface.

MLD is enabled as shown.

Trap 2: This router is not the MLD querier.

The output states 'this system' is the querier.

Trap 3: MLD version 1 is in use.

Current MLD version is 2.

A
MLD is disabled on this interface.
Why wrong: MLD is enabled as shown.
B
This router is not the MLD querier.
Why wrong: The output states 'this system' is the querier.
C
There are no multicast listeners on this tunnel interface.
0 joins indicates no hosts have joined any multicast groups.
D
MLD version 1 is in use.
Why wrong: Current MLD version is 2.

Review the full OSPF breakdown →

Question 14mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.1.2       1   FULL/DR        00:00:32     FE80::2         Tunnel0
192.168.1.3       1   FULL/BDR       00:00:35     FE80::3         Tunnel0

Based on this output, which statement is correct?

Trap 1: OSPFv3 is not running on this router.

Neighbors are shown, so OSPFv3 is running.

Trap 2: The router with Neighbor ID 192.168.1.3 is the Designated Router.

192.168.1.2 is DR, 192.168.1.3 is BDR.

Trap 3: The tunnel is using IPv4 transport.

The output does not indicate transport; it shows OSPFv3 neighbor information.

A
OSPFv3 is not running on this router.
Why wrong: Neighbors are shown, so OSPFv3 is running.
B
The router with Neighbor ID 192.168.1.3 is the Designated Router.
Why wrong: 192.168.1.2 is DR, 192.168.1.3 is BDR.
C
OSPFv3 adjacencies are fully established over the tunnel.
Both neighbors are in FULL state.
D
The tunnel is using IPv4 transport.
Why wrong: The output does not indicate transport; it shows OSPFv3 neighbor information.

Open the full BGP breakdown →

Question 15mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 bgp summary

BGP router identifier 192.168.1.1, local AS number 65001 BGP table version is 10, main routing table version 10 5 network entries using 720 bytes of memory 5 path entries using 400 bytes of memory 3/2 BGP path/bestpath attribute entries using 456 bytes of memory 1 BGP AS-PATH entries using 24 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 1600 total bytes of memory BGP activity 10/5 prefixes, 10/5 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

2001:DB8::2 4 65002 100 99 10 0 0 00:45:12 3

Based on this output, which statement is correct?

Trap 1: BGP is not configured for IPv6.

The neighbor is an IPv6 address and prefixes are received.

Trap 2: The BGP session is down.

The session is up with prefixes received.

Trap 3: The router is in AS 65002.

Local AS is 65001.

A
BGP is not configured for IPv6.
Why wrong: The neighbor is an IPv6 address and prefixes are received.
B
The BGP session is down.
Why wrong: The session is up with prefixes received.
C
BGP is peering over IPv6 and 3 prefixes are learned from the neighbor.
The neighbor is IPv6 and PfxRcd shows 3.
D
The router is in AS 65002.
Why wrong: Local AS is 65001.

Study the full EIGRP explanation →

Question 16mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 eigrp neighbors

IPv6-EIGRP neighbors for process 100 H Address Interface Hold Uptime SRTT RTO Q Seq (sec) (ms) Cnt Num 0 FE80::A8BB:CCFF:FE00:2 Tunnel0 13 00:23:45 10 200 0 12 1 FE80::A8BB:CCFF:FE00:3 Tunnel1 12 00:22:10 15 200 0 15

Based on this output, which statement is correct?

Trap 1: EIGRP is not configured for IPv6.

Neighbors are shown, so EIGRP for IPv6 is configured.

Trap 2: The neighbors are using global unicast addresses.

The addresses are link-local (FE80::/10).

Trap 3: The tunnels are using GRE encapsulation.

The output does not specify encapsulation type.

A
EIGRP is not configured for IPv6.
Why wrong: Neighbors are shown, so EIGRP for IPv6 is configured.
B
The neighbors are using global unicast addresses.
Why wrong: The addresses are link-local (FE80::/10).
C
EIGRP for IPv6 adjacencies are established over the tunnels.
Neighbors are present with uptime and hold time.
D
The tunnels are using GRE encapsulation.
Why wrong: The output does not specify encapsulation type.

Question 17mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 mtu

IPv6 MTU per interface:

Interface    MTU

Tunnel0 1476 Tunnel1 1476 GigabitEthernet0/0 1500

Based on this output, what is the most likely reason for the reduced MTU on the tunnel interfaces?

Trap 1: The tunnels are configured with a manual MTU.

While possible, the default behavior is to reduce MTU for encapsulation.

Trap 2: The tunnels are using IPsec, which adds more overhead.

IPsec overhead is typically larger and variable; 24 bytes is consistent with GRE.

Trap 3: The tunnels are 6to4 tunnels, which do not reduce MTU.

6to4 tunnels also reduce MTU due to IPv4 encapsulation.

A
The tunnels are configured with a manual MTU.
Why wrong: While possible, the default behavior is to reduce MTU for encapsulation.
B
The tunnels are using GRE encapsulation, which adds 24 bytes of overhead.
GRE adds 4 bytes to the 20-byte IPv4 header, totaling 24 bytes overhead.
C
The tunnels are using IPsec, which adds more overhead.
Why wrong: IPsec overhead is typically larger and variable; 24 bytes is consistent with GRE.
D
The tunnels are 6to4 tunnels, which do not reduce MTU.
Why wrong: 6to4 tunnels also reduce MTU due to IPv4 encapsulation.

Question 18mediummultiple choice

A network engineer runs the following command on Router R1:

R1# show ipv6 traffic | include tunnel

0 tunnel packets received 0 tunnel packets sent 0 tunnel packets dropped

Based on this output, what can be concluded?

Trap 1: The tunnel is passing traffic normally.

Zero packets indicate no traffic.

Trap 2: The tunnel is dropping all packets.

Dropped packets are also zero.

Trap 3: The tunnel is using IPsec encryption.

The output does not indicate encryption.

A
The tunnel is passing traffic normally.
Why wrong: Zero packets indicate no traffic.
B
The tunnel is not carrying any IPv6 traffic.
All counters are zero, meaning no traffic.
C
The tunnel is dropping all packets.
Why wrong: Dropped packets are also zero.
D
The tunnel is using IPsec encryption.
Why wrong: The output does not indicate encryption.

Question 19mediummultiple choice

Examine the following partial configuration on Router R1: ```

interface Tunnel0

ipv6 address 2001:DB8:1::1/64 tunnel source GigabitEthernet0/0 tunnel destination 2001:DB8:2::2 tunnel mode ipv6ip ``` What is the effect of this configuration?

Trap 1: The tunnel will successfully encapsulate IPv6 packets in IPv4 and…

The tunnel destination is an IPv6 address, but ipv6ip mode requires an IPv4 destination.

Trap 2: The tunnel will use IPv6 as the transport protocol, encapsulating…

ipv6ip mode encapsulates IPv6 in IPv4, not the reverse.

Trap 3: The configuration is correct for a 6to4 tunnel.

6to4 tunnels use a different tunnel mode and automatic destination calculation.

A
The tunnel will successfully encapsulate IPv6 packets in IPv4 and forward them.
Why wrong: The tunnel destination is an IPv6 address, but ipv6ip mode requires an IPv4 destination.
B
The tunnel will not come up because the tunnel destination is an IPv6 address instead of an IPv4 address.
In ipv6ip mode, both tunnel source and destination must be IPv4 addresses.
C
The tunnel will use IPv6 as the transport protocol, encapsulating IPv4 packets.
Why wrong: ipv6ip mode encapsulates IPv6 in IPv4, not the reverse.
D
The configuration is correct for a 6to4 tunnel.
Why wrong: 6to4 tunnels use a different tunnel mode and automatic destination calculation.

Question 20mediummultiple choice

Consider this configuration on Router R2: ```

interface Tunnel0

ipv6 address 2001:DB8:3::1/64 tunnel source GigabitEthernet0/0 tunnel mode gre ipv6 tunnel destination 2001:DB8:4::2 ``` Which statement is true?

Trap 1: This tunnel can only carry IPv6 traffic.

GRE can carry multiple protocols; the transport is IPv6, but payload can be IPv4, IPv6, etc.

Trap 2: The tunnel source and destination must be IPv4 addresses.

With 'gre ipv6', the transport is IPv6, so source and destination are IPv6.

Trap 3: The tunnel mode should be 'ipv6ip' for IPv6 transport.

ipv6ip is for IPv6 over IPv4, not GRE over IPv6.

A
This tunnel can only carry IPv6 traffic.
Why wrong: GRE can carry multiple protocols; the transport is IPv6, but payload can be IPv4, IPv6, etc.
B
The tunnel source and destination must be IPv4 addresses.
Why wrong: With 'gre ipv6', the transport is IPv6, so source and destination are IPv6.
C
This tunnel will encapsulate packets using GRE over an IPv6 transport.
GRE over IPv6 is correctly configured, allowing multiprotocol payload.
D
The tunnel mode should be 'ipv6ip' for IPv6 transport.
Why wrong: ipv6ip is for IPv6 over IPv4, not GRE over IPv6.