300-410 IPv6 Tunneling Techniques Practice Test 3 — 15 Questions

Question 1

An MPLS network with IPv6 over MPLS (6PE) is experiencing loss of IPv6 routes from a remote provider edge (PE) router. Router PE1 has the following relevant configuration: interface GigabitEthernet0/0 ipv6 address 2001:DB8:1::1/64 mpls ip interface Loopback0 ip address 192.0.2.1 255.255.255.255 router ospf 1 router-id 192.0.2.1 redistribute bgp 65000 subnets. Router PE2 shows: PE2# show bgp ipv6 unicast 2001:DB8:2::/64 % Network not in table. PE2# show mpls forwarding-table 192.0.2.1 Label: 16, Interface: GigabitEthernet0/1. What is the root cause?

Accepted Answer

PE1 is missing the network 2001:DB8:2::/64 command under router bgp for IPv6 unicast address family.. The correct answer is A because the output shows that PE2 has an MPLS label (16) for PE1's loopback (192.0.2.1) and can forward labeled traffic, but the IPv6 route 2001:DB8:2::/64 is missing from the BGP table. This indicates that PE1 is not advertising the IPv6 prefix into BGP. The missing `network 2001:DB8:2::/64` command under the IPv6 unicast address family on PE1 prevents the prefix from being injected into BGP, even though the interface is configured with the IPv6 address and OSPF redistribution is in place.

Answer

The MPLS label distribution between PE1 and PE2 is failing due to LDP mismatch.

Answer

OSPF is not redistributing the IPv6 prefix correctly.

Answer

The IPv6 address family is not enabled under router bgp on PE1.

Question 2

A dual-stack network using IPv6 over IPv4 GRE tunnels with OSPFv3 is experiencing routing loops. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 ipv6 ospf network point-to-point ipv6 ospf 1 area 0. Router R2 shows: R2# show ipv6 ospf neighbor Neighbor ID Pri State Dead Time Interface ID Interface 192.0.2.2 1 FULL/ - 00:00:33 7 Tunnel0. R2# show ipv6 route 2001:DB8:3::/64 O 2001:DB8:3::/64 [110/2] via 2001:DB8:1::1, Tunnel0. What is the root cause?

Accepted Answer

R1 has a higher metric for the same prefix via another interface, causing a routing loop due to OSPF's SPF algorithm.. The routing loop is caused by R1 having a higher metric for the prefix 2001:DB8:3::/64 via another interface, which OSPF's SPF algorithm still installs in the routing table. Since R2's best path to that prefix points to R1 via Tunnel0 (metric 2), and R1's best path points back to R2 via a different interface with a higher metric, traffic bounces between them. This is a classic OSPF metric asymmetry issue in a dual-stack GRE tunnel environment.

Answer

The OSPF network type on Tunnel0 should be broadcast to match the underlying GRE tunnel.

Answer

The tunnel destination on R1 is incorrect, causing OSPF adjacencies to form with the wrong router.

Answer

IPv6 unicast-routing is not enabled on R1.

Question 3

An enterprise is using IPv6-to-IPv4 translation tunneling (NAT64) but users report that they cannot reach IPv6-only servers. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 nat64 enable. Router R2 shows: R2# show nat64 translations % No translations. R2# debug nat64 all NAT64: No mapping found for packet. What is the root cause?

Accepted Answer

The NAT64 prefix is not configured under the tunnel interface.. Option A is correct because NAT64 requires a NAT64 prefix to be explicitly configured under the tunnel interface using the 'nat64 prefix' command. Without this prefix, the router cannot construct the IPv6 representation of IPv4 destinations, so no translation mappings are created, and packets are dropped. The debug output confirms 'No mapping found for packet', which directly points to the missing prefix configuration.

Answer

The tunnel mode should be ipv6ip for NAT64 to work.

Answer

The IPv4 address of the destination server is not reachable.

Answer

NAT64 is not supported on tunnel interfaces.

Question 4

A network using IPv6 over IPv4 ISATAP tunnels is experiencing connectivity issues. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::/64 eui-64 tunnel source 192.0.2.1 tunnel mode ipv6ip isatap. Router R2 shows: R2# show ipv6 interface Tunnel0 Tunnel0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::5EFE:192.0.2.2 Global unicast address(es): 2001:DB8:1::5EFE:192.0.2.2. What is the root cause?

Accepted Answer

The tunnel mode should be ipv6ip isatap on both ends, but R2's address shows correct ISATAP format, indicating R1's address is wrong.. Option B is correct because ISATAP tunnels require both endpoints to use the same tunnel mode (ipv6ip isatap) and to form the IPv6 interface identifier from the IPv4 tunnel source using the format ::5EFE:w.x.y.z. R2's global address 2001:DB8:1::5EFE:192.0.2.2 correctly embeds its IPv4 address 192.0.2.2, but R1's configuration with the eui-64 keyword causes it to derive an interface identifier from its MAC address instead of the required ISATAP format. This mismatch prevents proper IPv6 neighbor discovery and connectivity over the tunnel.

Answer

The IPv6 address on R1 should be configured without the eui-64 keyword to allow manual assignment.

Answer

The tunnel source on R1 should be the IPv4 address of R2.

Answer

ISATAP requires the use of a link-local address only.

Question 5

A service provider is using 6rd (IPv6 Rapid Deployment) tunnels to provide IPv6 to customers. Customer router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel mode ipv6ip 6rd ipv6 6rd prefix 2001:DB8::/32 6rd-br 198.51.100.1. Router R2 (BR) shows: R2# show ipv6 route 2001:DB8:1::/64 % Route not found. What is the root cause?

Accepted Answer

The 6rd prefix on R1 should match the BR's configured 6rd prefix, and the BR must have a route for the customer's delegated prefix.. The 6rd (IPv6 Rapid Deployment) tunnel requires both the customer router (R1) and the border relay (BR, R2) to agree on the same 6rd prefix. R1 is configured with prefix 2001:DB8::/32, but the BR has no route for the delegated prefix 2001:DB8:1::/64, which is derived from R1's IPv4 address (192.0.2.1) and the 6rd prefix. Without this route in the BR's IPv6 routing table, the BR cannot forward traffic to the customer's 6rd tunnel, causing the 'Route not found' error.

Answer

The tunnel mode should be ipv6ip 6rd on both ends, but the BR is missing the 6rd configuration.

Answer

The customer's IPv4 address is not reachable from the BR.

Answer

The 6rd prefix length should be /64 instead of /32.

Question 6

A network using IPv6 over IPv4 L2TPv3 tunnels is experiencing packet drops. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 tunnel mode l2tpv3 ipv6. Router R2 shows: R2# show l2tp tunnel Tunnel id 1, 1 session Session id 1, state established. R2# show ipv6 interface Tunnel0 Tunnel0 is up, line protocol is up. What is the root cause?

Accepted Answer

The MTU on the tunnel interface is too high, causing fragmentation of IPv6 packets.. The correct answer is A. In an L2TPv3 tunnel carrying IPv6 traffic, the tunnel interface MTU defaults to 1500 bytes, but the actual path MTU between the tunnel endpoints (192.0.2.1 and 198.51.100.1) is likely lower due to the added L2TPv3, UDP, and IP headers. This causes IPv6 packets larger than the effective path MTU to be dropped because IPv6 routers do not fragment packets; they rely on ICMPv6 Packet Too Big messages, which may be blocked or not processed, leading to packet loss.

Answer

The tunnel mode should be l2tpv3 ipv4 instead of ipv6.

Answer

The session ID on R1 does not match R2's session ID.

Answer

IPv6 routing is not enabled on R2.

Question 7

A network using IPv6 over IPv4 Teredo tunnels is experiencing intermittent connectivity. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:0:4136:E378:8000:63BF:3C57:DD0B/128 tunnel source 192.0.2.1 tunnel mode ipv6ip teredo. Router R2 shows: R2# show ipv6 route 2001:0:4136:E378::/64 % Route not found. What is the root cause?

Accepted Answer

The Teredo server IPv4 address is not configured on R1.. The Teredo tunnel on R1 requires a Teredo server to facilitate the initial configuration and to help the client discover its public IPv4 address and the Teredo relay. Without the 'tunnel teredo server-ip' command specifying the Teredo server's IPv4 address, R1 cannot complete the Teredo setup, leading to an incomplete or non-functional tunnel. This results in the IPv6 prefix 2001:0:4136:E378::/64 not being installed in R2's routing table, as the tunnel interface never becomes fully operational.

Answer

The tunnel mode should be ipv6ip teredo relay instead of teredo.

Answer

The IPv4 address of the Teredo relay is not reachable.

Answer

Teredo is not supported on Cisco IOS.

Question 8

What is the default tunnel mode for an IPv6 tunnel configured on Cisco IOS-XE?

Accepted Answer

GRE. The default tunnel mode for an IPv6 tunnel on Cisco IOS-XE is GRE (Generic Routing Encapsulation), as specified by the 'tunnel mode gre ipv6' command. When you create a tunnel interface and configure an IPv6 tunnel source and destination without explicitly setting the tunnel mode, the router defaults to GRE encapsulation, which provides a multiprotocol transport capable of carrying both IPv4 and IPv6 payloads.

Answer

IPv6IP

Answer

ISATAP

Answer

6to4

Question 9

In a 6to4 tunnel, what is the default IPv6 prefix assigned to the tunnel interface?

Accepted Answer

2002:IPv4-address::/48. In a 6to4 tunnel, the default IPv6 prefix assigned to the tunnel interface is 2002:IPv4-address::/48, where the IPv4 address of the tunnel source is embedded in the prefix. This is defined in RFC 3056, which specifies that the 6to4 prefix is 2002::/16, and the next 32 bits are the tunnel source's IPv4 address, resulting in a /48 prefix for the 6to4 site.

Answer

2002::/16

Answer

2001::/32

Answer

3ffe::/16

Question 10

What is the default keepalive interval for a GRE tunnel interface on Cisco IOS-XE?

Accepted Answer

10 seconds. The default keepalive interval for a GRE tunnel interface on Cisco IOS-XE is 10 seconds. This is defined by the default behavior of the `keepalive` command on tunnel interfaces, which sends periodic keepalive packets to verify the reachability of the tunnel destination. If no keepalive reply is received within three consecutive intervals (30 seconds by default), the tunnel interface is marked as down.

Answer

5 seconds

Answer

15 seconds

Answer

20 seconds

Question 11

Which statement correctly describes the behavior of ISATAP tunneling regarding host configuration?

Accepted Answer

ISATAP embeds the IPv4 address into the interface identifier (last 64 bits) of the IPv6 address.. ISATAP (Intra-Site Automatic Tunnel Addressing Protocol) automatically generates the IPv6 address by embedding the host's IPv4 address into the interface identifier (the last 64 bits of the IPv6 address). This allows hosts to obtain a complete IPv6 address without manual configuration of the full 128-bit address, as the IPv4 address is used to form the unique interface ID. Option C correctly describes this behavior.

Answer

ISATAP requires manual configuration of the entire IPv6 address on each host.

Answer

ISATAP uses the prefix 2002::/16 for global addresses.

Answer

ISATAP is only used for site-to-site tunnels.

Question 12

In a manual IPv6 over IPv4 tunnel (tunnel mode ipv6ip), what is the default maximum number of hops (TTL) for the IPv4 header?

Accepted Answer

255. In a manual IPv6 over IPv4 tunnel (tunnel mode ipv6ip), the IPv4 header's TTL (Time to Live) defaults to 255. This is because the tunnel is treated as a directly connected virtual link, and the IPv4 TTL is set to the maximum value of 255 to minimize the chance of the tunneled packet being dropped due to TTL expiry within the transit IPv4 network. This behavior is defined in RFC 2473 and is the default for IPv6-in-IPv4 tunnels on Cisco IOS.

Answer

64

Answer

128

Answer

16

Question 13

Which loop prevention mechanism is inherent to 6to4 tunneling?

Accepted Answer

Embedded IPv4 address validation. 6to4 tunneling uses an embedded IPv4 address in the IPv6 prefix (2002::/16) to automatically derive the tunnel destination. This inherent validation prevents routing loops by ensuring that a 6to4 router only accepts packets whose source IPv4 address matches the embedded address in the IPv6 source prefix, rejecting mismatched or spoofed traffic that could cause loops.

Answer

Split horizon

Answer

Reverse path forwarding (RPF) check

Answer

TTL decrement

Question 14

What is the default metric for a static route pointing to a tunnel interface in Cisco IOS-XE?

Accepted Answer

0. In Cisco IOS-XE, a static route pointing to a tunnel interface (e.g., a GRE or IPsec tunnel) uses a default administrative distance of 0. This is because the route is considered directly connected to the tunnel interface, and directly connected routes have an administrative distance of 0, overriding any other route type. This behavior is specific to tunnel interfaces and differs from static routes pointing to next-hop IP addresses, which default to an administrative distance of 1.

Answer

1

Answer

10

Answer

20

Question 15

Which authentication method is supported by default for GRE tunnels in Cisco IOS-XE?

Accepted Answer

No authentication by default. GRE tunnels in Cisco IOS-XE do not include any built-in authentication mechanism by default. The GRE protocol (RFC 2784) defines a simple encapsulation method without authentication or encryption; any security features must be added externally, such as using IPsec to protect the tunnel traffic. Therefore, the correct answer is that no authentication is enabled by default.

Answer

MD5 authentication

Answer

SHA-256 authentication

Answer

Plain-text password authentication