300-410 Practice Test 10 — 10 Questions

Question 1

An engineer configures DHCPv6 prefix delegation on a Cisco router acting as a requesting client. The router receives a valid IA_PD prefix from the server, but no route for the delegated prefix appears in the routing table. Which is the most likely explanation?

Accepted Answer

The router's 'ipv6 dhcp client pd' command does not install a route; the engineer must manually configure a static route for the delegated prefix.. DHCPv6 prefix delegation does not automatically install a route for the delegated prefix. The engineer must configure a static route or use a routing protocol to advertise the prefix. This is a common oversight.

Answer

DHCPv6 prefix delegation automatically installs a connected route; the issue is a missing 'ipv6 route' command to point the prefix to the DHCP-learned next-hop.

Answer

The DHCPv6 server must also send a route option; the requesting client cannot install a route without it.

Answer

The router's 'ipv6 unicast-routing' is disabled, preventing route installation.

Question 2

A network engineer configures a DHCP snooping trusted port on a switch to allow a legitimate DHCP server. However, clients connected to untrusted ports on the same VLAN cannot obtain IP addresses. The DHCP server is reachable and the switch shows no DHCP snooping drops. Which is the most likely explanation?

Accepted Answer

The DHCP server is not configured to trust the relay agent information option (option 82), causing it to discard the requests.. DHCP snooping requires IP source guard or DHCP snooping binding to be populated. If the switch has 'ip dhcp snooping information option' enabled, the DHCP server may drop packets due to option 82 being inserted, unless the server is configured to trust it.

Answer

The trusted port is in the wrong VLAN; DHCP snooping only works when the trusted port is in the same VLAN as the clients.

Answer

The switch has 'ip dhcp snooping verify mac-address' enabled, causing MAC address mismatches.

Answer

The DHCP server is not configured with the correct subnet for the client VLAN.

Question 3

An engineer configures a DHCP server on a Cisco router to assign IP addresses from a pool. Clients receive addresses, but when the engineer checks 'show ip dhcp binding', no bindings are displayed. Which is the most likely explanation?

Accepted Answer

The 'ip dhcp excluded-address' command includes the entire pool range, so no addresses are assigned and no bindings are created.. The 'show ip dhcp binding' command only displays bindings that are currently active. If the lease time is very short or the clients release addresses, bindings may not appear. However, a more subtle edge case is when the DHCP server is configured with 'ip dhcp excluded-address' that includes the pool range, causing the server to not assign addresses but still respond with NAKs.

Answer

The DHCP pool is configured with 'lease infinite', causing bindings to be stored in a different database.

Answer

The router's DHCP server is configured in 'database' mode, which stores bindings externally.

Answer

The clients are using DHCPv6, not DHCPv4.

Question 4

A router configured as a DHCPv6 relay agent is not forwarding DHCPv6 requests from clients to the server. The relay interface has 'ipv6 dhcp relay destination' configured. Clients are on a different VLAN. Which is the most likely explanation?

Accepted Answer

The relay destination is configured on the client-facing interface; it should be on the server-facing interface.. DHCPv6 relay requires that the relay agent has a route to the DHCP server. If the relay interface is not in the same VRF as the server, or if the server is reachable via a different interface, the relay may fail. A common edge case is that the relay destination is configured on the wrong interface (e.g., the client-facing interface instead of the server-facing interface).

Answer

The DHCPv6 server is not reachable via the relay agent's routing table.

Answer

The relay agent must have 'ipv6 dhcp server' configured to act as a server.

Answer

The clients are using DHCPv4, not DHCPv6.

Question 5

An engineer configures DHCPv4 on a router with multiple pools for different subnets. Clients in subnet A receive addresses correctly, but clients in subnet B receive addresses from subnet A's pool. The router has 'ip dhcp relay' configured. Which is the most likely explanation?

Accepted Answer

The DHCP server selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool.. When a DHCP relay agent forwards requests, it inserts the gateway IP address (giaddr) based on the interface that received the request. If the relay agent is not configured correctly, or if the DHCP server does not have a pool matching the giaddr, it may assign addresses from a different pool. However, a subtle edge case is that the DHCP server uses the giaddr to select the pool; if the relay agent is not configured to set the giaddr (e.g., using 'ip helper-address' without proper interface configuration), the giaddr may be 0.0.0.0, causing the server to use a default pool.

Answer

The pools are configured in the wrong order; the server uses the first pool that matches the client's MAC address.

Answer

The router has 'ip dhcp smart-relay' enabled, which overrides pool selection.

Answer

The DHCP server is configured with 'network' statements that overlap.

Question 6

A router is configured as a DHCPv6 client on an interface using 'ipv6 address dhcp'. The router receives a valid IPv6 address from the server, but the address is not used for outgoing traffic. Which is the most likely explanation?

Accepted Answer

The router has a static IPv6 address configured on the same interface, and the DHCP-assigned address is not preferred for source selection.. When a router uses 'ipv6 address dhcp', it requests a non-temporary address (IANA). However, the router may also have a link-local address and possibly a static address. The source address selection for outgoing traffic is based on the destination and the routing table. If the router has a static global unicast address configured, it may prefer that over the DHCP-assigned address. A common edge case is that the DHCP-assigned address is not preferred due to the 'ipv6 dhcp client' command not setting the address as the primary.

Answer

The DHCPv6 server assigned a temporary address (IA_TA) instead of a non-temporary address (IA_NA).

Answer

The router's 'ipv6 unicast-routing' is disabled.

Answer

The DHCP-assigned address is in a different subnet than the default gateway.

Question 7

An engineer configures DHCP snooping on a switch to prevent rogue DHCP servers. After enabling, legitimate DHCP clients on VLAN 10 cannot obtain addresses. The DHCP server is connected to a trusted port. The switch shows 'show ip dhcp snooping binding' with no entries. Which is the most likely explanation?

Accepted Answer

The DHCP server is on a different subnet, and no DHCP relay is configured; the switch only snoops broadcast traffic within the VLAN.. DHCP snooping builds a binding database by snooping DHCPACK messages. If the switch does not see the DHCPACK (e.g., due to asymmetric routing or the server responding on a different VLAN), no bindings are created. A common edge case is that the DHCP server is on a different subnet and the relay agent is not configured, so the DHCP requests are broadcast and not forwarded.

Answer

The trusted port is configured as an access port in VLAN 10, but the server is in VLAN 20.

Answer

The switch has 'ip dhcp snooping information option' disabled, preventing binding creation.

Answer

The DHCP server is using a different MAC address than expected, causing the switch to drop the packets.

Question 8

An engineer configures IPv6 uRPF strict mode on an interface that is used for both IPv6 traffic and OSPFv3 routing. The router is an ABR with multiple areas. OSPFv3 adjacencies form correctly, but some IPv6 data traffic is dropped. The show ipv6 interface command shows uRPF is enabled. Which is the most likely explanation?

Accepted Answer

The router has a default route pointing to a different interface, and uRPF strict mode without 'allow-default' drops packets whose source address is reachable via the default route.. uRPF strict mode checks the source address of incoming packets against the routing table. For a router that is an ABR, some routes may be learned via OSPFv3 inter-area or external routes. If the routing table has multiple paths, the reverse path may not be via the incoming interface. Additionally, OSPFv3 uses link-local addresses for adjacencies, which are not affected because uRPF is not applied to OSPFv3 packets if the router uses 'ipv6 uRPF' but OSPFv3 packets are sourced from link-local addresses that are not in the routing table, but the engineer may have configured 'allow-default' or not. The edge case: uRPF strict mode can drop traffic that arrives on a different interface than the best reverse path, especially if the router has multiple interfaces in the same area or different areas. However, a more subtle issue: if the router has a default route via another interface, uRPF strict mode without 'allow-default' will drop packets whose source address matches the default route because the default route points to a different interface.

Answer

OSPFv3 adjacencies use link-local addresses, which are not checked by uRPF, but data traffic uses global addresses that are incorrectly filtered by the OSPFv3 process.

Answer

The router has 'ipv6 uRPF allow-default' configured, but the default route is not installed, causing all traffic to be dropped.

Answer

The interface has an IPv6 ACL that denies traffic from certain prefixes, overriding uRPF.

Question 9

A router configured as a DHCPv4 server uses a pool with 'bootfile' and 'next-server' options for PXE boot. Clients receive the DHCP offer with the correct bootfile, but they fail to download it. Which is the most likely explanation?

Accepted Answer

The 'next-server' is configured with a hostname, but the client does not have DNS resolution capabilities at boot time.. The 'next-server' option specifies the TFTP server IP address. If the TFTP server is not reachable from the client's subnet, or if the client cannot route to it, the download fails. A subtle edge case is that the 'next-server' is set to a hostname instead of an IP address, and the client cannot resolve it.

Answer

The 'bootfile' name is case-sensitive, and the client is requesting a different case.

Answer

The DHCP server is not configured with the 'option 150' for TFTP server.

Answer

The client's subnet does not have a route to the TFTP server, but the DHCP server cannot control that.

Question 10

An engineer configures DHCPv6 prefix delegation on a router with multiple requesting interfaces. The router receives a /48 prefix from the server, but only one interface gets a /64 sub-prefix. Which is the most likely explanation?

Accepted Answer

Only one interface is configured with 'ipv6 address prefix-name ::1:0:0:0:1/64' using the delegated prefix; other interfaces lack this configuration.. When using DHCPv6 PD, the router can delegate sub-prefixes to downstream interfaces using the 'ipv6 dhcp client pd' command with a hint. If only one interface has the 'ipv6 address' command with the prefix, only that interface gets an address. The other interfaces may not have the correct configuration to use the delegated prefix.

Answer

The DHCPv6 server only delegates a single /64 prefix, not a /48.

Answer

The router's 'ipv6 dhcp client pd' command is missing the 'rapid-commit' option.

Answer

The router has 'ipv6 dhcp client information refresh' set to a high value, delaying updates.