300-410 DHCP (IPv4 and IPv6) — Timed 10 Questions

Question 1

A network engineer is troubleshooting a DHCPv4 issue on a Cisco router configured as a DHCP server. Clients on VLAN 10 are unable to obtain IP addresses. The engineer verifies that the DHCP pool is correctly configured and that the router interface facing the clients has 'ip helper-address 192.168.1.1' pointing to the DHCP server. However, the DHCP server is on a different subnet and the router's interface is in a VRF. The DHCP DISCOVER messages are not reaching the server. What is the most likely cause?

Accepted Answer

The 'ip helper-address' command must be configured under the interface with the VRF name using 'ip helper-address vrf

'.. The DHCP relay agent (the router) must be configured to forward DHCP broadcasts across VRFs using the 'ip helper-address vrf ' command, otherwise the packets are dropped because the interface is in a VRF and the helper address is not VRF-aware.

Answer

The DHCP pool is missing the 'default-router' command.

Answer

The router needs the 'ip dhcp relay information option' command to insert Option 82.

Answer

The DHCP server is not configured with a scope for the client subnet.

Question 2

An engineer is troubleshooting an IPv6 deployment where hosts on a subnet are not receiving IPv6 addresses via SLAAC. The router is configured with 'ipv6 unicast-routing' and the interface has 'ipv6 address 2001:db8:1::1/64' and 'ipv6 nd other-config-flag'. The hosts are sending Router Solicitations but receive no Router Advertisements. What is the root cause?

Accepted Answer

The 'ipv6 nd ra suppress' command is configured on the interface.. For SLAAC to work, the router must send Router Advertisements (RAs). The 'ipv6 nd other-config-flag' only sets the 'O' flag for additional information (like DNS), but does not enable RA generation. The interface must have an 'ipv6 enable' command or a configured unicast address to automatically enable RA generation, but here the address is configured, so RA generation should be on by default. However, the 'ipv6 nd prefix' command may be missing or the interface may be administratively down. The most common cause is that the interface is not 'no shutdown' or the RA interval is set to 0. Given the symptom, the likely cause is that the interface is in a state where RAs are suppressed, such as when 'ipv6 nd ra suppress' is configured.

Answer

The interface is missing the 'ipv6 enable' command.

Answer

The 'ipv6 nd prefix' command is missing for the subnet.

Answer

The hosts are using DHCPv6 instead of SLAAC.

Question 3

A network engineer is troubleshooting a DHCPv4 relay scenario where clients on subnet 10.1.1.0/24 are unable to obtain IP addresses from a DHCP server at 192.168.1.10. The router interface Gi0/0 (10.1.1.1/24) has 'ip helper-address 192.168.1.10' configured. The engineer captures packets and sees DHCP DISCOVER messages sourced from 10.1.1.1 being sent to 192.168.1.10, but no replies are seen. The server is reachable via ping from the router. What is the most likely cause?

Accepted Answer

The DHCP server is not configured with a scope for subnet 10.1.1.0/24.. The DHCP server may not have a route back to the client subnet (10.1.1.0/24) or the server's default gateway does not have a route. The relay agent sets the gateway IP address (giaddr) to the interface IP (10.1.1.1), and the server sends the OFFER back to that giaddr. If the server cannot reach 10.1.1.1, the reply is lost. However, the engineer confirms the server is reachable via ping, so the issue is likely that the server is not configured with a scope for the 10.1.1.0/24 subnet, causing it to drop the DISCOVER.

Answer

The DHCP server does not have a route to 10.1.1.0/24.

Answer

The 'ip helper-address' command should be configured on the server-facing interface, not the client-facing interface.

Answer

The router needs the 'ip dhcp relay information option' command.

Question 4

An engineer is troubleshooting a DHCPv6 stateful (DHCPv6) deployment. The router is configured as a DHCPv6 server with a pool for prefix 2001:db8:2::/64. Clients on the LAN are configured to use DHCPv6, but they are not receiving IPv6 addresses. The router interface has 'ipv6 address 2001:db8:2::1/64' and 'ipv6 dhcp server DHCP_POOL'. The engineer sees that the clients are sending SOLICIT messages, but the router sends no REPLY. What is the issue?

Accepted Answer

The DHCPv6 pool is missing the 'address prefix 2001:db8:2::/64' command.. For stateful DHCPv6, the router must also send Router Advertisements with the 'M' (Managed) flag set to indicate that clients should use DHCPv6. Without the 'ipv6 nd managed-config-flag' command, clients may not request addresses via DHCPv6, or the server may not respond appropriately. However, the symptom says clients are sending SOLICIT messages, so they are attempting DHCPv6. The router is not replying, which could be due to a misconfigured pool or the server not being enabled. The most likely cause is that the DHCPv6 pool does not have an 'address prefix' statement, so the server has no addresses to assign.

Answer

The interface is missing the 'ipv6 nd managed-config-flag' command.

Answer

The router needs the 'ipv6 dhcp relay' command on the interface.

Answer

The 'ipv6 unicast-routing' command is missing globally.

Question 5

A network engineer is troubleshooting a DHCPv4 issue where clients on a subnet are getting IP addresses from the correct pool, but they cannot reach the default gateway. The router is configured as a DHCP server with pool 'POOL' that includes 'default-router 192.168.1.1'. The router's interface IP is 192.168.1.1/24. Clients receive the address and default gateway, but pings to 192.168.1.1 fail. What is the most likely cause?

Accepted Answer

The router interface Gi0/0 is administratively down.. If the router's interface is up and has the correct IP, but clients cannot ping the gateway, the issue is often that the interface is in a different VLAN or the clients are on a different broadcast domain. However, the most common misconfiguration is that the 'default-router' command points to an IP that is not actually configured on the router's interface, or the interface is down. Given that the symptom is specific to the gateway, the likely cause is that the interface is shut down or has a different IP.

Answer

The DHCP pool has the wrong subnet mask.

Answer

The 'ip helper-address' command is interfering with DHCP.

Answer

The clients have a static ARP entry for the gateway.

Question 6

An engineer is troubleshooting a DHCPv4 issue where a Cisco router acting as a DHCP client on interface Gi0/0 is not receiving an IP address from an ISP modem. The router has 'ip address dhcp' on the interface. The engineer sees that the interface is up/up, but no IP address is assigned. Debug shows that the router is sending DHCP DISCOVER messages but receives no OFFER. The ISP modem is known to work with other devices. What is the most likely cause?

Accepted Answer

The router is sending the client identifier as the MAC address in a non-standard format; the modem expects the client identifier to be the MAC address only.. The router is sending DHCP DISCOVER messages but receiving no OFFER, indicating the ISP modem is ignoring the requests. By default, Cisco routers send the client identifier in DHCP packets as a concatenation of the hardware type (1 for Ethernet) and the MAC address (e.g., '0100.1122.3344'). Many ISP modems expect the client identifier to be just the MAC address (without the hardware type prefix), or they may not use client identifier matching at all. The 'ip dhcp client client-id' command can be used to override this behavior, making the router send only the MAC address as the client identifier, which resolves the incompatibility.

Answer

The router needs the 'ip dhcp client broadcast-flag' command.

Answer

The router's interface is in a VRF, and the DHCP client needs VRF awareness.

Answer

The ISP modem requires DHCP Option 82 to be present.

Question 7

A network engineer is troubleshooting a DHCPv4 issue where a router configured as a DHCP server is not assigning addresses from a pool to clients on a specific VLAN. The pool is configured with 'network 10.1.1.0 255.255.255.0' and 'default-router 10.1.1.1'. The router's interface Gi0/0.10 (subinterface) has encapsulation dot1Q 10 and IP 10.1.1.1/24. Clients send DISCOVER messages, but the router does not respond. The engineer notices that the router has multiple DHCP pools configured. What is the most likely cause?

Accepted Answer

The DHCP pool is configured under a VRF, but the interface is not in that VRF.. The router has multiple DHCP pools configured, and the pool for VLAN 10 is likely bound to a VRF. When a DHCP pool is configured under a VRF, the router only responds to DHCP DISCOVER messages received on interfaces that belong to that same VRF. Since the subinterface Gi0/0.10 is not in the VRF, the router ignores the client broadcasts, even though the IP address and subnet match the pool.

Answer

The subinterface is missing the 'ip helper-address' command.

Answer

The 'ip dhcp server' command is missing globally.

Answer

The encapsulation dot1Q 10 is misconfigured, causing the router to not receive broadcasts.

Question 8

An engineer is troubleshooting a DHCPv6 prefix delegation scenario. The router (R1) is a DHCPv6 client on its WAN interface and is supposed to receive a /48 prefix from the ISP's DHCPv6 server to delegate to LAN interfaces. The WAN interface is configured with 'ipv6 address dhcp' and 'ipv6 dhcp client pd ISP-PREFIX'. The LAN interfaces have 'ipv6 address ISP-PREFIX 0:0:0:1::/64 eui-64'. The engineer sees that the WAN interface gets a global IPv6 address, but the LAN interfaces do not get any IPv6 address. What is the most likely cause?

Accepted Answer

The ISP's DHCPv6 server is not configured to delegate a prefix.. The WAN interface successfully obtains a global IPv6 address via DHCPv6, but the LAN interfaces lack IPv6 addresses because the DHCPv6 client process on R1 has not received a delegated prefix. The 'ipv6 dhcp client pd ISP-PREFIX' command requests a prefix delegation from the ISP's DHCPv6 server; if the server is not configured to delegate prefixes (e.g., missing the 'prefix-delegation' pool or 'ipv6 dhcp server' with prefix assignment), the client never receives a prefix to assign to LAN interfaces. This is the most likely cause given the symptom.

Answer

The LAN interfaces are missing the 'ipv6 enable' command.

Answer

The 'ipv6 dhcp client pd' command should be 'ipv6 dhcp client pd ISP-PREFIX hint ::/48'.

Answer

The WAN interface needs the 'ipv6 nd other-config-flag' command.

Question 9

A network engineer is troubleshooting a DHCPv4 issue where a router configured as a DHCP server is not assigning addresses to clients on a subnet that is reachable via a different router (relay). The relay router (R2) has 'ip helper-address 10.1.1.1' on its client-facing interface, and the DHCP server is at 10.1.1.1 (R1). The engineer sees that R2 is sending DHCP DISCOVER messages with giaddr set to the client-facing interface IP, but R1 is not responding. R1 has a DHCP pool for the client subnet. The engineer pings 10.1.1.1 from R2 successfully. What is the most likely cause?

Accepted Answer

The DHCP server does not have a route to the client subnet (the giaddr subnet).. The DHCP server may not have a route back to the client subnet (the giaddr subnet). Even though the server's interface IP is reachable, the server needs to send the OFFER to the giaddr (which is the relay agent's interface IP). If the server does not have a route to that subnet, the OFFER will be dropped. The ping from R2 to R1 succeeds because R1's interface is directly connected, but the return traffic from R1 to the giaddr (which is on a different subnet) may fail if R1 does not have a route.

Answer

The relay agent R2 is missing the 'ip dhcp relay information option' command.

Answer

The DHCP pool on R1 is missing the 'default-router' command.

Answer

The 'ip helper-address' on R2 should point to the server's loopback address, not the interface IP.

Question 10

A network engineer runs the following command on Router R1:

R1# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address       Client-ID/              Lease expiration        Type
                 Hardware address/
                 User name
192.168.1.10     0063.6973.636f.2e30.    Mar 01 2020 12:00 AM    Automatic
                 3030.3030.2e30.3030
                 312e.3130.3030.2e30
                 3030.312d.4574.682d
                 302f.31
192.168.1.11     0063.6973.636f.2e30.    Mar 01 2020 12:00 AM    Automatic
                 3030.3030.2e30.3030
                 312e.3130.3030.2e30
                 3030.312d.4574.682d
                 302f.31

Based on this output, which statement is correct?

Accepted Answer

The DHCP server has a duplicate client-id issue, likely caused by two clients using the same client identifier.. The output shows two DHCP bindings with identical client identifiers (the long hexadecimal string). This indicates that two clients are using the same client-ID, which violates RFC 2131 and causes the DHCP server to treat them as the same client, leading to duplicate address assignments or conflicts. Option B correctly identifies this duplicate client-id issue.

Answer

The DHCP server is functioning correctly with two unique bindings.

Answer

The DHCP server has run out of addresses in the pool.

Answer

The DHCP server is not assigning addresses because the lease time is set to 0.