300-410 DHCP (IPv4 and IPv6) — 30 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.

Question 11

A network engineer runs the following command on Router R1:

R1# show ip dhcp conflict
IP address       Detection method     Detection time        VRF
192.168.1.20     Ping                 Mar 01 2020 01:00 AM  
192.168.1.21     Gratuitous ARP       Mar 01 2020 01:05 AM

Based on this output, what is the problem?

Accepted Answer

The DHCP server has detected IP address conflicts, meaning another device on the network is using the same IP addresses.. The `show ip dhcp conflict` command displays IP addresses that the DHCP server has detected as already in use on the network. The detection methods (Ping and Gratuitous ARP) confirm that another device is responding to these addresses, indicating a conflict. This output directly shows that the DHCP server is functioning but has identified conflicts, meaning another host is using the same IP addresses.

Answer

The DHCP server is working normally; conflicts are automatically resolved.

Answer

The DHCP server is not responding to client requests.

Answer

The DHCP pool is misconfigured with overlapping subnets.

Question 12

A network engineer runs the following command on Router R1:

R1# show ipv6 dhcp binding
Client: FE80::A8BB:CCFF:FE01:0200
  DUID: 00030001AABBCC010200
  Username: unassigned
  IA NA: IA ID 0x00010001, T1 302400, T2 483840
    Address: 2001:DB8:1::1000
      Preferred lifetime 604800, valid lifetime 2592000
      Expires at Mar 08 2020 12:00 AM (2592000 seconds)
Client: FE80::A8BB:CCFF:FE01:0300
  DUID: 00030001AABBCC010300
  Username: unassigned
  IA NA: IA ID 0x00010001, T1 302400, T2 483840
    Address: 2001:DB8:1::1001
      Preferred lifetime 604800, valid lifetime 2592000
      Expires at Mar 08 2020 12:00 AM (2592000 seconds)

Based on this output, which statement is correct?

Accepted Answer

The DHCPv6 server is functioning correctly with two active bindings.. The output shows two DHCPv6 clients with unique link-local addresses and DUIDs, each assigned a distinct IPv6 address from the 2001:DB8:1::/64 prefix. The presence of valid lifetimes and T1/T2 timers indicates the DHCPv6 server is operating normally, maintaining two active bindings. Option B correctly identifies this as proper server behavior.

Answer

The DHCPv6 server has assigned duplicate addresses to the clients.

Answer

The DHCPv6 server is not using a pool; addresses are statically assigned.

Answer

The DHCPv6 server has a DUID conflict.

Question 13

A network engineer runs the following command on Router R1:

R1# show ip dhcp server statistics
Memory usage         26140
Address conflicts    0

Pool statistics

Pool              IP addresses    Requests     Offers      Acks     Naks     Declines     Releases
  POOL1             10-20           50           45          40       5        2            3

Based on this output, which statement is correct?

Accepted Answer

The DHCP server is rejecting requests (Naks) and clients are declining offers, indicating possible pool exhaustion or address conflicts.. The output shows 5 NAKs and 2 Declines, indicating that the DHCP server is rejecting requests (NAKs) and clients are declining offers (Declines). NAKs typically occur when a client requests an IP address that is no longer valid or available, while Declines happen when a client detects an address conflict via ARP. This combination strongly suggests pool exhaustion or address conflicts, making option B correct.

Answer

The DHCP server is operating without any issues.

Answer

The DHCP server has a memory problem.

Answer

The DHCP server has no address conflicts.

Question 14

A network engineer runs the following command on Router R1:

R1# show ip dhcp pool POOL1
Pool POOL1 :
  Utilization mark (high/low)    : 100 / 0
  Subnet size (first/next)       : 0 / 0
  Total addresses                : 10
  Leased addresses               : 10
  Pending event                  : none
  1 subnet is currently in the pool :
  Current index        IP address range                    Leased addresses
  192.168.1.11         192.168.1.10    - 192.168.1.19       10

Based on this output, which statement is correct?

Accepted Answer

The DHCP pool is fully utilized; no more addresses are available.. The output shows that all 10 addresses in the pool (192.168.1.10–192.168.1.19) are leased, and the current index is 192.168.1.11, which is beyond the first address. This means no free addresses remain, so the pool is fully utilized. Option B correctly states this condition.

Answer

The DHCP pool has available addresses for new clients.

Answer

The DHCP pool is configured with a /24 subnet.

Answer

The DHCP server has a pending event causing address allocation to fail.

Question 15

A network engineer runs the following command on Router R1:

R1# show ip dhcp snooping binding
MacAddress          IpAddress        Lease(sec)  Type           VLAN  Interface
AA:BB:CC:01:02:03   192.168.1.10     86400       dhcp-snooping   10    GigabitEthernet0/1
AA:BB:CC:01:02:04   192.168.1.11     86400       dhcp-snooping   10    GigabitEthernet0/1
AA:BB:CC:01:02:05   192.168.1.12     86400       dhcp-snooping   10    GigabitEthernet0/2

Based on this output, which statement is correct?

Accepted Answer

DHCP snooping is enabled and has recorded bindings for three clients.. Option A is correct because the output shows three DHCP snooping bindings with MAC addresses, IP addresses, lease times, and associated VLANs and interfaces, which indicates that DHCP snooping is enabled and has successfully recorded these bindings for clients. The 'dhcp-snooping' type confirms these are dynamically learned from DHCP messages, and the presence of multiple clients on the same interface (GigabitEthernet0/1) is valid as long as they are on the same VLAN.

Answer

DHCP snooping has detected a rogue DHCP server on GigabitEthernet0/1.

Answer

DHCP snooping is not functioning because the bindings are not trusted.

Answer

DHCP snooping has a conflict because two clients are on the same interface.

Question 16

A network engineer runs the following command on Router R1:

R1# show ipv6 dhcp interface GigabitEthernet0/1
GigabitEthernet0/1 is in server mode
  Using pool: POOL6
  Preference value: 0
  Hint from client: ignored
  Rapid-Commit: disabled

Based on this output, which statement is correct?

Accepted Answer

The interface is configured as a DHCPv6 server using pool POOL6.. The command output explicitly states 'GigabitEthernet0/1 is in server mode' and 'Using pool: POOL6', which confirms that the interface is acting as a DHCPv6 server. The DHCPv6 server assigns IPv6 addresses and other configuration parameters from the specified pool to requesting clients.

Answer

The interface is configured as a DHCPv6 client.

Answer

The interface is using rapid-commit for faster address assignment.

Answer

The interface is in DHCPv6 relay mode.

Question 17

A network engineer runs the following command on Router R1:

R1# show ip dhcp relay information trusted
Interface                Trusted
GigabitEthernet0/1       Yes
GigabitEthernet0/2       No

Based on this output, which statement is correct?

Accepted Answer

DHCP relay information is trusted on GigabitEthernet0/1, so option 82 packets are accepted on that interface.. The command 'show ip dhcp relay information trusted' displays the trust status of each interface for DHCP relay information (option 82). When an interface is marked as 'Yes' under the Trusted column, it means the router will accept and forward DHCP packets that already contain option 82 information from that interface. Therefore, on GigabitEthernet0/1, option 82 packets are accepted.

Answer

DHCP relay information is trusted on both interfaces.

Answer

DHCP relay information is not configured on any interface.

Answer

DHCP relay information is trusted on GigabitEthernet0/2, so option 82 packets are dropped.

Question 18

A network engineer runs the following command on Router R1:

R1# show ip dhcp database
URL : flash:/dhcpdb
Read/write : Read/Write
Last updated : Mar 01 2020 00:00:00
Status : Last read succeeded. Next write scheduled in 0 seconds.

Based on this output, which statement is correct?

Accepted Answer

The DHCP database is stored in flash and is functioning correctly.. The output shows the DHCP database is stored in flash (URL: flash:/dhcpdb) with read/write access, and the status indicates the last read succeeded with a write scheduled in 0 seconds. This confirms the database is functioning correctly, as the router can read and write DHCP bindings to flash memory, which is the default storage location for DHCP database persistence.

Answer

The DHCP database is stored in NVRAM and is read-only.

Answer

The DHCP database write failed and needs to be rescheduled.

Answer

The DHCP database is empty because no bindings exist.

Question 19

Examine the following DHCP configuration on router R1:

ip dhcp excluded-address 192.168.1.1 192.168.1.10
!
ip dhcp pool POOL1
 network 192.168.1.0 255.255.255.0
 default-router 192.168.1.1
 dns-server 8.8.8.8
 lease 0 2 30
!
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip helper-address 192.168.2.2
 no shutdown

What is the effect of this configuration?

Accepted Answer

Clients on the 192.168.1.0/24 subnet will receive IP addresses from the local DHCP pool, and the ip helper-address will forward DHCP requests to 192.168.2.2, causing duplicate offers.. Option A is correct because the router R1 is configured as a DHCP server for the 192.168.1.0/24 subnet via the local pool POOL1, and the ip helper-address 192.168.2.2 on the same interface will forward broadcast DHCP requests from clients to the remote DHCP server at 192.168.2.2. This results in both the local pool and the remote server offering IP addresses, leading to duplicate offers on the subnet.

Answer

The ip helper-address command will be ignored because the router is the DHCP server for that subnet.

Answer

The excluded-address range is misconfigured because it includes the router's own interface IP, which will prevent the router from functioning.

Answer

The DHCP pool will not assign addresses because the network statement does not match the interface subnet exactly.

Question 20

Consider the following DHCPv6 configuration on router R2:

ipv6 dhcp pool DHCP6_POOL
 dns-server 2001:db8::1
 domain-name example.com
!
interface GigabitEthernet0/0
 ipv6 address 2001:db8:1::1/64
 ipv6 dhcp server DHCP6_POOL
 ipv6 nd other-config-flag
 no shutdown

What is the effect of this configuration?

Accepted Answer

Hosts on this subnet will use SLAAC to obtain their IPv6 address and then use DHCPv6 to get DNS and domain information.. Option A is correct because the configuration uses the `ipv6 nd other-config-flag` command, which sets the 'Other Configuration' flag (O-flag) in Router Advertisement (RA) messages. This tells hosts to use Stateless Address Autoconfiguration (SLAAC) for their IPv6 address (based on the prefix in the RA) and then use DHCPv6 (stateless DHCPv6) only to obtain additional parameters like DNS server and domain name, as defined in the DHCPv6 pool.

Answer

Hosts will obtain both their IPv6 address and DNS information from the DHCPv6 pool.

Answer

The DHCPv6 pool is missing the address prefix, so it will not provide any configuration to clients.

Answer

The ipv6 nd other-config-flag command is ignored because the DHCPv6 server is configured on the interface.