CCNA Infrastructure Services Questions

The ipv6 dhcp relay destination command configures the interface as a DHCPv6 relay agent and specifies the destination DHCPv6 server address. This command is applied to the interface facing the clients. The router will then forward DHCPv6 messages from clients to the specified server and relay responses back. Without this command, the router will not relay DHCPv6 messages, and clients will not receive addresses from the remote server.

The crypto pki authenticate command retrieves and installs the CA certificate, which is necessary for the router to validate client certificates. It authenticates the CA by obtaining its self-signed certificate and installing it into the router's certificate store. This step is mandatory before the router can trust certificates issued by that CA, enabling proper certificate chain validation for the VPN client.

In Cisco IOS XE, when using MP-BGP for MPLS L3VPN, each VRF must have an address family configured under the BGP process. Specifically, the 'address-family ipv4 vrf CUSTOMER' command must be present, and the eBGP neighbor must be activated within that address family. Without this, BGP will not exchange routes for that VRF, even if the global BGP session is up.

VRRPv2 uses a virtual MAC address derived from the VRRP group ID. The format is 0000.0c07.acXX, where XX is the group ID in hexadecimal. This ensures that the virtual MAC remains consistent regardless of which router is the master. The virtual IP and MAC are maintained during failover, providing seamless redundancy. This is a key characteristic of VRRP, distinguishing it from HSRP, which uses a different MAC format.

The ip nhrp shortcut command on the spoke enables it to intercept transit traffic that the hub redirects. When the hub sends an NHRP redirect message, the spoke creates a shortcut entry in its NHRP mapping table and can then establish a direct tunnel to the destination spoke. Without this command, the spoke continues to forward all traffic through the hub.

IP SLA operations can monitor reachability by sending probes such as ICMP echoes. The track object tracks the state of the SLA operation. When the tracked object goes down, any static route referencing that object is removed from the routing table, allowing the backup route to be used. When the SLA recovers, the primary route is reinstalled. This provides fast failover and automatic restoration, meeting the requirements.

The 'ipv6 nd managed-config-flag' command sets the Managed Address Configuration flag in Router Advertisement messages, instructing clients to use DHCPv6 for address assignment. Without this flag, clients will use stateless autoconfiguration and ignore DHCPv6 for addresses. This is required for stateful DHCPv6 operation.

The command ipv6 ospf 1 area 0 is used in interface configuration mode to enable OSPFv3 on an interface and assign it to area 0. This command activates OSPFv3 for IPv6 on that interface, allowing it to form adjacencies. It is the correct way to enable OSPFv3 on a per-interface basis.

The 'set ip next-hop' command in a route map specifies the next-hop IP address for policy-based routing. When applied to an interface, it overrides the normal routing table for matching traffic. This is the correct action to direct traffic from 192.168.1.0/24 to 10.1.1.2. It is the standard method for PBR next-hop configuration.

MPLS Layer 3 VPNs require MP-BGP to exchange VPNv4 routes between PE routers. The `address-family vpnv4` configuration enables the VPNv4 address family, and neighbors must be activated within that address family. This allows the PE routers to advertise customer routes with route targets and VPN labels. Without this, VPNv4 routes cannot be exchanged, and the VPN will not function.

The ip helper-address command, configured on the interface receiving the DHCP broadcasts, forwards these UDP broadcasts (ports 67 and 68) to the specified server as unicast packets. This allows a single DHCP server to service multiple subnets without being directly attached to each. It is the correct and standard method for DHCP relay on Cisco IOS.

This command creates an SNMPv3 user 'admin' with SHA authentication and AES 128-bit encryption. The 'auth sha' specifies SHA authentication, and 'priv aes 128' specifies AES encryption with a 128-bit key. This meets the requirement for both authentication and encryption.

The default-router command in DHCP pool configuration mode specifies the default gateway address that DHCP clients receive. In this scenario, the router's interface on the remote subnet is 10.10.10.1, so this is the correct gateway. This command is essential for clients to reach other networks, and it is a standard part of Cisco IOS DHCP server configuration. Without it, clients would lack a default route.

The Route Distinguisher (RD) is an 8-byte value prepended to an IPv4 prefix to make it unique within the MPLS L3VPN domain. It allows different customers to use the same address space without conflict. Each VRF is assigned an RD, which is used in MP-BGP VPNv4 updates. Without an RD, overlapping customer prefixes would be ambiguous in the provider's BGP table.

NHRP redirect is a DMVPN Phase 3 feature that enables the hub to send an NHRP redirect message to the originating spoke when it detects traffic being routed through the hub to another spoke. The redirect instructs the spoke to initiate an NHRP resolution request for the destination spoke's NBMA address, allowing the spoke to build a direct tunnel. This reduces latency and hub load, and it is the correct mechanism for the hub to inform the spoke of the optimal path.

The router needs a default route to forward traffic to the Internet. Without it, the router does not know where to send packets destined for outside networks. The NAT configuration only translates addresses; it does not provide routing. Adding a default route via the next-hop ISP router (203.0.113.1) enables the router to forward translated packets, allowing inside hosts to reach the Internet.

For DHCP relay to work, the DHCP server must have a route back to the client subnet to send the DHCPOFFER. If the server lacks a route to 192.168.1.0/24, it cannot deliver the offer, and clients will not receive addresses. This is a common oversight when the server is on a different subnet and no default gateway or static route is configured.

The 'ip helper-address' command is used on the interface facing the DHCP clients to forward broadcast DHCP requests to a specific DHCP server. It converts the broadcast to a unicast packet destined for the server. This is the correct command to enable DHCP relay functionality on the client-facing interface.

The 'ip ospf message-digest-key' command with the 'md5' keyword configures an MD5 key for OSPFv2 authentication on an interface. The key is not sent in clear text; instead, an MD5 hash is used. Multiple keys can be configured with different key IDs, allowing for graceful key rollover without disrupting the adjacency. This meets both requirements.

By default, the 'ip helper-address' command forwards UDP broadcasts for a predefined set of services, including DHCP (ports 67/68), TFTP (69), DNS (53), NetBIOS (137/138), and others like BOOTP and TACACS. This allows clients to reach servers on different subnets without additional configuration.

The global command 'ipv6 unicast-routing' enables IPv6 unicast routing on the router. Without it, the router can still have IPv6 addresses on interfaces, but it will not forward IPv6 packets or participate in IPv6 routing protocols. This command is required to make the router act as an IPv6 router. It is the correct answer for enabling IPv6 routing globally.

RSVP-TE extends RSVP to support traffic engineering by reserving resources (bandwidth) along the explicit path of a TE tunnel. The headend router sends PATH messages that include the requested bandwidth and other constraints. Each router along the path reserves the requested resources if available and forwards the PATH message. This reservation ensures that the TE tunnel has the required bandwidth, enabling deterministic traffic handling.

Applying an inbound route-map on the PE-CE BGP session allows matching the community and setting local preference before the route is installed in the VRF BGP table. This ensures only desired routes are accepted with the correct preference.

This command enables RSVP-TE signaling on the interface and reserves 100,000 kbps (100 Mbps) of bandwidth for RSVP-TE LSPs. It is required to signal an LSP with a specific bandwidth requirement. The bandwidth value is specified in kilobits per second, so 100 Mbps equals 100,000 kbps. This command allows the interface to participate in RSVP-TE signaling and reserve the requested bandwidth.

To preserve the OSPF domain ID across the MPLS backbone, the PE router must redistribute OSPF into MP-BGP and use the domain ID feature. Configuring the same domain ID under router ospf for the VRF on all PE routers ensures that OSPF routes retain their domain identity, preventing loops and allowing proper route redistribution into BGP.

For RSVP-TE to reserve bandwidth, MPLS TE must be enabled on the interface, and the ip rsvp bandwidth command must be configured to define the amount of reservable bandwidth. Without this, the TE tunnel cannot signal reservations, and path computation will fail because no bandwidth is available for reservation, even if the physical link has capacity.

The tunnel mpls traffic-eng bandwidth command under the tunnel interface sets the bandwidth value that RSVP-TE signals along the path. This reservation ensures that the required resources are available on each link. Without it, the tunnel may not receive the necessary guarantees for delay-sensitive traffic, making it a critical configuration step.

In DMVPN Phase 3, NHRP redirect on the hub allows the hub to inform the spoke that a better path exists directly to another spoke. NHRP shortcut on the spokes allows them to install a direct route to the destination spoke based on the redirect message. Without these, spoke-to-spoke traffic will continue to traverse the hub even if a direct path is available. Enabling these features is essential for Phase 3 direct spoke-to-spoke communication.

A unique RD per VRF ensures that identical customer prefixes from different VRFs do not conflict in the MP-BGP table. Route targets then define which VRFs import or export those routes, allowing the same customer routes to be imported into multiple VRFs. This is the standard MPLS L3VPN design for overlapping address spaces and shared services.

This is the correct behavior of a DHCP relay agent. The router receives the DHCP discover message from the client, inserts the giaddr (the router's interface IP on the client subnet), and forwards it to the DHCP server. The server uses the giaddr to determine the correct subnet and sends the DHCP offer back to the router's giaddr. The router then forwards the offer to the client. This process is defined in RFC 2131 and is essential for DHCP to work across subnets.

NHRP redirect is essential for DMVPN Phase 3. When the hub receives a packet from one spoke destined to another spoke, it sends an NHRP redirect message to the source spoke, informing it of a better path. The source spoke then initiates an NHRP resolution for the destination spoke's tunnel IP, allowing direct spoke-to-spoke tunnel establishment. Without NHRP redirect, spokes continue sending traffic through the hub even if a direct path exists.

The ip helper-address command is configured on the interface receiving the DHCP broadcasts. It causes the router to forward UDP broadcasts for specific ports, including DHCP, to the specified server. This is the correct and standard way to configure a DHCP relay agent on Cisco IOS. Without it, DHCP discover messages would not reach the server.

LDP (Label Distribution Protocol) is used to distribute labels for internal routes within the MPLS core. It maps IP prefixes to labels, enabling label switching for core routing. In an MPLS L3VPN, LDP is responsible for establishing label-switched paths (LSPs) between PE routers, which is essential for forwarding VPN traffic across the core.

The ip dhcp client client-id ascii BRANCH-ROUTER command under the interface configuration sets the DHCP client identifier to the ASCII string BRANCH-ROUTER. This allows the ISP to identify the router uniquely, which is often required for address assignment or authentication. Other options either configure server-side parameters or use incorrect syntax for the client identifier.

A GRE tunnel interface requires an IP address and subnet to route traffic over the tunnel. This subnet should be unique and not overlap with other interfaces to avoid routing conflicts. It is typically a private subnet dedicated to the tunnel, allowing dynamic routing protocols to run over the tunnel and exchange routes between sites.

The 'ip dhcp excluded-address' command is used in global configuration mode to prevent the DHCP server from assigning specific IP addresses. Specifying a single address excludes only that address. This is the correct syntax to exclude 10.10.10.1 from the DHCP pool, ensuring it is not offered to clients.

The ip nhrp redirect command is essential for DMVPN Phase 3. It allows the hub to send NHRP redirect messages to spokes, informing them of a more optimal path directly to the destination spoke. Without this, spokes continue sending traffic through the hub, defeating the purpose of Phase 3. This command works in conjunction with ip nhrp shortcut on the spokes.

The 'ip dhcp excluded-address' command specifies a range of IP addresses that the DHCP server will not assign to clients. This is the correct way to reserve addresses for static devices such as servers and printers, ensuring they are not dynamically allocated.

The VPNv4 unicast address family is specifically designed to carry MPLS Layer 3 VPN routes. It includes the route distinguisher and route target extended communities necessary for VPN segmentation. Configuring this address family under BGP allows the PE routers to exchange VPNv4 prefixes, enabling MPLS L3VPN functionality. This is the correct address family for this scenario.

NHRP redirect is a key component of DMVPN Phase 3. The hub sends an NHRP redirect message to the spoke, informing it that a more optimal path exists to the destination spoke. This allows the spoke to initiate a direct tunnel to the other spoke using NHRP resolution, bypassing the hub for data traffic.

For VPNv4 routes to be exchanged, the BGP process must have the VPNv4 address family activated and the PE routers must be configured as BGP neighbors within that address family. Without this, MP-BGP will not carry VPNv4 NLRIs. This is the fundamental step for MPLS L3VPN route distribution.

The 'ip helper-address' command is used on the interface facing the DHCP clients to forward DHCP broadcast requests to a specified DHCP server. It converts the broadcast to a unicast packet destined to the server's IP address. This is the standard method for DHCP relay on Cisco IOS routers and is essential for clients on remote subnets to obtain addresses from a centralized server.

If the tunnel destination IP address is not reachable, the tunnel interface will show up/down. The tunnel source is up, so the interface state is up, but without a route to the destination, the line protocol remains down. This is a common issue when the destination is a loopback that is not advertised or when there is no route to it.

This command configures PAT by referencing an access list (list 1) that defines the internal traffic to be translated and using the interface GigabitEthernet0/0's IP address as the public address. The overload keyword enables PAT, allowing multiple internal hosts to share the single public IP address by using different source ports. This is the correct command for the scenario.

The show ip bgp vpnv4 all command displays the VPNv4 routing table, including all prefixes learned from other PE routers. It shows the route targets, MPLS labels, and next-hops, which are crucial for verifying that VPNv4 prefixes are exchanged correctly. This is the primary command for troubleshooting MPLS L3VPN route propagation.

The instability is caused by redistributing the customer's OSPF routes into the provider's global OSPF. Removing this redistribution stops the customer routes from entering the provider's IGP. The customer routes can still be advertised across the MPLS core by redistributing them into MP-BGP, which is the correct method for L3VPN route propagation.

For the spoke to build a direct tunnel, it must have a route to the destination spoke's tunnel IP address pointing out the tunnel interface. This is usually achieved through a dynamic routing protocol running over the DMVPN cloud. Without such a route, the spoke cannot forward traffic directly even if it has the NHRP mapping.

This configuration defines a RADIUS server group named AD-SERVER with the correct IPv4 address and standard ports 1812/1813, sets the shared key, and applies it to the default login authentication list. The username is sent as entered because no stripping or manipulation is configured. This meets all requirements for authenticating VPN users against Active Directory via RADIUS.

To redistribute routes from a VRF into MP-BGP, you must enter the VRF address family under router bgp and use the redistribute command. This injects the connected routes from that VRF into BGP, where they are then advertised as VPNv4 prefixes to remote PEs. The address-family ipv4 vrf CUSTOMER command is the correct context for redistribution. Without this, the routes are not advertised.

Without ip nhrp map multicast dynamic, the hub cannot replicate multicast packets (such as EIGRP hellos) to all registered spokes. EIGRP uses multicast to discover and maintain neighbors. If the hub does not forward these multicasts, spokes will not receive hellos and adjacencies will not form. This command is essential for dynamic routing protocols over DMVPN.

This command specifies the SNMP trap recipient at 192.168.1.200 using SNMPv2c and the community string 'public'. It is the correct command to configure the destination for SNMP notifications. The 'version 2c' keyword ensures SNMPv2c is used, and 'public' is the community string for authentication.

CoPP is implemented by attaching a service policy to the control-plane interface (control-plane). This interface represents the route processor's traffic. By applying the policy there, you can filter and rate-limit traffic destined to the control plane, such as ICMP echo requests to the router's IP addresses. This ensures that only traffic intended for the router itself is policed, not transit traffic.

Route targets are extended BGP communities used to control import and export of routes between VRFs. They are required to define which VPN routes are accepted into which VRF on remote PEs. Without route targets, MP-BGP would not know which customer routes to import, and VPN connectivity would fail. They are a fundamental part of MPLS Layer 3 VPN configuration.

The 'ip dhcp excluded-address' command is used in global configuration mode to specify a range of IP addresses that the DHCP server should not assign to clients. This is typically used for addresses that are statically assigned to servers, printers, or routers. The command takes a start and end IP address to define the exclusion range.

For DHCP to work across subnets, the DHCP server must have a route to the remote subnet to send the DHCPOFFER and DHCPACK messages. If the server lacks a route, it cannot respond to the client's request. The `ip helper-address` forwards the initial DHCPDISCOVER, but the server's reply must be routable back to the client's subnet. Thus, a missing route on the server is a common cause.

A DHCPv6 pool defines the address prefix, DNS servers, domain name, and other options that clients receive. Without a pool, the router has no address space or parameters to assign, so stateful DHCPv6 cannot function. This is a fundamental configuration step for a DHCPv6 server.

The network command in DHCP pool configuration specifies the subnet and mask for the pool. This ensures that the router only assigns addresses from the 192.168.1.0/24 range to clients on that segment. Without this command, the pool would not know which addresses to offer, and the DHCP server would not function correctly for that subnet.

The 'mpls ip' command enables MPLS forwarding on the interface, allowing it to send and receive labeled packets. This is essential for the interface to participate in the MPLS core. It is the correct command to enable MPLS on a core-facing interface in an MPLS L3VPN deployment.

The 'ip helper-address' command configures the interface to forward UDP broadcasts, including DHCP requests, to the specified server address. It is the standard method for DHCP relay. When applied to the client-facing interface, the router will relay DHCP Discover messages to the server, allowing clients on that subnet to obtain addresses.

For OSPF routes from a VRF to be advertised across the MPLS VPN to other PEs, they must be redistributed into MP-BGP. If redistribution is not configured, the routes remain only in the VRF routing table and are not exported as VPNv4 routes. This is the most likely cause when routes are present in the VRF but missing from MP-BGP.

On a DMVPN Phase 3 spoke, 'ip nhrp shortcut' enables the spoke to intercept traffic and send an NHRP resolution request to the hub for a remote spoke. The hub replies with a redirect, and the spoke installs a shortcut route, allowing direct spoke-to-spoke communication. Without this command, the spoke continues to forward traffic through the hub even if the hub is configured with 'ip nhrp redirect'.

In Cisco IOS, to exchange VPNv4 routes, you must enter the VPNv4 address family configuration mode using 'address-family vpnv4' and then activate the neighbor with 'neighbor <ip> activate'. This enables the BGP session to carry VPNv4 routes, which include the route distinguisher and extended communities.

The excluded-address range prevents the DHCP server from offering those specific IP addresses in the pool. It does not reserve them for any particular device; they remain available for static assignment. This is commonly used to avoid conflicts with gateways, servers, or printers that are manually configured.

In MPLS L3VPN, route targets (RTs) control the import and export of VPNv4 routes between VRFs. If the export RT on one PE does not match the import RT on the remote PE, the routes will not be imported into the remote VRF, even though the MP-BGP session is up and the routes are exchanged. This is a common misconfiguration and directly causes missing customer routes.

DHCPv6 Guard relies on IPv6 snooping to function. IPv6 snooping builds a binding table that tracks legitimate DHCPv6 servers and clients. Without IPv6 snooping enabled, DHCPv6 Guard cannot inspect DHCPv6 messages or enforce policies. Therefore, enabling IPv6 snooping globally is a prerequisite for DHCPv6 Guard to operate correctly and block rogue servers.

The dns-server command in DHCP pool configuration provides the IP address of the DNS server to clients. This is necessary for name resolution. The scenario explicitly requires the router to provide the DNS server address, so this command must be configured. Multiple DNS servers can be specified in a single command.

In DMVPN Phase 3, when a spoke needs to reach another spoke's network, it initially sends packets to the hub. The hub forwards them and sends an NHRP redirect to the source spoke. The source spoke then sends an NHRP resolution request for the destination spoke's NBMA address, waits for a reply, and then builds a direct tunnel. During this process, the first few packets may be dropped or delayed. This is a normal behavior and not a configuration error.

The Reply message is used by the DHCPv6 server to provide configuration information, including IPv6 addresses and DNS server addresses, to the client. It is sent in response to a Request, Renew, Rebind, or Information-request message. This is the correct message type for delivering DNS server information.

An extended ACL is used to define the interesting traffic that should be encrypted by IPsec. In this scenario, the ACL would permit IP traffic from 192.168.1.0/24 to 10.0.0.0/24. This ACL is referenced in the crypto map. Traffic matching the ACL is encrypted; traffic not matching is sent unencrypted. Thus, the ACL is the configuration element that defines the traffic.

The EXPLICIT_ROUTE object (ERO) is used in RSVP-TE Path messages to specify the explicit path that the TE tunnel should take. It contains a list of hops (usually IP addresses or autonomous system numbers) that the tunnel must traverse. In this scenario, the engineer needs to include a specific link in the path, and the ERO is the correct object to carry that information.

This command sets the source IP address for syslog messages to the loopback0 interface's IP address. It ensures that the syslog server sees a consistent and stable source address, which is especially useful for logging correlation and filtering. The loopback interface is always up, so the source address remains reachable even if physical interfaces flap.

To enable the router to act as a DHCPv6 server on an interface, the ipv6 dhcp server <pool-name> command must be configured on that interface. This binds the DHCPv6 pool to the interface and allows the router to respond to DHCPv6 solicit messages from clients. Without this command, the router will not process DHCPv6 requests on VLAN 20, even though the pool exists. This is the missing piece to make the server operational.

For NAT to function, interfaces must be designated as inside or outside using the ip nat inside and ip nat outside commands. Without these, the router does not know which interfaces to translate. Even if the NAT statement and access list are correct, missing interface designations will prevent translation, causing connectivity failure.

The 'service dhcp' command in global configuration mode enables the DHCP server and relay agent functionality on the router. By default, this service is enabled, but it may have been disabled. Without it, the router will not process DHCP requests even if a pool is configured. This command is essential to activate the DHCP service.

A VRF (Virtual Routing and Forwarding) instance creates separate routing and forwarding tables per VPN. The route distinguisher (RD) makes the customer prefix unique within the MPLS domain, while route targets (RTs) control import and export of routes between VRFs. This combination ensures isolation and proper route leaking only where intended. Without VRFs, customer routes would mix with the global table or with other customers' routes, violating the isolation requirement.