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Nokia NRS II practice questions

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Question 1IP networks

In an IPv4 header, which field is decremented by each router that forwards the packet, and what happens when it reaches zero?

  1. The Type of Service field is decremented; the packet is queued with lower priority.
  2. The TTL field is decremented; the packet is discarded and an ICMP Time Exceeded message is returned. Correct answer.
  3. The Header Checksum field is decremented; the packet is silently dropped at the next hop.
  4. The Identification field is decremented; the packet is fragmented on the next link.

Explanation

The Time to Live field is decremented by one at every router that forwards the packet. When it hits zero the router discards the packet and sends ICMP Time Exceeded back to the source, which prevents stale packets from looping forever. Traceroute deliberately exploits this behavior.

Question 2IP networks

A host is configured with address 192.168.10.70/26. What is the directed broadcast address of its subnet?

  1. 192.168.10.64
  2. 192.168.10.63
  3. 192.168.10.95
  4. 192.168.10.127 Correct answer.

Explanation

A /26 mask leaves 6 host bits, so each subnet has 64 addresses. The subnets are .0-.63, .64-.127, .128-.191 and .192-.255; the host .70 falls in the second block, whose broadcast address is the last address of the block, 192.168.10.127.

Question 3IP networks

Which statement correctly describes the role of ARP in an IPv4 LAN?

  1. It resolves domain names into IPv4 addresses for application clients.
  2. It maps a MAC address to the IPv4 address assigned by a DHCP server.
  3. It distributes IPv6 prefix information to hosts on the link.
  4. It maps a known IPv4 address to the MAC address of the device that owns it. Correct answer.

Explanation

ARP broadcasts a request asking who owns a given IPv4 address, and the owner replies with its link-layer address so the sender can build the frame. The resulting binding is cached for a period. Reverse mapping and name resolution are handled by other protocols.

Question 4IP networks

A router receives a packet destined to 10.20.30.40 and has the following routes: 10.0.0.0/8 via A, 10.20.0.0/16 via B, 10.20.30.0/24 via C, and a default route via D. Which next hop is chosen?

  1. C Correct answer.
  2. B
  3. A
  4. D

Explanation

IPv4 forwarding uses longest-prefix match, so among all routes that match the destination the one with the most specific mask wins regardless of metric. The /24 route via C is the longest match for 10.20.30.40. The default route only applies when no more specific route matches.

Question 5IP networks

What is the primary purpose of the IPv4 Differentiated Services Code Point in the header?

  1. To carry the fragment offset of the datagram.
  2. To signal per-hop forwarding treatment such as queue selection. Correct answer.
  3. To identify the upper-layer protocol carried in the payload.
  4. To detect corruption of the header during transit.

Explanation

The DSCP is a 6-bit field in the former Type of Service byte that classifies packets into behavior aggregates so routers can apply per-hop queuing and drop treatment. It is the marking used by DiffServ QoS architectures. It does not carry protocol identity or error detection.

Question 6IP networks

Which address range is reserved for IPv4 link-local autoconfiguration (APIPA) and is never forwarded by routers?

  1. 169.254.0.0/16 Correct answer.
  2. 192.0.2.0/24
  3. 172.16.0.0/12
  4. 127.0.0.0/8

Explanation

169.254.0.0/16 is the link-local block used when a host cannot obtain an address from DHCP, and routers do not forward traffic sourced from or destined to it. It is distinct from private RFC 1918 space, which is routable within an enterprise.

Question 7IP networks

In IPv6, which address type is used for one-to-nearest-group delivery where the packet is delivered to only one member of the group?

  1. Multicast
  2. Unicast
  3. Broadcast
  4. Anycast Correct answer.

Explanation

An anycast address is assigned to multiple nodes, and a packet sent to it is delivered to the nearest member according to routing metrics. IPv6 has no broadcast type; multicast delivers to all members of a group. Unicast delivers to a single interface.

Question 8IP networks

Which IPv6 mechanism lets a host derive its interface identifier from its 48-bit MAC address?

  1. Privacy extension randomization
  2. Stateless DHCPv6
  3. Modified EUI-64 Correct answer.
  4. Neighbor Unreachability Detection

Explanation

Modified EUI-64 inserts FFFE in the middle of the MAC address and flips the universal/local bit to form a 64-bit interface identifier. Combined with a /64 prefix it produces a full IPv6 address without a server. Privacy extensions instead randomize the identifier for anonymity.

Question 9IP networks

A network engineer must summarize the four subnets 10.1.4.0/24, 10.1.5.0/24, 10.1.6.0/24 and 10.1.7.0/24 into one route. Which prefix is correct?

  1. 10.1.4.0/22 Correct answer.
  2. 10.1.4.0/23
  3. 10.1.6.0/22
  4. 10.1.0.0/21

Explanation

The four /24s span 10.1.4.0 through 10.1.7.255, which is four contiguous /24 blocks. Removing two host bits from the mask gives /22, and the network address must have those bits zero, so 10.1.4.0/22 covers exactly the range.

Question 10IP networks

What does an administrative distance value represent in a router's routing table?

  1. The trustworthiness of a route source when comparing routes from different protocols. Correct answer.
  2. The bandwidth cost of the outgoing interface.
  3. The number of hops to the destination network.
  4. The age of the routing entry in seconds.

Explanation

Administrative distance is a locally significant preference value used to choose among routes to the same prefix learned from different sources, with lower values preferred. It is compared before any metric because metrics from different protocols are not comparable. It is not a hop count or a timer.

Question 11IP networks

Which OSPF packet type is used by a router to confirm that a neighbor is still alive on a broadcast segment after adjacency is formed?

  1. Link State Update
  2. Link State Request
  3. Database Description
  4. Hello Correct answer.

Explanation

Hello packets are sent periodically on OSPF-enabled interfaces to discover neighbors and to act as a keepalive once the adjacency is up. If no Hello is received within the dead interval the neighbor is declared down. The other packet types carry database synchronization information.

Question 12IP networks

In OSPF, what is the purpose of the designated router on a multi-access broadcast network?

  1. It reduces the number of adjacencies by acting as the central point for LSA exchange. Correct answer.
  2. It assigns IP addresses to OSPF neighbors.
  3. It authenticates all Hello packets on the segment.
  4. It floods all external LSAs into the area as an ASBR.

Explanation

Without a DR, every router on the segment would form a full adjacency with every other, giving n(n-1)/2 adjacencies. Electing a DR and BDR limits full adjacencies to the DR/BDR and centralizes LSA flooding, which improves scaling and stability.

Question 13IP networks

A router running IS-IS is configured as Level 1-2. What does this configuration enable?

  1. It acts as a pure Level 1 router and drops all backbone traffic.
  2. It participates in both a Level 1 area and the Level 2 backbone, leaking or advertising routes between them. Correct answer.
  3. It disables the DIS election on all broadcast interfaces.
  4. It advertises only Level 2 routes and ignores Level 1 adjacencies.

Explanation

A Level 1-2 router maintains separate link-state databases for Level 1 and Level 2 and forms adjacencies in both. It can advertise Level 1 routes into Level 2 and set the attached bit, allowing inter-area reachability. This is the IS-IS equivalent of an area border router.

Question 14IP networks

Which BGP attribute is well-known discretionary and influences path selection by indicating the local preference for an exit point from the AS?

  1. LOCAL_PREF Correct answer.
  2. AS_PATH
  3. NEXT_HOP
  4. MED

Explanation

LOCAL_PREF is a well-known discretionary attribute propagated within an autonomous system; higher values are preferred when choosing among multiple exit points. It is not advertised to external peers. MED is optional non-transitive and influences inbound traffic instead.

Question 15IP networks

Two BGP routes to the same prefix have identical LOCAL_PREF, AS_PATH length and origin. Which attribute is compared next in the standard decision process?

  1. MED Correct answer.
  2. Router ID
  3. Cluster list length
  4. Originator ID

Explanation

After LOCAL_PREF, locally originated preference, AS_PATH length and origin type, the standard process compares MED, preferring the lower value. Router ID and cluster list are considered later. Knowing the ordering matters when tuning outbound and inbound policy.

Question 16IP networks

What is the effect of configuring a static route with a next-hop address that is not reachable through any interface?

  1. The route is redistributed into all dynamic protocols.
  2. The route remains in the table only if the next hop resolves via a connected route. Correct answer.
  3. The route is always installed and used regardless of next-hop reachability.
  4. The route is automatically converted into a default route.

Explanation

A static route with an unresolved next hop is not installed in the forwarding table because the router cannot determine an outgoing interface. Resolution normally requires a matching connected or recursive route to the next hop. Once resolution succeeds the route becomes active.

Question 17IP networks

A service provider wants customer traffic to be forwarded without inspecting the full IPv4 header at each core hop. Which technology achieves this by stacking a short fixed-length shim header?

  1. VLAN stacking with 802.1ad
  2. MPLS label switching Correct answer.
  3. IPsec transport mode
  4. GRE tunneling

Explanation

MPLS inserts a 32-bit shim label between the link header and the IP packet, and core routers forward on the label rather than the IP header. This enables traffic engineering and fast forwarding. GRE and IPsec still require IP header processing at each hop.

Question 18IP networks

Which ICMP message type is returned to a source when a packet arrives at a router with a destination that is unreachable because no route exists?

  1. Redirect
  2. Echo Reply
  3. Parameter Problem
  4. Destination Unreachable Correct answer.

Explanation

ICMP Destination Unreachable, with a code indicating the reason such as network unreachable, is generated when a router cannot forward a packet. It lets the source learn about the failure. Redirect and Parameter Problem serve different purposes.

Question 19IP networks

Which field in the IPv6 header replaces the function of the IPv4 header checksum and fragmentation fields?

  1. The Payload Length field
  2. The Hop Limit field
  3. The Next Header field, which chains extension headers Correct answer.
  4. The Flow Label field

Explanation

IPv6 removes the header checksum and moves fragmentation into extension headers. The Next Header field identifies the next header in the chain, which may be a routing, fragment or upper-layer header. This simplifies the base header and speeds processing.

Question 20IP networks

A network uses VRRP to provide a default gateway. Which statement describes the virtual router behavior?

  1. The virtual MAC address changes each time a new master is elected.
  2. Only the master responds to ARP for the virtual IP and forwards traffic for the group. Correct answer.
  3. The backup router responds to ARP but drops forwarded packets.
  4. All routers in the group forward traffic simultaneously for load sharing.

Explanation

In VRRP the master owns the virtual IP and virtual MAC and answers ARP requests, while backups monitor Hellos. If the master fails, a backup takes over the same virtual addresses so hosts see no change. This provides first-hop redundancy.

Question 21IP networks

An engineer must prevent hosts on a switch port from using a statically configured IP address that conflicts with the DHCP scope. Which feature is designed for this purpose?

  1. DHCP snooping with IP source guard Correct answer.
  2. Dynamic ARP Inspection
  3. Port security with sticky MAC addresses
  4. BPDU guard

Explanation

DHCP snooping builds a binding table of legitimate address assignments, and IP source guard uses that table to drop traffic whose source IP or MAC does not match. This blocks statically configured or spoofed addresses on access ports. Port security only limits MAC addresses.

Question 22IP networks

Which statement describes the difference between a routing table and a forwarding table (FIB)?

  1. The routing table holds all candidate routes from all sources; the forwarding table holds the selected best entries used per packet. Correct answer.
  2. The forwarding table contains only directly connected routes.
  3. The routing table is stored in hardware; the forwarding table is stored only in software.
  4. The routing table is used for IPv6 and the forwarding table for IPv4.

Explanation

The routing information base collects every learned route along with attributes from each protocol. The best route for each prefix is then programmed into the forwarding information base, which is consulted at line rate for each packet. This separation lets policy and convergence happen without disturbing fast-path forwarding.

Question 23IP networks

A packet arrives at a router with a destination of 2001:db8:10::1. The router has routes 2001:db8::/32 via A, 2001:db8:10::/48 via B, and ::/0 via C. Which next hop is selected?

  1. A
  2. It is dropped for lack of a route.
  3. B Correct answer.
  4. C

Explanation

IPv6 forwarding also uses longest-prefix match. The /48 route via B matches the destination and is more specific than the /32 route via A and the default route via C, so B is selected. The default route would only apply if no other prefix matched.

Question 24IP networks

What is the function of the Path MTU Discovery process in IP networks?

  1. It negotiates the TCP window size between endpoints.
  2. It determines the smallest MTU along a path so the source can size packets to avoid in-path fragmentation. Correct answer.
  3. It fragments every packet to 1280 bytes before transmission.
  4. It sets the TTL to the number of hops in the path.

Explanation

PMTUD relies on the Don't Fragment bit and ICMP 'fragmentation needed' messages to learn the smallest MTU on the path. The source then limits packet size so no router needs to fragment. In IPv6 this process is mandatory because routers do not fragment.

Question 25IP networks

An engineer observes that a route learned via eBGP is preferred over an identical prefix learned via OSPF in the same router. Which principle explains this?

  1. eBGP routes bypass the routing table and are placed directly in the FIB.
  2. OSPF routes are never installed when BGP is enabled.
  3. eBGP routes always have a lower metric than OSPF routes.
  4. Administrative distance prefers eBGP over OSPF when comparing routes from different sources. Correct answer.

Explanation

When routes to the same prefix come from different protocols, the router compares administrative distance before any metric. The default eBGP distance of 20 is lower than the default OSPF distance of 110, so the eBGP route wins. Metrics from different protocols cannot be compared directly.

Question 26IP networks

In an IPv4 header, which field allows a router to prevent a packet from looping indefinitely when a routing loop exists?

  1. Time to Live Correct answer.
  2. Header Checksum
  3. Total Length
  4. Identification

Explanation

The Time to Live field is decremented by each router that forwards the packet, and the packet is discarded with an ICMP Time Exceeded message when the value reaches zero. This bounds the lifetime of a looping packet and also underpins traceroute operation.

Question 27IP networks

A router receives a packet whose destination address matches no entry in its routing table and no default route is configured. What does the router do?

  1. Drop the packet and return ICMP Destination Unreachable (network unreachable) Correct answer.
  2. Hold the packet until a matching route is learned
  3. Forward the packet to the lowest-metric route in the table
  4. Flood the packet out all interfaces

Explanation

Without a matching route or default route, the router has no forwarding decision and discards the packet. It normally signals the source with an ICMP Destination Unreachable message carrying the network-unreachable code.

Question 28IP networks

Which statement correctly describes the purpose of the IPv4 subnet mask associated with an interface address?

  1. It defines the maximum transmission unit for the link
  2. It encrypts the payload of packets leaving the interface
  3. It identifies which portion of the address is the network prefix and which is the host portion Correct answer.
  4. It lists the DNS servers available on the segment

Explanation

The mask is a contiguous series of ones followed by zeros; the ones mark the network prefix bits and the zeros mark host bits. A host uses it to decide whether a destination is on-link or must be sent to a gateway.

Question 29IP networks

How many usable host addresses are available in the subnet 192.168.10.0/27?

  1. 14
  2. 62
  3. 32
  4. 30 Correct answer.

Explanation

A /27 leaves 5 host bits, giving 32 addresses in the block. Subtracting the network and directed-broadcast addresses leaves 30 assignable host addresses.

Question 30IP networks

Which IPv6 address is the loopback address that a node uses to send a packet to itself?

  1. ::1 Correct answer.
  2. ff02::1
  3. fe80::1
  4. ::

Explanation

The address ::1/128 is the IPv6 loopback and must never be assigned to a physical interface or forwarded. A packet sent to ::1 is delivered internally to the same node.

Question 31IP networks

Which protocol provides a mechanism to map a known IPv4 address to a link-layer (MAC) address on a LAN?

  1. IGMP
  2. ARP Correct answer.
  3. DHCP
  4. ICMP

Explanation

ARP broadcasts a request asking which station owns a given IPv4 address and caches the reply mapping in the ARP table. This mapping is required to build the Layer 2 frame that carries the IP packet.

Question 32IP networks

A host is configured with the address 172.16.5.10 and mask 255.255.255.0 and needs to reach 172.16.6.20. What does the host do with the packet?

  1. Drop it because the destination is in a different subnet
  2. Send it directly on the local segment using ARP for 172.16.6.20
  3. Send it to its configured default gateway Correct answer.
  4. Send an ICMP redirect to the destination

Explanation

The mask /24 places 172.16.5.0 and 172.16.6.0 in different subnets, so the destination is off-link. The host therefore forwards the frame to its default gateway, which must be on-link.

Question 33IP networks

Which address block is reserved for IPv4 private use under RFC 1918?

  1. 192.0.2.0/24
  2. 100.64.0.0/10
  3. 198.18.0.0/15
  4. 172.16.0.0/12 Correct answer.

Explanation

RFC 1918 defines 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 as private address space that is not routed on the public Internet. 172.16.0.0/12 spans 172.16.0.0 through 172.31.255.255.

Question 34IP networks

On a router, two routes to the same prefix exist: a static route with administrative distance 1 and an OSPF route with administrative distance 110. Which route is installed in the forwarding table?

  1. Both routes, and traffic is load-balanced equally
  2. The OSPF route, because dynamic routes are preferred
  3. Neither route, until the metrics are equalized
  4. The static route, because the lower administrative distance wins Correct answer.

Explanation

Administrative distance is compared only among routes from different sources to the same prefix, and the lowest value is preferred. Static routes have a default distance of 1, so they beat OSPF routes with distance 110.

Question 35IP networks

Which statement describes the behaviour of a floating static route?

  1. It is used only for IPv6 traffic while dynamic routes handle IPv4
  2. It is configured with a higher administrative distance than the primary route so it is used only if the primary fails Correct answer.
  3. It is always preferred because static routes are trusted most
  4. It is installed only when the primary route flaps more than three times

Explanation

A floating static route deliberately uses a higher administrative distance than the dynamic protocol providing the primary path. It stays out of the forwarding table while the better route exists and is installed when that route is withdrawn.

Question 36IP networks

A router with three equal-cost OSPF paths to a prefix is configured for per-packet load balancing. What is the result for a single TCP flow?

  1. The flow is dropped because OSPF does not support multiple paths
  2. The router signals the sender to reduce its window size
  3. All packets follow one path because TCP is connection-oriented
  4. Packets may arrive out of order because successive packets can take different paths Correct answer.

Explanation

Per-packet load balancing distributes consecutive packets across the equal-cost next hops regardless of flow identity. Because different paths have different delays, packets of one TCP flow can arrive out of order, which typically degrades throughput.

Question 37IP networks

In OSPFv2, which multicast address carries Hello packets on a broadcast-capable interface?

  1. 224.0.0.5 Correct answer.
  2. 224.0.0.10
  3. 224.0.0.9
  4. 224.0.0.6

Explanation

OSPFv2 routers listen on 224.0.0.5 (AllSPFRouters) for Hellos and other packets sent to all routers on the segment. The address 224.0.0.6 (AllDRouters) is used by non-DR routers when reporting to the DR and BDR.

Question 38IP networks

What is the effect of an OSPF interface being placed in a passive state?

  1. The interface only accepts Hello packets and never sends them
  2. The interface is removed from the OSPF process entirely
  3. The interface advertises its network but does not form adjacencies or send Hellos on it Correct answer.
  4. The interface stops advertising its connected network in LSAs

Explanation

A passive OSPF interface still has its network included in router LSAs so other routers can reach it, but Hellos are suppressed and no neighbour adjacency forms. This is typically used on LANs facing end hosts.

Question 39IP networks

Which statement best describes the purpose of the OSPF router ID?

  1. It uniquely identifies the router within the OSPF domain for LSA origination Correct answer.
  2. It sets the cost of all interfaces on the router
  3. It identifies the area in which the router resides
  4. It is the MAC address used for multicast Hellos

Explanation

The router ID is a 32-bit value that uniquely identifies each router originating LSAs within the OSPF domain. It is normally chosen as the highest configured loopback address or the highest active interface address if no loopback exists.

Question 40IP networks

A network engineer must design addressing so that a single summarized route covers 10.8.0.0/24 through 10.15.0.0/24. Which prefix achieves this with the minimum number of addresses?

  1. 10.8.0.0/23
  2. 10.8.0.0/20
  3. 10.8.0.0/21 Correct answer.
  4. 10.0.0.0/13

Explanation

The range 10.8.0.0 through 10.15.255.255 spans 8 consecutive /24 blocks, which aligns on a /21 boundary. A /21 contains exactly those eight /24s, so it is the minimal summary.

Question 41IP networks

A router has routes 2001:db8::/32 via next hop A, 2001:db8:1::/48 via next hop B and 2001:db8:1:2::/64 via next hop C. A packet destined for 2001:db8:1:2::99 arrives. Which next hop is used?

  1. The packet is dropped because the routes overlap
  2. C Correct answer.
  3. A
  4. B

Explanation

IPv6 forwarding uses longest-prefix match. The /64 route via C is the most specific match for the destination, so it wins over the /48 and /32 routes even though those also match.

Question 42IP networks

Which mechanism allows an IPv6 host to obtain a globally routable prefix from a router without a DHCPv6 server?

  1. SLAAC using Router Advertisement prefix information Correct answer.
  2. VRRP
  3. IGMP snooping
  4. ARP proxy

Explanation

With stateless address autoconfiguration, routers send Router Advertisements carrying prefix information with the autonomous flag set. Hosts combine that prefix with a locally formed interface identifier to build a global address without a stateful server.

Question 43IP networks

Which statement correctly describes the difference between the control plane and the data plane of a router?

  1. The control plane builds routing state while the data plane forwards packets using the resulting forwarding table Correct answer.
  2. The data plane computes shortest paths and the control plane stores them
  3. The control plane forwards packets while the data plane runs routing protocols
  4. Both planes run the same routing protocol but on different interfaces

Explanation

The control plane runs routing protocols and builds the routing information base, from which the forwarding table is derived. The data plane performs the actual per-packet lookup and forwarding at line rate.

Question 44IP networks

A router has an interface with MTU 1500 bytes and must forward a 4000-byte IPv4 packet with the DF bit clear. What happens?

  1. The packet is forwarded whole because fragmentation is optional
  2. The packet is dropped and an ICMP Fragmentation Needed message is sent
  3. The packet is queued until the MTU is raised
  4. The packet is fragmented into pieces that fit the MTU and forwarded Correct answer.

Explanation

When the DF bit is clear and the packet exceeds the outgoing MTU, the router fragments it and sets the fragment offset and MF flag fields appropriately. The receiver reassembles the fragments using the identification field.

Question 45IP networks

What is the primary purpose of the IPv6 flow label field?

  1. To carry the hop limit for the packet
  2. To allow a source to label packets belonging to a flow so routers can treat them consistently Correct answer.
  3. To identify the upper-layer protocol
  4. To store the fragmentation offset

Explanation

The 20-bit flow label lets a source mark a sequence of packets as one flow, enabling routers and middleboxes to apply consistent handling such as load balancing or QoS without deep inspection. It is set by the source and should not be modified in transit.

Question 46IP networks

A network uses BFD alongside OSPF. What benefit does this provide?

  1. It detects path failures faster than OSPF Hello timers allow, triggering quicker reconvergence Correct answer.
  2. It encrypts OSPF Hellos on broadcast segments
  3. It assigns a lower administrative distance to OSPF routes
  4. It replaces OSPF LSA flooding with a link-state database sync

Explanation

BFD sends lightweight, sub-second probes over the same path and notifies the routing protocol when the session fails. Because OSPF Hellos are typically measured in seconds, BFD allows much faster detection of failures.

Question 47IP networks

A router receives an IPv4 packet with TTL 1 that must be forwarded to a remote network. What does the router do?

  1. Forward it with TTL reset to 255
  2. Fragment it and forward the fragments
  3. Forward it with TTL 0
  4. Drop it and send ICMP Time Exceeded to the source Correct answer.

Explanation

The router decrements the TTL, which would reach zero, so the packet is discarded. It returns an ICMP Time Exceeded message to the source, which is exactly the behaviour traceroute relies on.

Question 48IP networks

An engineer observes that a prefix learned via eBGP is preferred over an identical prefix learned via OSPF on the same router. Which explanation is correct?

  1. The router prefers the route with the longer AS_PATH
  2. A lower configured route preference for the eBGP route can make it preferred over the OSPF route Correct answer.
  3. OSPF routes are only installed when BGP is disabled
  4. OSPF has a lower administrative distance, but the eBGP route was redistributed into OSPF

Explanation

Nokia SR OS prefers the eligible route with the lower numerical route preference. Its defaults favor OSPF internal routes at 10 and OSPF external routes at 150 over BGP at 170. Adjusting route preferences so the eBGP route has the lower value can explain the observation.

Question 49IP networks

A service provider wants to avoid dropping large packets when a path MTU shrinks in the middle of the network. Which combination of behaviours supports this?

  1. Increasing the TTL on packets that traverse small-MTU links
  2. Configuring all interfaces with an MTU of 9000 regardless of the underlying media
  3. Allowing ICMP Packet Too Big or Fragmentation Needed messages to reach the source so it can reduce its packet size Correct answer.
  4. Setting DF on all packets and disabling ICMP generation on routers

Explanation

Path MTU Discovery depends on the source receiving ICMP Packet Too Big (IPv6) or Fragmentation Needed (IPv4) messages when a packet with DF set exceeds a downstream MTU. Filtering those ICMP messages breaks PMTUD and causes black-holed large packets.

Question 50IP networks

A router runs both IS-IS and OSPF and learns the same prefix from both with default administrative distances. Which route is preferred and why?

  1. The routes are load-balanced because the metrics are equal
  2. Neither route is installed until a tie-breaker is configured
  3. The OSPF route, because OSPF has a default distance of 110 and IS-IS has 115 Correct answer.
  4. The IS-IS route, because IS-IS has a default distance of 115 and OSPF has 110

Explanation

With default values, OSPF's administrative distance of 110 is lower than IS-IS's 115, so the OSPF route is installed. The comparison is made on distance first, before any metric comparison between protocols.