Cisco 300-420 Exam Dumps & Practice Test Questions

Question 1:

Which two Border Gateway Protocol (BGP) features enable successful route exchange between external BGP (eBGP) neighbors that share the same Autonomous System (AS) number? (Select two.)

A advertise-best-external
B bestpath as-path ignore
C client-to-client reflection
D as-override
E allow-as-in

Answer: D, E

Explanation:

BGP is the primary protocol for exchanging routing information between different networks, generally across the internet. Typically, eBGP neighbors belong to different Autonomous Systems (AS), and BGP enforces this by rejecting routes where the AS number matches the local AS to prevent routing loops. However, there are scenarios where eBGP neighbors share the same AS number, and in such cases, special BGP features are required to enable proper route exchange.

The as-override feature allows an eBGP router to replace the AS number in the AS path attribute with its own when advertising routes to a peer within the same AS. This “overriding” prevents the router from seeing its own AS in the path, which would normally cause it to reject the route to avoid loops. By using as-override, routers can exchange routes even though they share the same AS number, enabling route propagation in complex network designs such as multi-homed or overlapping AS environments.

The allow-as-in feature permits a BGP router to accept routes that already include the local AS number in the AS path. Normally, BGP drops these routes to avoid loops, but allow-as-in provides an exception by allowing a set number of occurrences of the local AS in incoming route paths. This feature is often used when eBGP neighbors share an AS number or in scenarios involving route reflection or certain forms of route redistribution.

Other options do not specifically address the problem of eBGP neighbors sharing the same AS:

• advertise-best-external deals with advertising external routes internally, unrelated to AS number conflicts.

• bestpath as-path ignore influences path selection rather than route acceptance or exchange.

• client-to-client reflection pertains to iBGP route reflectors, not eBGP.

In summary, as-override and allow-as-in are crucial for enabling route exchange between eBGP neighbors within the same AS by preventing AS path loop rejections and facilitating route acceptance, making them the correct choices.

Question 2:

A client operates an IPv4-only network but wants to enable IPv6 connectivity while keeping their current IPv4 network intact during the transition. Their goal is to gradually migrate services to IPv6 and eventually retire the IPv4 setup. 

Which network topology best supports these requirements?

A dual stack
B 6VPE
C 6to4
D NAT64

Answer: A

Explanation:

The situation describes a network environment where IPv6 must be introduced alongside an existing IPv4 infrastructure to allow a seamless migration. The customer wants to maintain IPv4 operations while incrementally adding IPv6, eventually phasing out IPv4 services. To achieve this, the network design must support both protocols simultaneously on the same devices and network paths.

The dual stack topology is the optimal solution here. In a dual-stack network, devices and routers are configured to run both IPv4 and IPv6 protocols concurrently. This means they have both IPv4 and IPv6 addresses, and can communicate over either protocol as necessary. This approach allows IPv6 traffic to be introduced gradually while IPv4 remains fully operational, making it ideal for incremental migration strategies. Services can be shifted to IPv6 at a controlled pace, reducing risk and ensuring compatibility during the transition period.

Other options do not fit this gradual migration model as effectively:

• 6VPE (IPv6 Virtual Private Edge) is designed for transporting IPv6 over MPLS networks, suited for service providers rather than simple enterprise migrations.

• 6to4 is a tunneling method that encapsulates IPv6 packets within IPv4. While useful for IPv6 connectivity over IPv4 networks, it’s typically a temporary transitional technology rather than a comprehensive migration solution. It also doesn’t support native IPv6 deployment alongside IPv4.

• NAT64 facilitates communication between IPv6-only clients and IPv4 servers by translating addresses. It’s more of a translation mechanism rather than a topology for dual operation or migration.

In conclusion, dual stack enables both IPv4 and IPv6 to coexist natively, providing the flexibility and compatibility needed for a smooth, staged migration. It fully aligns with the customer’s need to preserve existing IPv4 services while enabling IPv6 connectivity and eventual IPv4 decommissioning.

Question 5:

What is the key design factor to consider when deploying EIGRP on Data Center switches?

A. Manually summarize routes on all Layer 3 interfaces to reduce routing table size.
B. Avoid unnecessary EIGRP neighborship formation across Switch Virtual Interfaces (SVIs).
C. Set EIGRP hello and hold timers to their minimum values to speed up route reconvergence.
D. Use multiple EIGRP autonomous systems to separate Data Center services and applications.

Correct Answer: B

Explanation:

When configuring EIGRP (Enhanced Interior Gateway Routing Protocol) in Data Center environments, the primary design concern revolves around controlling how neighborships form, especially across Switch Virtual Interfaces (SVIs). SVIs are logical Layer 3 interfaces that connect VLANs and enable routing on switches. Because Data Center switches may have many SVIs, unchecked EIGRP neighborships could form between interfaces that don’t require adjacency, causing unnecessary routing traffic and increased CPU load.

Preventing these unnecessary neighborships helps maintain a clean routing topology and improves network stability. This is typically done through careful interface-level configuration and filtering mechanisms to allow EIGRP to form neighborships only where truly needed. By limiting neighbor relationships, the network avoids redundant routing updates and potential loops, which are critical in a performance-sensitive Data Center setting.

Let’s consider the other options:
A suggests manual summarization on all Layer 3 interfaces to minimize routing table size. While summarization is a useful optimization, it is not the foremost design consideration in Data Centers using EIGRP. Automatic summarization often suffices, and manual summarization on all interfaces may be impractical or counterproductive in dynamic environments.

C involves lowering hello and hold timers to speed route convergence. Although faster convergence can be beneficial, reducing these timers to their minimum values can cause instability by making the protocol overly sensitive to transient network issues, leading to route flapping.

D recommends using multiple EIGRP autonomous systems to segment traffic. This approach adds complexity and administrative overhead. Usually, a single autonomous system with route filtering and summarization is preferable for Data Centers.

In summary, the best design approach when deploying EIGRP on Data Center switches is to avoid unnecessary neighborships across SVIs to maintain efficient and stable routing. Therefore, option B is the correct answer.

Question 6:

What is a critical design principle to remember when implementing IPv6 overlay tunnels?

A. IPv6 overlay tunnels are intended as the permanent architecture for IPv6 networks.
B. Overlay tunnels serve primarily as a temporary transition method toward a full IPv6 solution.
C. Overlay tunnels should be limited to border devices and require only the IPv6 protocol stack.
D. Overlay tunnels encapsulate IPv4 packets within IPv6 packets to traverse an IPv6 infrastructure.

Correct Answer: B

Explanation:

IPv6 overlay tunnels are commonly deployed to facilitate communication between IPv6 networks over an existing IPv4 infrastructure during a transition phase. The most important design principle is recognizing that these tunnels are not meant to be a permanent solution. Rather, they are a temporary technique that allows organizations to gradually migrate to native IPv6 connectivity without disrupting existing IPv4 networks.

Overlay tunnels encapsulate IPv6 packets inside IPv4 packets to enable IPv6 traffic to traverse IPv4-based networks. This encapsulation allows isolated IPv6 domains to communicate even when the underlying network does not fully support IPv6. However, as organizations move towards full IPv6 adoption, the reliance on these tunnels should diminish, with the goal of achieving a native IPv6 environment that eliminates the need for tunneling.

Now, examining the other options:
A incorrectly states that overlay tunnels are the final IPv6 network design, which is misleading because they are a transitional technology, not the end goal.

C suggests that overlay tunnels should only be configured between border devices and use solely IPv6 stacks, which is inaccurate. These tunnels can be implemented on various devices, and they encapsulate IPv6 inside IPv4, meaning both protocols are involved.

D is incorrect because it reverses the typical tunneling process. The usual IPv6 overlay tunnels encapsulate IPv6 packets within IPv4 packets, not IPv4 within IPv6.

In conclusion, the correct understanding is that IPv6 overlay tunnels are primarily a transition mechanism toward full IPv6 implementation, making B the right answer.

Question 7:

Which two types of circuits does the IS-IS routing protocol support? (Select two.)

A. nonbroadcast multiaccess
B. multiaccess
C. point-to-multipoint
D. nonbroadcast
E. point-to-point

Correct Answer: D, E

Explanation:

The IS-IS (Intermediate System to Intermediate System) routing protocol supports different circuit types depending on the network design and link characteristics. It is commonly deployed in large-scale networks, such as service provider infrastructures, and works efficiently across both LAN and WAN environments.

Two primary circuit types supported by IS-IS are point-to-point and nonbroadcast circuits:

• Point-to-point (Option E): This circuit type connects exactly two routers directly. It is the simplest form of link with no intermediary devices or multiple endpoints. Point-to-point circuits simplify routing because they avoid the complexity of multiple neighbors on a single link, which is ideal for IS-IS environments seeking efficiency and straightforward topology.

• Nonbroadcast (Option D): In nonbroadcast networks, routers do not automatically send link-state updates to all neighbors via broadcast or multicast. Instead, neighbors must be manually configured. This circuit type is common in WAN technologies such as Frame Relay or X.25, where native broadcast capability is absent. IS-IS supports these because it can handle manually defined adjacencies and still maintain robust routing.

On the other hand:

• Nonbroadcast multiaccess (Option A) is a term mostly associated with OSPF, describing networks like Frame Relay with multiple endpoints but no broadcast. IS-IS does not categorize circuits this way.

• Multiaccess (Option B) refers to networks like Ethernet, where multiple devices share a common broadcast medium. While IS-IS can operate on multiaccess LANs, it handles them differently, not using that specific term.

• Point-to-multipoint (Option C) is not a typical IS-IS circuit classification. IS-IS emphasizes point-to-point or nonbroadcast links rather than point-to-multipoint.

Therefore, the two correct circuit types supported by IS-IS are nonbroadcast and point-to-point.

Question 8:

Which Cisco-specific BGP path attribute is used to control the outbound flow of traffic?

A. Local Preference
B. MED
C. Weight
D. AS Path
E. Community

Correct Answer: C

Explanation:

In BGP (Border Gateway Protocol), various path attributes influence how routers select the best routes for traffic. When a network connects to multiple ISPs, managing outbound traffic is vital to optimize performance and costs. Among these attributes, some influence inbound traffic, while others affect outbound traffic.

• Local Preference (Option A): This attribute is used within an Autonomous System (AS) to prioritize exit points for outgoing traffic. While it helps influence routing decisions internally, it is not a Cisco-specific attribute and mainly affects traffic decisions within the AS.

• MED (Multi-Exit Discriminator) (Option B): MED signals to external AS neighbors which exit point is preferred when multiple interconnections exist. It mainly influences inbound traffic from other ASes, not outbound decisions made locally.

• Weight (Option C): This is a Cisco-proprietary attribute that affects outbound traffic flow. Unlike other attributes, weight is local to the router where it is configured and is not advertised to other routers. By assigning a higher weight to a particular path, a Cisco router prefers that route for outgoing traffic. This attribute allows precise control over how outbound traffic exits the network, especially useful in multi-ISP environments.

• AS Path (Option D): This attribute lists the autonomous systems a route has traversed and is primarily used for loop prevention and route selection. It doesn't provide a mechanism to control outbound traffic flow.

• Community (Option E): Communities are tags applied to routes for policy management and grouping. While they can influence routing decisions through policies, they do not directly control outbound traffic flow as clearly or simply as weight does.

In summary, the Weight attribute (Option C) is the best tool for Cisco devices to control outbound traffic because it locally influences which path is preferred for traffic leaving the router without propagating that preference to other routers.

Question 9:

When designing an enterprise network for unified communications, which two considerations are critical for ensuring Quality of Service (QoS) for voice and video traffic? (Choose two.)
A Prioritize voice and video packets using low-latency queuing
B Enable TCP retransmission for voice traffic to ensure reliability
C Use DiffServ code points (DSCP) to classify and mark packets
D Compress voice packets to reduce bandwidth consumption
E Apply static routing for all voice traffic paths

Answer: A, C

Explanation:

In the Cisco 300-420 exam (Implementing Cisco Enterprise Network Core Technologies), a key topic is ensuring Quality of Service (QoS) for latency-sensitive applications such as voice and video. QoS mechanisms help guarantee that these types of traffic receive priority treatment to maintain call quality and reduce jitter and delay.

A. Prioritize voice and video packets using low-latency queuing:
Voice and video are real-time traffic types highly sensitive to delay and jitter. Low-Latency Queuing (LLQ) allows these packets to be placed in a priority queue, ensuring they are transmitted ahead of other traffic. This reduces latency and jitter, which are critical for maintaining call quality and smooth video playback.

C. Use DiffServ code points (DSCP) to classify and mark packets:
DiffServ uses DSCP values in the IP header to classify traffic into different priority levels. By marking voice and video packets with high-priority DSCP values (like EF for voice), network devices along the path can recognize and apply appropriate QoS policies. This classification ensures consistent prioritization end-to-end across the network.

The other options are less appropriate:
B — TCP retransmission is unsuitable for voice because it introduces delays; voice uses UDP for real-time delivery.
D — Voice packet compression might reduce bandwidth but can degrade quality or increase CPU load; it's less about QoS.
E — Static routing isn’t a QoS mechanism and doesn’t ensure priority or low latency; routing protocols or dynamic methods are preferred for scalability.

In summary, prioritizing traffic with LLQ and classifying packets using DSCP are foundational QoS strategies to guarantee high-quality voice and video services in enterprise networks.

Question 10:

A network engineer is implementing a resilient Layer 3 design between two data centers using OSPF. Which two features improve convergence time and network stability? (Choose two.)

A Enable OSPF fast hello timers
B Configure OSPF passive interfaces on all links
C Use OSPF stub areas where applicable
D Disable OSPF authentication to reduce overhead
E Tune OSPF SPF timers to faster values

Answer: A, E

Explanation:

For the Cisco 300-420 exam, understanding routing protocol design and optimization is critical. When implementing OSPF between data centers, rapid convergence and network stability are key to maintaining availability and minimizing downtime.

A. Enable OSPF fast hello timers:
Fast hello packets allow routers to detect neighbor failures more quickly than the default hello interval (typically 10 seconds on broadcast networks). For example, reducing hello timers to 1 second can significantly speed up failure detection, triggering faster convergence and rerouting in case of link or node failure.

E. Tune OSPF SPF timers to faster values:
The Shortest Path First (SPF) algorithm recalculates the best routes when a topology change occurs. Tuning SPF timers (such as SPF delay and hold time) to lower values reduces the wait time before recalculation begins, enabling quicker route updates. However, these timers should be balanced carefully to avoid excessive CPU load from frequent SPF calculations.

Other options are less appropriate:
B — Passive interfaces prevent OSPF adjacencies on an interface, so enabling it on all links would break neighbor relationships, reducing connectivity.
C — Stub areas help reduce routing table size but don’t directly affect convergence speed.
D — Disabling authentication reduces security and does not improve convergence; authentication overhead is minimal and beneficial for securing routing updates.

In essence, adjusting hello intervals and SPF timers directly enhances OSPF’s responsiveness, leading to faster failover and improved network stability, which are essential for mission-critical data center environments.