Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Free Practice Test — 30 Questions

Question 1

Consider a large enterprise network where OSPF is deployed across multiple areas. Router R1 serves as an Area Border Router (ABR) connecting OSPF Area 0 to Area 1. A new high-speed Ethernet link is established between R1 and Router R2, which is also located within Area 1. R1 is also redistributing an external default route into OSPF as a Type 5 LSA. Network engineers are concerned that this new link, due to its default OSPF cost, might inadvertently become the preferred path for traffic originating in Area 1 destined for prefixes in Area 0 or other external networks advertised via R1, thereby creating suboptimal routing and potentially increasing latency for critical applications. Which configuration adjustment on R1\'s interface connecting to R2 would most effectively mitigate this risk and maintain optimal routing behavior?

Accepted Answer

Increase the OSPF cost on the interface of R1 connecting to R2 within Area 1.

Question 2

Consider a network administrator configuring BGP on a Cisco IOS XE router. The router receives two distinct paths for the destination prefix 192.168.1.0/24 from two different BGP neighbors. The path from neighbor 10.1.1.1 has been assigned a local weight attribute of 400, and the path from neighbor 10.1.2.2 has a local preference attribute of 350. Both paths have an AS_PATH length of 2 and an origin code of IGP. Which path will the router install in its routing table as the best path to reach 192.168.1.0/24?

Accepted Answer

The path learned from neighbor 10.1.1.1 due to the higher weight attribute.

Question 3

A network administrator is troubleshooting a Border Gateway Protocol (BGP) deployment where traffic destined for a specific external prefix is consistently taking a path with higher latency and lower bandwidth than an alternative path available through a different peering session. The administrator has already configured local preference attributes on their edge routers to favor the path with better link characteristics. Despite these configurations, BGP continues to select the suboptimal route. What fundamental BGP attribute, if longer for the preferred path, could be overriding the administrator\'s intended traffic engineering, leading to the selection of the path with poorer performance metrics?

Accepted Answer

AS_PATH length

Question 4

A network administrator is configuring BGP on router R1, which is connected to two different Internet Service Providers (ISPs), ISP-A and ISP-B. R1 is advertising its internal network prefix $192.168.1.0/24$ to both ISPs. R1 has a direct BGP peering with ISP-A, and ISP-A advertises the prefix $192.168.1.0/24$ back to R1 with a Multi-Exit Discriminator (MED) value of 100. R1 also establishes a BGP peering with ISP-B, which advertises the same prefix $192.168.1.0/24$ to R1 with a MED value of 50. Assuming all other BGP attributes are identical for these two routes as seen by R1, which path will R1 select for inbound traffic destined for the $192.168.1.0/24$ network, and why?

Accepted Answer

R1 will select the path advertised by ISP-B because a lower MED value is preferred in BGP path selection when comparing routes from different ASes.

Question 5

A network administrator for a large enterprise is troubleshooting intermittent reachability issues to a critical external prefix. Their network utilizes BGP with multiple ISPs. Analysis of BGP routing tables reveals that paths through ISP-A are consistently preferred over paths through ISP-B, even when the AS_PATH length to ISP-B is shorter and the local preference is identical. Further investigation shows that an inbound route map applied to the BGP session with ISP-B sets the Multi-Exit Discriminator (MED) to 200 for all prefixes originating from a specific customer segment, while no explicit MED manipulation is performed for prefixes received from ISP-A. The enterprise\'s internal policy aims to utilize ISP-B for this prefix due to its superior peering arrangements. What configuration aspect is most likely contributing to the observed suboptimal path selection and intermittent connectivity?

Accepted Answer

The inbound route map setting a high MED on prefixes received from ISP-B is making those paths less attractive to the enterprise's routers compared to paths with lower or no MEDs from ISP-A.

Question 6

A network administrator is troubleshooting a corporate network experiencing significant degradation in real-time application performance, including voice and video conferencing, alongside general sluggishness. The network utilizes OSPF for routing and has a sophisticated QoS policy applied to critical WAN links. Initial checks confirm OSPF is converging correctly and no routing loops are present. The administrator suspects a QoS misconfiguration. Upon reviewing the output of `show policy-map interface GigabitEthernet0/1`, which of the following observations would most strongly indicate a QoS misconfiguration that is directly contributing to the observed application performance issues?

Accepted Answer

A substantial number of packets are being dropped in the `VOICE_TRAFFIC` class, which is configured with a strict bandwidth guarantee.

Question 7

Consider an enterprise network using BGP for connectivity to multiple Internet Service Providers (ISPs). Router R1, located in AS 65001, is peering with R2 in AS 65002 and R3 in AS 65003. Both R2 and R3 are advertising the network prefix 192.168.1.0/24 to R1. R2 advertises this prefix with a BGP Multi-Exit Discriminator (MED) value of 100, and R3 advertises the same prefix with a MED value of 50. Assuming all other BGP attributes (AS_PATH length, Local Preference, Weight, etc.) are equal or not configured to influence this specific path selection, which path will R1\'s BGP process install into its routing table as the preferred route to reach 192.168.1.0/24?

Accepted Answer

The route learned from R3 via AS 65003, due to the lower MED value.

Question 8

During a network audit, a senior engineer observes that a critical internal application hosted in a multi-homed data center is experiencing intermittent connectivity issues. Analysis of the BGP routing tables on edge routers reveals that for a specific destination prefix, multiple inbound paths are advertised with identical weight and local preference values. The engineer also confirms that the origin attribute and AS-path length are identical for all these competing paths. What is the subsequent attribute BGP will utilize to select the preferred path to this destination?

Accepted Answer

The lowest BGP router ID of the next-hop

Question 9

A network administrator is tasked with resolving intermittent connectivity problems affecting several internal subnets, impacting critical applications. While unicast reachability to these subnets appears stable from the core, users report sporadic failures in accessing services that rely on multicast for discovery and communication. The troubleshooting steps taken so far include verifying IP reachability, examining routing tables for any anomalies, and confirming the health of unicast routing protocols. Despite these efforts, the issue persists, particularly when specific multicast-dependent services are utilized. Which of the following areas of investigation is most likely to reveal the root cause of this selective connectivity loss?

Accepted Answer

Protocol Independent Multicast (PIM) configuration and operational state

Question 10

Consider a network topology where AS 65001 peers with AS 65002 using eBGP multihop. Router R1 in AS 65001 has two internal routes to reach a specific destination prefix: one via an OSPF learned route with a metric of 50, and another via an internal static route with a metric of 75. R1 selects the OSPF route as its best path. R1 then advertises this prefix to router R3 in AS 65002. R3 also has an alternative eBGP peering with router R4 in AS 65002, which also advertises the same destination prefix. However, the IP address R1 uses for its eBGP multihop peering with R3 is not directly routable by R3 through any internal routing protocol or static configuration. Which of the following outcomes is most likely to occur regarding R3\'s BGP table and forwarding decision for the destination prefix?

Accepted Answer

R3 will install the route learned from R1 into its BGP table but will not install it into its routing table for forwarding because the next-hop IP address is unreachable. R3 will then rely on the route learned from R4, assuming it is valid and has a reachable next-hop.

Question 11

A network administrator for a large enterprise network is tasked with optimizing traffic flow. The company\'s AS has two eBGP peers, `Peer-1` and `Peer-2`, providing connectivity to the internet. The administrator wants to ensure that outbound traffic from the AS to a specific remote network prefers the path through `Peer-1`. Concurrently, inbound traffic from the remote network to the AS should also prefer the path originating from `Peer-1`. To achieve this, the administrator plans to configure BGP attributes. Considering the typical BGP path selection process and attribute precedence, which set of attribute manipulations would most effectively achieve both the outbound preference via `Peer-1` and the inbound preference for the AS from `Peer-1`\'s perspective, assuming standard BGP attribute weighting and default configurations elsewhere?

Accepted Answer

Increase the Weight attribute on the router advertising the path from Peer-1, increase the Local Preference attribute on an internal BGP peer for routes learned from Peer-1, and prepend the AS number on advertisements sent to Peer-2.

Question 12

A multinational corporation operates a network with two distinct Internet Service Providers (ISPs), AlphaNet and BetaConnect, providing redundant connectivity. The network engineering team needs to implement a traffic engineering policy that favors BetaConnect for all outbound traffic originating from their network and prioritizes AlphaNet for inbound traffic destined to their network. Considering the standard BGP attributes used for path selection and influence, which configuration strategy would best achieve this dual objective?

Accepted Answer

Configure a higher LOCAL_PREF value for routes learned from BetaConnect and advertise a lower MED value to AlphaNet for the organization's prefixes.

Question 13

A network administrator is troubleshooting a Border Gateway Protocol (BGP) deployment where prefixes originating from Autonomous System (AS) 65002 are being advertised to a customer (AS 65003) through a route reflector (AS 65001), despite an outbound route-map intended to filter these specific prefixes. The route-map applied to the route reflector\'s peering session with AS 65003 is configured with several permit and deny sequences, including one that sets a higher local preference for prefixes originating from AS 65002. However, the problematic prefixes are still visible in customer B\'s routing table. What is the most precise configuration adjustment required to prevent the advertisement of these specific prefixes from AS 65002 to AS 65003, while maintaining the intended local preference for other prefixes from AS 65002 that should be advertised?

Accepted Answer

Implement a prefix-list that precisely matches the problematic prefixes originating from AS 65002 and add a `deny` sequence using this prefix-list in the route-map applied outbound from the route reflector to AS 65003, placed before any general permit statements for AS 65002 prefixes.

Question 14

A network administrator is troubleshooting an issue where a subnet at Site B (192.168.2.0/24) is intermittently unreachable from Site A, despite stable BGP peering between the Provider Edge (PE) routers. The network utilizes MPLS VPNs to connect the two sites. Configuration review reveals that on the PE router serving Site A, a route-map named `BLOCK-SPECIFIC-SUBNETS` is applied inbound to the BGP neighbor session originating from Site B\'s PE. This route-map contains a sequence `deny 10` that matches a prefix-list `PRE-BLOCK-192-168-2`, which explicitly denies routes for `192.168.2.0/24`. Concurrently, on the PE router serving Site B, an outbound route-map `SET-METRIC` is applied to the BGP neighbor session for Site A\'s PE, which sets the BGP metric to 150 for advertised routes. Which configuration adjustment is most likely required to restore consistent reachability for the 192.168.2.0/24 subnet from Site A?

Accepted Answer

Modify the inbound route-map `BLOCK-SPECIFIC-SUBNETS` on Site A's PE router to permit the 192.168.2.0/24 subnet.

Question 15

Consider a scenario where a network administrator for a large enterprise network is tuning BGP path selection to favor a more stable upstream Internet Service Provider (ISP). The enterprise router has established BGP peering sessions with two different ISPs, ISP-Alpha and ISP-Beta, both advertising the same set of external routes. To ensure that routes learned from ISP-Alpha are consistently preferred over those from ISP-Beta, assuming all other BGP attributes are equal, what specific local configuration action on the enterprise router would achieve this objective without influencing the BGP attributes advertised to other autonomous systems?

Accepted Answer

Assigning a higher "Weight" attribute to routes learned from ISP-Alpha.

Question 16

Consider a multinational corporation, "Aether Dynamics," operating AS 65001, which has established dual eBGP peering sessions with a key service provider, "Global Connect," operating AS 65002. The peering sessions are maintained via two distinct edge routers, "Titan" and "Atlas," within Aether Dynamics\' network. Global Connect advertises a critical internal network prefix, $10.10.0.0/16$, to Aether Dynamics. Through the Titan router, Global Connect advertises this prefix with a MED of 75. Conversely, through the Atlas router, Global Connect advertises the same prefix with a MED of 120. Aether Dynamics\' network engineers have configured their internal routing policies to prioritize inbound traffic from Global Connect based on the lowest MED value. However, they are also concerned with optimizing outbound traffic from their internal network towards Global Connect\'s advertised prefix. Which statement accurately describes the impact of the MED values advertised by Global Connect on Aether Dynamics\' outbound traffic routing decisions?

Accepted Answer

The MED values advertised by Global Connect will not directly influence Aether Dynamics' selection of an exit router for outbound traffic destined for the $10.10.0.0/16$ prefix.

Question 17

A network administrator is configuring BGP on several edge routers within a large enterprise network. The goal is to ensure that traffic destined for a specific external network is primarily routed through Router A, which has a direct, high-bandwidth connection to the external provider. Router A is advertising a path to this destination with a local `weight` attribute set to 500. Router B, another edge router, is advertising the same destination network with a `weight` attribute of 100. Router C, a third edge router, advertises the same destination without any specific `weight` configuration. Considering the BGP path selection process, which path will the internal routers select as the best path for this destination network?

Accepted Answer

The path advertised by Router A due to its higher local weight.

Question 18

Consider a scenario where an enterprise network has a BGP peering with an external Service Provider (SP). Router R1 within the enterprise network receives two distinct routes for the same destination prefix from the SP\'s Autonomous System (AS 65001). The first route arrives with a local preference of 120 and an AS_PATH attribute consisting of [65001, 65002, 65003]. The second route arrives with a local preference of 150 and an AS_PATH attribute of [65001, 65001, 65001, 65001, 65001, 65002, 65003]. Additionally, the second route carries a BGP community `65001:100`. Based on standard BGP path selection criteria, which route will R1 select, and why?

Accepted Answer

R1 will prefer the route with a local preference of 150 and an AS_PATH of 5 hops, as local preference is the highest-weighted attribute in BGP path selection.

Question 19

A network administrator is troubleshooting BGP convergence issues within an enterprise network. The router has learned two distinct paths to the 192.168.1.0/24 network. Path 1 originates from an internal BGP peer and has a local preference of 350, a weight of 100, and a route dampening suppression count of 5. Path 2 is learned from an external BGP peer, has a local preference of 300, a weight of 0, and a route dampening suppression count of 0. Path 2 also has a MED of 75. Which path will the router select as the best path for the 192.168.1.0/24 network, and what is the primary reason for this selection over the alternative?

Accepted Answer

Path 2 will be selected because the significantly lower route dampening suppression count indicates greater stability, overriding the higher local preference of Path 1.

Question 20

A network administrator is troubleshooting intermittent performance degradation for real-time applications across a multi-homed enterprise network. Analysis of BGP routing tables reveals multiple paths to external destinations, with some paths exhibiting higher latency and lower available bandwidth than others. The administrator has implemented policies to influence BGP path selection but observes that traffic is not consistently utilizing the most optimal routes, leading to occasional congestion and packet loss. The network is configured with several BGP attributes to influence path selection, and in cases where primary attributes are equal, the BGP router ID serves as the final tie-breaker. To proactively steer traffic towards the higher bandwidth, lower latency path for critical applications, which BGP path selection attribute should be primarily manipulated on inbound route advertisements to ensure consistent preference within the Autonomous System?

Accepted Answer

Local Preference

Question 21

Consider a scenario where an enterprise network administrator initiates a planned reload of an OSPF router that has been configured with the Graceful Restart (GR) capability. Upon receiving the OSPF `Grace-LSAs` from the restarting router, what is the immediate and most critical operational outcome for the OSPF neighbors that also support this feature?

Accepted Answer

Neighbors will maintain their existing OSPF adjacencies and forwarding states with the restarting router for a predefined grace period, continuing to use the last known routing information.

Question 22

An enterprise network administrator is troubleshooting intermittent connectivity issues impacting a critical financial application. Users report high latency and occasional packet loss when accessing services hosted beyond a distribution switch (DS1), which is directly connected to the core router (R1). Initial checks by the administrator have confirmed that Layer 1 and Layer 2 connectivity between R1 and DS1 are stable, and basic IP reachability is established. No similar issues are reported for other network segments or applications. The administrator suspects an advanced routing or service configuration issue is at play. Which of the following scenarios most plausibly explains the observed intermittent problems on the R1-DS1 link?

Accepted Answer

An IP SLA monitor configured on R1 is detecting performance degradation on the R1-DS1 link and is actively rerouting traffic via a route-map.

Question 23

A network administrator is troubleshooting persistent BGP session instability between two core routers, R1 and R2, within a large enterprise network that utilizes BGP confederations. Symptoms include frequent BGP neighbor flaps and inconsistent route advertisements. An examination of R1\'s configuration reveals the command `bgp suppress-four-byte-as-capability`. R2 is configured with a private AS number, and the confederation structure involves multiple member ASes, some of which are represented by AS numbers that fall within the four-byte range. Considering the potential implications of this configuration on BGP peering and route exchange within a confederated environment, what is the most probable underlying cause for the observed BGP instability?

Accepted Answer

The suppression of the four-byte AS number capability prevents R1 from correctly negotiating BGP sessions with R2 or other confederation members that rely on extended AS number formats, leading to session flaps.

Question 24

A multinational corporation relies on a provider-managed MPLS WAN for connectivity between its global branch offices. Recently, users have reported intermittent reachability issues and occasional packet loss between sites, which network engineers suspect is related to route flapping within the provider\'s backbone. The corporation\'s network administrators have observed that the problem seems to be exacerbated when specific routes are advertised with varying attributes. To improve stability and ensure optimal path selection, the corporation wants to influence how the provider\'s BGP routers choose paths for traffic destined to their network. Considering the goal of influencing the preferred path for traffic entering the provider\'s network from the corporation\'s AS, which BGP attribute is most effectively used by the service provider to signal to external BGP peers the preferred path for inbound traffic, and which attribute is primarily used internally within an AS to influence outbound path selection?

Accepted Answer

MED is used by the provider to signal preferred ingress paths from the corporation's AS, while Local Preference is used internally by the provider to prefer specific exit paths to the corporation's network.

Question 25

A network administrator is configuring BGP on an enterprise edge router, R1. R1 has learned two distinct paths to the prefix 192.168.10.0/24 from two different iBGP peers, R2 and R3. Both R2 and R3 are advertising the same prefix. Upon inspection of R1’s BGP table, it is observed that both paths have identical Weight, Local Preference, AS_PATH length, Origin type, and MED attributes. The network administrator needs to ensure that R1 selects the path that leverages the most efficient internal routing for reaching the BGP next-hop. Which BGP path attribute, when evaluated after all other common tie-breakers have been exhausted for iBGP learned routes, will determine the best path selection in this scenario?

Accepted Answer

IGP cost to the BGP next-hop

Question 26

Consider a scenario where a multinational corporation\'s regional offices rely on a hybrid WAN architecture connecting to a central data center. The branch office (Site B), utilizing a Cisco IOS XR router, experiences sporadic connectivity issues with the main data center (Site A), where a Cisco IOS XE router is deployed. Both sites are configured with EIGRP (Autonomous System 100) for internal routing. EIGRP authentication using MD5 digests is enforced on the links connecting to the WAN. Upon investigation, the network administrators discover that while both routers are configured to use EIGRP authentication with key chains, the key strings associated with one of the key IDs are different between the two sites. Specifically, Site A\'s key chain includes key ID 1 with a key string of \"MySecretKey123\" and key ID 2 with a key string of \"AnotherKey456\". Site B\'s key chain has key ID 1 with \"MySecretKey123\" and key ID 2 with \"WrongKey789\". Given these configurations, what is the most direct cause of the intermittent reachability issues between Site A and Site B, and what specific action is required to rectify it?

Accepted Answer

The key string for key ID 2 on Site B's EIGRP authentication key chain must be changed from "WrongKey789" to "AnotherKey456" to match Site A's configuration.

Question 27

A multinational corporation\'s enterprise network, spanning multiple continents, is experiencing significant degradation in Voice over IP (VoIP) call quality between its remote branch offices and the central data center. Network monitoring indicates that while overall bandwidth utilization is within acceptable limits, packet loss and increased latency are prevalent for VoIP traffic. The network employs BGP for inter-AS routing between the corporate WAN and its regional hubs, and also for iBGP within the larger AS. The IT infrastructure team has identified that the current BGP path selection is not consistently favoring the most direct or lowest-latency routes for this critical application. To address this, they need to implement a BGP configuration that prioritizes optimal path selection for latency-sensitive applications originating from the branch offices. Which BGP attribute, when configured on the edge routers at the branch offices, would most effectively influence the exit path selection to ensure the most direct and performant route to the data center is consistently chosen?

Accepted Answer

Increasing the Local Preference for routes learned from the data center AS

Question 28

Consider a large enterprise network employing OSPF within its campus and BGP for its WAN edge. Users report sporadic connectivity issues to a critical internal financial application hosted on a server in the data center. Network monitoring reveals frequent OSPF adjacency flaps between distribution switches and the core router, leading to route instability. A security audit of the network perimeter and internal segmentation firewalls is initiated. Analysis of firewall logs and traffic patterns indicates that the firewall positioned between the distribution layer and the core router is exhibiting unusual traffic drops specifically targeting OSPF packets. This firewall is configured with advanced threat prevention features that perform deep packet inspection on various protocols. Which of the following explanations most accurately describes the likely root cause of the OSPF adjacency instability, considering the firewall\'s behavior?

Accepted Answer

The firewall's deep packet inspection engine is incorrectly classifying OSPF traffic (IP Protocol 89) as a UDP-based protocol and applying aggressive session timeouts, causing Hello packets to be dropped and adjacencies to reset.

Question 29

A network engineer is troubleshooting a persistent BGP route flapping issue affecting a critical application server\'s reachability across an inter-AS peering. Initial investigations reveal that the MED attribute for the prefixes pointing to the server is fluctuating rapidly, causing BGP sessions to re-converge and alternate between different paths. The engineer has already implemented route-maps to influence the MED on outbound advertisements to the peer, but the instability continues. What is the most effective BGP configuration change to definitively stabilize the BGP path selection for these specific prefixes, given the observed MED fluctuation?

Accepted Answer

Apply a route-map on the inbound advertisements from the peer to unconditionally set a fixed MED value for the affected prefixes.

Question 30

A multinational corporation\'s network utilizes MPLS L3VPNs to connect its branch offices. Network administrators observe sporadic reachability problems between two key branches, Branch Alpha and Branch Beta. Troubleshooting reveals that while routes are correctly learned via BGP between the Provider Edge (PE) routers, the Customer Edge (CE) routers at each branch intermittently lose connectivity to each other. Packet captures indicate that PE routers are advertising routes to each other with the next-hop attribute pointing to an interface that is not consistently reachable via the MPLS core. This behavior is particularly problematic when the PE routers\' loopback interfaces, used for BGP peering, are not explicitly set as the next hop for routes advertised between them. What BGP configuration change on the PE routers is most critical to resolve this intermittent reachability issue by ensuring that the PE routers can always resolve the next hop for VPN routes learned from other PEs?

Accepted Answer

Applying the `neighbor next-hop-self` command to the BGP neighbor configuration between the PE routers.

Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Free Practice Test — 30 Questions

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