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Question 1
Which Junos OS routing protocol is used to exchange routing information between autonomous systems on the Internet?
A) OSPF
B) IS-IS
C) BGP
D) RIP
Answer: C
Explanation:
Routing protocols operate at different scales and serve different purposes in network architectures. Interior gateway protocols manage routing within single administrative domains, while exterior gateway protocols handle routing between independent networks. Understanding which protocol addresses inter-domain routing is fundamental to comprehending Internet routing architecture.
BGP (Border Gateway Protocol) is the exterior gateway protocol used to exchange routing information between autonomous systems on the Internet. An autonomous system represents a collection of IP networks under single administrative control presenting a common routing policy to the Internet. BGP enables these independent networks to exchange reachability information, make policy-based routing decisions, implement traffic engineering, and maintain the global Internet routing table. BGP differs fundamentally from interior protocols by emphasizing policy control over optimal path selection, supporting very large routing tables with hundreds of thousands of routes, enabling complex routing policies based on AS path, communities, and other attributes, and providing mechanisms for preventing routing loops in the absence of a common metric. ISPs and large enterprises use BGP to connect to multiple upstream providers, control inbound and outbound traffic paths, and implement multi-homing for redundancy. BGP’s path vector algorithm maintains information about the sequence of autonomous systems a route traverses, enabling policy decisions and loop prevention. The protocol’s scalability and policy flexibility make it the only viable protocol for Internet-scale routing.
A is incorrect because OSPF (Open Shortest Path First) is an interior gateway protocol designed for routing within single autonomous systems, not between them; it optimizes paths based on cost metrics within administrative domains. B is incorrect because IS-IS (Intermediate System to Intermediate System) is also an interior gateway protocol used within autonomous systems, particularly by large ISPs for their internal routing. D is incorrect because RIP (Routing Information Protocol) is a distance vector interior protocol with limited scalability, used within small networks, not for inter-AS routing on the Internet.
Question 2
In Junos OS, which command displays the active routing table?
A) show route
B) show ip route
C) display route table
D) get route active
Answer: A
Explanation:
Network troubleshooting and verification require examining routing tables to understand how devices forward traffic. Different network operating systems use different command syntax, and understanding Junos OS-specific commands is essential for effectively managing Juniper equipment.
The “show route” command displays the active routing table in Junos OS, presenting routes learned through various sources including directly connected networks, static routes, and dynamic routing protocols. The routing table shows destination prefixes, next-hop addresses, route preferences (administrative distance), route metrics, and the protocols or sources that installed routes. Output includes active routes actually used for forwarding, inactive routes available but not selected due to lower preference or higher metric, and hidden routes suppressed by routing policies. The command supports numerous options for filtering output such as “show route protocol ospf” to display only OSPF routes, “show route table inet.0” to show specific routing tables, or “show route 10.1.1.0/24” to examine routes for particular destinations. Understanding routing table contents is fundamental to troubleshooting routing issues, verifying routing protocol operation, and confirming traffic forwarding behavior. The routing table in Junos OS distinguishes between routing tables like inet.0 for IPv4 unicast and inet6.0 for IPv6 unicast.
B is incorrect because “show ip route” is Cisco IOS syntax, not Junos OS; while the functionality is similar, Juniper uses different command structure. C is incorrect because “display route table” is not valid Junos OS syntax; Junos uses “show” as the primary verb for displaying operational information. D is incorrect because “get route active” is not valid Junos syntax; the command structure follows “show” verb patterns consistent across Junos OS.
Question 3
Which OSPF area type does not allow external routes (Type 5 LSAs) but does allow summary routes?
A) Backbone area
B) Stub area
C) Totally stubby area
D) Not-so-stubby area (NSSA)
Answer: B
Explanation:
OSPF area design uses different area types to control routing information distribution, reduce routing table size, and optimize protocol overhead. Understanding how different area types filter LSAs helps network designers implement appropriate topologies for specific requirements.
Stub areas do not allow external routes (Type 5 LSAs) from being advertised into the area but do permit summary routes (Type 3 LSAs) from other OSPF areas. Type 5 LSAs represent external routes redistributed into OSPF from other protocols like BGP or static routes. By blocking Type 5 LSAs, stub areas reduce routing table size and LSA flooding in parts of the network that don’t require full external routing information. Routers in stub areas reach external destinations through a default route injected by the Area Border Router (ABR). However, stub areas still receive Type 3 summary LSAs allowing routers to learn about networks in other OSPF areas within the same OSPF domain. This design suits branch locations that need connectivity to corporate networks (via summaries) but don’t need to know about all external routes. Configuring stub areas requires enabling the stub designation on all routers in the area, as mixed stub/non-stub configurations cause adjacency failures. Stub areas reduce memory and CPU requirements on routers with limited resources while maintaining internal OSPF connectivity.
A is incorrect because the backbone area (Area 0) allows all LSA types including external routes; it serves as the central hub connecting all other areas and must carry complete routing information. C is incorrect because totally stubby areas are more restrictive than stub areas, blocking both external routes and summary routes, only allowing a default route from the ABR. D is incorrect because NSSA (Not-So-Stubby Area) is designed specifically to allow limited external route redistribution within stub areas through Type 7 LSAs while still blocking Type 5 LSAs from other areas.
Question 4
Which Junos OS configuration hierarchy contains commands for configuring routing protocols?
A) [edit interfaces]
B) [edit protocols]
C) [edit routing-options]
D) [edit system]
Answer: B
Explanation:
Junos OS uses a hierarchical configuration structure organizing related settings into logical groupings. Understanding configuration hierarchy helps administrators efficiently navigate to appropriate sections when configuring different features and maintains consistent configuration management.
The [edit protocols] hierarchy contains configuration commands for routing protocols including OSPF, BGP, IS-IS, RIP, and other protocols. Within this hierarchy, each protocol has its own sub-hierarchy for protocol-specific settings. For example, [edit protocols ospf] contains OSPF configuration including area definitions, interface settings, and protocol parameters, while [edit protocols bgp] contains BGP configuration like neighbor definitions, AS numbers, and BGP policies. The protocols hierarchy organizes all dynamic routing protocol configurations in a single logical location making it intuitive to find and configure routing protocols. Protocol configuration typically includes enabling the protocol, defining areas or neighbors, specifying authentication, and configuring protocol-specific timers or policies. The hierarchical structure allows administrators to use “edit protocols” to navigate to the protocols level, then configure specific protocols within that context. This organizational approach provides clear separation between protocol configuration, interface configuration, and routing policy application.
A is incorrect because [edit interfaces] configures physical and logical interface properties like IP addresses, VLAN tags, and interface parameters, not routing protocol operation. C is incorrect because [edit routing-options] configures global routing parameters like static routes, autonomous system numbers, and router ID, but not the routing protocols themselves. D is incorrect because [edit system] configures system-level settings like hostname, time zone, and management access, not routing protocols.
Question 5
What is the default priority value for OSPF interfaces in Junos OS?
A) 1
B) 64
C) 100
D) 128
Answer: D
Explanation:
OSPF designated router election on broadcast and non-broadcast multi-access networks uses priority values to determine which router becomes the DR and BDR. Understanding default values helps predict election outcomes and enables deliberate DR/BDR placement when defaults are used.
The default OSPF interface priority value in Junos OS is 128. Priority ranges from 0 to 255, with higher values being preferred in DR/BDR elections. A priority of 0 makes a router ineligible to become DR or BDR. During DR election, the router with highest priority becomes DR, second-highest becomes BDR, and remaining routers become DROTHERs. When priorities are equal, router ID serves as the tiebreaker with higher router ID winning. The default priority of 128 allows administrators flexibility to influence elections by setting higher priorities on routers that should become DR or lower priorities on routers that should not. Many network designs deliberately configure DR/BDR placement by adjusting priorities rather than relying on defaults, particularly when routers have different capabilities or when specific topological arrangements are desired. Priority changes don’t cause immediate re-election; existing DR/BDR remain until failure or the OSPF process restarts. Understanding default priority helps troubleshoot unexpected election outcomes and plan appropriate priority configurations for network designs.
A is incorrect because 1 is not the default OSPF priority in Junos OS, though some network engineers might configure this low priority on routers that should not become DR. B is incorrect because 64 is not used as a default in Junos OSPF priority settings. C is incorrect because while 100 is used as a default in some other Junos contexts like BGP preference, it is not the OSPF priority default.
Question 6
Which BGP attribute is used to prevent routing loops in AS path vector routing?
A) Local Preference
B) AS Path
C) MED
D) Origin
Answer: B
Explanation:
BGP’s path vector approach to routing differs from distance vector and link-state protocols by maintaining complete path information. Understanding how BGP prevents loops without distance metrics or topology databases helps comprehend the protocol’s design and behavior.
The AS Path attribute prevents routing loops in BGP by maintaining a list of autonomous systems through which a route advertisement has passed. When a BGP router advertises a route to a neighbor, it prepends its own AS number to the AS Path. Upon receiving a route, a BGP router examines the AS Path and rejects any route containing its own AS number, as this would indicate a routing loop. This mechanism works because the AS Path grows with each AS the route traverses, and any router seeing its own AS number in the path knows the route would create a loop if accepted. AS Path serves multiple purposes beyond loop prevention including path length as a tie-breaking factor in route selection where shorter AS paths are preferred, policy routing where administrators can filter or prefer routes based on AS Path contents, and traffic engineering through AS Path prepending to make routes less attractive. The AS Path’s role in loop prevention is fundamental to BGP’s scalability, enabling the protocol to operate without the convergence issues that plague distance vector protocols lacking path information.
A is incorrect because Local Preference is used for choosing exit points from an AS and affects path selection within an AS but doesn’t prevent loops. C is incorrect because MED (Multi-Exit Discriminator) influences path selection between autonomous systems but doesn’t provide loop prevention. D is incorrect because Origin indicates the source of the route (IGP, EGP, or incomplete) but doesn’t prevent routing loops.
Question 7
In Junos OS, which command commits configuration changes to the active configuration?
A) save
B) apply
C) commit
D) write
Answer: C
Explanation:
Junos OS configuration management uses a candidate configuration model providing separation between configuration editing and activation. Understanding the commit process is fundamental to safely managing Juniper devices and avoiding configuration errors that could disrupt network operation.
The “commit” command activates configuration changes by moving the candidate configuration to become the active configuration. Junos OS maintains two configurations including the candidate configuration where administrators make and edit changes, and the active configuration actually running on the device. Administrators can make multiple configuration changes in the candidate without affecting operation, review all pending changes using “show | compare”, and validate syntax before activation. The commit process validates configuration syntax, checks for configuration errors, and if successful, applies changes to the active configuration and running system. If validation fails, Junos displays errors and retains the candidate configuration for correction without disrupting the active configuration. This model prevents accidental partial configurations from being activated and provides opportunity to review all changes before impact. Advanced commit options include “commit confirmed” which requires a second commit within a specified time or automatically rolls back, and “commit and-quit” which commits and exits configuration mode in one command. The commit model provides safety and predictability in configuration management.
A is incorrect because “save” is not a Junos command for activating configuration; Junos automatically saves committed configurations. B is incorrect because “apply” is not used for committing configuration in Junos OS; it is used in some contexts for applying configuration groups but not for activating candidate configuration. D is incorrect because “write” is Cisco IOS syntax for saving configuration, not Junos syntax for committing configuration.
Question 8
Which VLAN tagging standard uses a 12-bit VLAN ID field in the Ethernet frame?
A) ISL
B)1Q
C) VTP
D) QinQ
Answer: B
Explanation:
VLAN tagging enables multiple virtual LANs to share physical network infrastructure by adding VLAN identification to Ethernet frames. Different tagging protocols exist with different characteristics, and understanding standard tagging is essential for interoperability in multi-vendor environments.
IEEE 802.1Q is the industry standard VLAN tagging protocol that inserts a 4-byte tag into Ethernet frames containing a 12-bit VLAN ID field. The 12-bit field allows 4,096 possible VLAN IDs (0-4095), though some values are reserved. The 802.1Q tag also includes a 3-bit priority field for Quality of Service and a 1-bit CFI (Canonical Format Indicator) field. 802.1Q inserts the tag between the source MAC address and EtherType/Length fields in the Ethernet frame, modifying the frame structure. The standard designates one VLAN as the native VLAN which carries untagged traffic on trunk links, though best practice often configures the native VLAN to an unused VLAN ID for security. 802.1Q’s standardization makes it the preferred tagging method for multi-vendor environments where interoperability is required. The protocol supports VLAN ranges from 1-4094 with VLAN 1 typically serving as the default and VLAN 4095 being reserved. Understanding 802.1Q frame structure is essential for troubleshooting VLAN-related issues and configuring trunk ports between switches.
A is incorrect because ISL (Inter-Switch Link) is Cisco’s proprietary VLAN tagging protocol that encapsulates entire Ethernet frames rather than inserting tags, and it has been largely deprecated in favor of 802.1Q. C is incorrect because VTP (VLAN Trunking Protocol) is Cisco’s protocol for distributing VLAN configuration information, not a tagging protocol. D is incorrect because QinQ (802.1ad) stacks 802.1Q tags for service provider networks but is not the basic standard VLAN tagging protocol.
Question 9
Which spanning tree protocol provides the fastest convergence for Layer 2 networks?
A) STP (802.1D)
B) RSTP (802.1w)
C) MSTP (802.1s)
D) PVST+
Answer: B
Explanation:
Spanning tree protocols prevent Layer 2 loops while providing redundancy, but different versions offer varying convergence speeds. Understanding convergence characteristics helps network designers select appropriate protocols based on uptime requirements and convergence time objectives.
RSTP (Rapid Spanning Tree Protocol, 802.1w) provides the fastest convergence among standard spanning tree protocols, achieving sub-second convergence in optimal conditions compared to 30-50 seconds for traditional STP. RSTP’s speed improvements come from several mechanisms including rapid transition to forwarding state for edge ports without waiting for timers, alternate and backup port roles that provide pre-calculated backup paths, new port states (discarding, learning, forwarding) that simplify state machine, and proposal/agreement handshake that speeds up convergence on point-to-point links. When topology changes occur, RSTP-enabled switches can determine and activate backup paths almost immediately rather than waiting through listening and learning states. Edge ports configured with PortFast-equivalent settings transition immediately to forwarding without negotiation. Point-to-point links between switches use the proposal/agreement process where the downstream switch synchronizes its ports before responding, enabling rapid reconvergence. RSTP maintains backward compatibility with 802.1D STP but operates in legacy mode when interoperating with older switches, losing rapid convergence benefits. Modern networks typically deploy RSTP or MSTP (which incorporates RSTP convergence mechanisms) rather than legacy STP.
A is incorrect because traditional STP (802.1D) has the slowest convergence, taking 30-50 seconds through listening and learning states before forwarding. C is incorrect because while MSTP (Multiple Spanning Tree Protocol) uses RSTP’s rapid convergence mechanisms, the question asks specifically about convergence speed, and MSTP’s primary benefit is multiple instance support, not faster convergence than RSTP. D is incorrect because PVST+ is Cisco’s per-VLAN STP implementation that can use rapid-PVST+ for faster convergence but is not inherently faster than standard RSTP.
Question 10
In Junos OS, which configuration statement enables an interface to participate in OSPF?
A) set protocols ospf interface interface-name
B) set interfaces interface-name protocols ospf
C) set routing-options ospf interface interface-name
D) set ospf interface interface-name enable
Answer: A
Explanation:
Configuring OSPF in Junos OS requires enabling specific interfaces to participate in the protocol. Understanding correct configuration syntax ensures successful OSPF deployment and helps avoid configuration errors that prevent neighbor adjacencies.
The correct syntax to enable an interface for OSPF in Junos OS is “set protocols ospf area area-id interface interface-name” where the interface statement appears within the protocols ospf hierarchy under a specific area. For example, “set protocols ospf area 0.0.0.0 interface ge-0/0/1.0” enables interface ge-0/0/1.0 in OSPF area 0. The configuration hierarchy reflects OSPF’s area-based design where interfaces belong to areas. Additional interface-specific parameters can be configured at this level including interface priority for DR election, interface cost for metric calculation, and authentication settings. Unlike some routing protocols where interfaces are implicitly included based on network statements matching interface IP addresses, OSPF in Junos explicitly requires listing interfaces under the protocol configuration. This explicit model provides clear visibility into which interfaces participate in OSPF and reduces ambiguity in configurations. The interface must have an IP address configured and be in an up state to establish OSPF adjacencies. Passive interfaces can be configured to advertise networks without forming adjacencies.
B is incorrect because OSPF configuration doesn’t occur under the interfaces hierarchy; protocol enablement happens under [edit protocols]. C is incorrect because while routing-options contains some global routing parameters, OSPF interface configuration happens under [edit protocols ospf]. D is incorrect because the configuration syntax doesn’t use standalone “ospf” statements at the top level; OSPF configuration must be within the protocols hierarchy.
Question 11
Which BGP message type is used to advertise, update, or withdraw routes?
A) OPEN
B) KEEPALIVE
C) UPDATE
D) NOTIFICATION
Answer: C
Explanation:
BGP operates through exchanging messages between peers, with different message types serving specific purposes in the protocol’s operation. Understanding message types helps comprehend BGP behavior and troubleshoot issues with route advertisement or peer relationships.
UPDATE messages advertise new routes, update existing route attributes, or withdraw previously advertised routes. Update messages contain multiple components including withdrawn routes listing prefixes no longer reachable, path attributes describing characteristics of advertised routes, and Network Layer Reachability Information (NLRI) listing prefixes being advertised. A single UPDATE message can advertise multiple prefixes sharing common attributes, or withdraw multiple prefixes no longer available. BGP’s incremental update model means routers send UPDATES only when routing information changes rather than periodically advertising their entire routing table. After initial full table exchange, peers exchange only incremental updates reflecting changes. UPDATE messages drive BGP’s core function of distributing routing information. Attributes in UPDATE messages include AS_PATH showing the route through autonomous systems, NEXT_HOP indicating where to forward traffic, LOCAL_PREF for route preference within an AS, and MED for influencing inbound traffic. Understanding UPDATE message structure is essential for troubleshooting why routes aren’t being advertised or why particular paths are chosen.
A is incorrect because OPEN messages establish BGP sessions, exchanging parameters like AS number, router ID, and hold time, but don’t advertise routes. B is incorrect because KEEPALIVE messages maintain sessions by periodically confirming peer liveness but don’t carry routing information. D is incorrect because NOTIFICATION messages report errors and close sessions but don’t advertise or withdraw routes.
Question 12
What is the administrative distance of OSPF routes in Junos OS?
A) 10
B) 20
C) 110
D) 150
Answer: D
Explanation:
Administrative distance or preference determines which routes are installed in the routing table when multiple protocols provide routes to the same destination. Understanding preference values in Junos OS helps predict route selection and enables appropriate route preference configuration.
OSPF routes have an administrative distance (called preference in Junos OS) of 150 for both internal and external OSPF routes by default. Junos uses the term “preference” rather than “administrative distance” but the concept is identical where lower values are preferred. When multiple protocols offer routes to the same destination, the route with lowest preference is installed in the routing table. Understanding Junos preference values helps predict route selection including direct and local routes with preference 0, static routes with preference 5, OSPF internal routes with preference 10 in traditional implementations but 150 in Junos default, and BGP with preference 170. The difference between Cisco and Juniper default preferences can surprise administrators familiar with one platform working on the other. Junos allows modifying preferences through routing policies, enabling administrators to prefer routes from specific protocols. For example, if both OSPF and static routes exist for a destination, the static route with preference 5 would be preferred over OSPF’s preference 150. Understanding preference interactions is crucial for proper route selection in networks running multiple protocols.
A is incorrect because 10 is used for OSPF internal routes in some other implementations and in traditional networking texts, but not Junos OS default preference. B is incorrect because 20 is not a standard preference value in Junos OS. C is incorrect because 110 is the Cisco IOS administrative distance for OSPF, not the Junos OS preference value.
Question 13
Which LACP mode actively sends LACP packets to negotiate link aggregation?
A) Passive
B) Active
C) On
D) Auto
Answer: B
Explanation:
Link aggregation control protocol enables dynamic negotiation of aggregated links providing higher bandwidth and redundancy. Understanding LACP modes helps configure link aggregation that establishes correctly between devices.
Active LACP mode actively sends LACP packets to negotiate link aggregation with remote devices. LACP operates in two primary modes where active mode initiates negotiation by sending LACP packets and responds to received packets, and passive mode doesn’t initiate but responds to received LACP packets. For successful LACP negotiation, at least one end must be in active mode; both ends can be active, but both ends cannot be passive as no negotiation would begin. Active mode is typically configured on at least one side of aggregated links to ensure negotiation occurs. LACP packets exchange information about system priority, port priority, and operational status, enabling both devices to agree on which ports participate in the aggregation. The protocol detects failures of individual links in the aggregate and redistributes traffic across remaining links. LACP also prevents misconfiguration by refusing to aggregate ports with inconsistent parameters like speed or duplex mismatches. Understanding active versus passive modes is important for ensuring aggregation establishes; many implementations use active mode on both ends for simplicity, though passive mode may be chosen when wanting the other device to control negotiation.
A is incorrect because passive mode responds to LACP packets but doesn’t initiate negotiation, requiring the other side to be active. C is incorrect because “On” mode forces aggregation without LACP negotiation, potentially creating issues if the other side isn’t configured correctly. D is incorrect because “Auto” is not a standard LACP mode term; the standard modes are active and passive.
Question 14
Which command displays the MAC address table on a Juniper switch running Junos OS?
A) show mac-address-table
B) show ethernet-switching table
C) display mac addresses
D) show bridge table
Answer: B
Explanation:
Troubleshooting Layer 2 connectivity and VLAN configuration requires examining the MAC address table to understand how switches learn and forward traffic. Knowing correct Junos commands for displaying switching information enables effective troubleshooting.
The “show ethernet-switching table” command displays the MAC address table on Juniper EX series switches, showing learned MAC addresses, associated VLANs, and interfaces where addresses were learned. The output includes MAC addresses in the table, VLAN IDs where each address was learned, logical interfaces associated with each MAC address, and entry age or type indicating dynamic learning or static configuration. This information is essential for troubleshooting MAC address learning issues, verifying VLAN membership, confirming proper trunk configuration, and investigating switching loops or broadcast storms. The command supports filtering options like “show ethernet-switching table vlan vlan-name” to display only specific VLAN entries, or “show ethernet-switching table interface interface-name” to show addresses learned on particular interfaces. Understanding MAC address table contents helps verify that switches are learning addresses on correct ports and VLANs. The table is dynamically built through learning source MAC addresses from received frames and ages out entries that haven’t been seen recently.
A is incorrect because “show mac-address-table” is Cisco IOS syntax, not Junos syntax for displaying the MAC table. C is incorrect because “display” is not the Junos verb for showing operational information; Junos uses “show” consistently. D is incorrect because “show bridge” is not the current Junos command for displaying the MAC table; “ethernet-switching table” is the correct command for EX series switches.
Question 15
In OSPF, which LSA type represents external routes redistributed into OSPF?
A) Type 1 – Router LSA
B) Type 2 – Network LSA
C) Type 3 – Summary LSA
D) Type 5 – AS External LSA
Answer: D
Explanation:
OSPF uses different LSA types to advertise different categories of routing information throughout the OSPF domain. Understanding LSA types helps comprehend OSPF operation, troubleshoot routing issues, and design appropriate area structures.
Type 5 AS External LSAs represent routes redistributed into OSPF from other routing protocols or sources external to the OSPF domain. These LSAs are originated by Autonomous System Boundary Routers (ASBRs) that redistribute external routes into OSPF. Type 5 LSAs flood throughout the entire OSPF domain except into stub areas, which specifically block Type 5 LSAs to reduce routing table size. External routes can be Type 1 external (E1) where the metric includes both the external cost and OSPF internal cost, or Type 2 external (E2, the default) where the metric includes only the external cost. Type 5 LSAs enable OSPF to integrate with other routing protocols and advertise routes learned from BGP, static routes, or other sources. The ASBR sets the metric and metric type in Type 5 LSAs based on redistribution configuration. All OSPF routers maintain Type 5 LSAs in their link state database and use them to calculate routes to external destinations. Understanding Type 5 LSA behavior is essential for implementing route redistribution and designing multi-protocol networks.
A is incorrect because Type 1 Router LSAs describe router links and are originated by every OSPF router within an area, not external routes. B is incorrect because Type 2 Network LSAs are originated by designated routers on broadcast networks describing attached routers, not external routes. C is incorrect because Type 3 Summary LSAs are originated by area border routers advertising routes between OSPF areas, not external redistributed routes.
Question 16
Which Junos OS routing policy action accepts a route and stops policy evaluation?
A) accept
B) permit
C) allow
D) next-policy
Answer: A
Explanation:
Junos routing policies control route advertisement and acceptance using a structured policy language. Understanding policy actions and their effects on policy evaluation flow is essential for implementing correct routing policies that achieve desired objectives.
The “accept” action in Junos routing policies accepts a route and stops policy evaluation for that route. When a route matches conditions in a term with the accept action, Junos accepts the route with any modifications made by the term, and skips evaluating remaining terms in the current policy and any subsequent policies in the chain. This behavior differs from some platforms where multiple policy statements may be evaluated. The definitive accept behavior means term order matters significantly in Junos policies since later terms never evaluate routes accepted by earlier terms. Policy structure includes terms containing from clauses specifying match conditions, then clauses applying actions like modifying attributes, and action specifications of accept, reject, or next term. The accept action can include attribute modifications in the then clause, so routes can be accepted with modified metrics, communities, or preferences. Understanding accept behavior helps avoid policy mistakes where administrators expect later terms to process already-accepted routes. Default policy behavior when no explicit action is configured depends on policy type, with some defaulting to accept and others to reject if no terms match.
B is incorrect because “permit” is terminology used in some other platforms’ routing policy syntax but not Junos, which uses “accept” for this function. C is incorrect because “allow” is not Junos policy action terminology. D is incorrect because “next-policy” continues to the next policy in a chain rather than accepting the route and stopping evaluation; it’s used when current policy has no definitive action.
Question 17
What is the default bridge priority value for spanning tree in Junos OS?
A) 0
B) 32768
C) 4096
D) 65535
Answer: B
Explanation:
Spanning tree root bridge election uses bridge priority to determine which switch becomes root. Understanding default priority values helps predict election outcomes and enables appropriate configuration of desired root bridges.
The default bridge priority in Junos OS spanning tree is 32768. Bridge priority values range from 0 to 65535, with lower values being preferred in root bridge election. The bridge priority combines with the MAC address to form the bridge ID used in root election where the switch with lowest bridge ID becomes root bridge, second lowest becomes secondary root, and remaining switches become non-root. When all switches use default priority 32768, MAC address becomes the tiebreaker with lowest MAC address winning. Network designs should explicitly configure priority on intended root bridges rather than relying on defaults and MAC addresses. Primary root bridges typically receive priority values of 4096 or 8192, secondary roots receive 12288 or 16384, ensuring these switches win election regardless of MAC addresses. Priority must be configured in multiples of 4096 in modern spanning tree implementations. Understanding default priority is crucial for predicting spanning tree behavior in networks and troubleshooting unexpected topology outcomes. Many network issues trace to unintended root bridge selection when priorities are left at defaults, causing suboptimal traffic paths.
A is incorrect because 0 is the lowest possible priority that could be configured to guarantee root bridge selection, not the default. C is incorrect because 4096 is a common configured value for primary root bridges but not the default. D is incorrect because 65535 is the highest possible priority, opposite of what would be a reasonable default.
Question 18
Which BGP attribute is used to influence inbound traffic from external autonomous systems?
A) Local Preference
B) AS Path
C) MED (Multi-Exit Discriminator)
D) Weight
Answer: C
Explanation:
BGP provides multiple attributes for traffic engineering and path selection. Understanding which attributes influence inbound versus outbound traffic helps implement effective BGP policies for controlling traffic flows in multi-homed networks.
MED (Multi-Exit Discriminator) influences inbound traffic from external autonomous systems by suggesting to neighboring AS which entry point they should use when multiple connections exist between the autonomous systems. When an AS has multiple connection points to a neighbor AS, it can use MED to indicate preferred entry points for traffic returning from that neighbor. Lower MED values are preferred, so advertising lower MED values makes that path more attractive to the neighboring AS for sending traffic inbound. For example, if AS 100 connects to AS 200 at two locations, AS 100 can advertise its routes with lower MED values at the preferred entry point, suggesting AS 200 should send traffic via that path. MED is a suggestion rather than a requirement since the receiving AS controls its own routing decisions. MED is compared only between routes received from the same neighboring AS, not between different neighbors. Organizations use MED for traffic engineering when they want to influence which link neighbors use to reach their networks, such as preferring traffic ingress through higher-capacity links or geographically closer connection points.
A is incorrect because Local Preference influences outbound traffic paths by controlling exit point selection within an AS, not inbound traffic from external neighbors. B is incorrect because while AS Path length influences path selection, AS Path prepending is typically used to make routes less attractive to external ASes, which is the opposite of standard inbound traffic control. D is incorrect because Weight is a Cisco-specific attribute that is local to the router and not propagated, so it cannot influence decisions made by external autonomous systems.
Question 19
A network engineer configures routing policies on Junos to manipulate BGP attributes. What is the default behavior when no routing policy matches a BGP route?
A) Import policy: reject routes, Export policy: accept routes
B) All routes are accepted regardless of policy
C) All routes are rejected regardless of policy
D) Random decision for each route
Answer: A
Explanation:
Junos routing policies control route advertisement and acceptance enabling manipulation of routing information for traffic engineering, security, and network optimization. Understanding default policy behavior is essential for avoiding unintended routing impacts. Routing policy structure consists of terms containing match conditions like route prefix, AS path, community, or next hop, and actions like accept, reject, or attribute modification. Policy direction determines when policies are evaluated with import policies applied to routes received from routing peers before installation in routing table, and export policies applied to routes sent to routing peers selecting which local routes are advertised. Default policy behavior differs between import and export reflecting different security models. For BGP import policies default behavior is reject meaning routes not explicitly accepted by policy are rejected preventing automatic acceptance of potentially harmful routes, requiring intentional policy allowing desired routes providing security through explicit allowlisting, and preventing accidental route acceptance from misconfigured or malicious peers. For BGP export policies default behavior is accept meaning locally originated routes are advertised by default enabling route propagation without explicit policy, requiring policies to prevent export of sensitive routes, and simplifying basic BGP configuration. This asymmetric default enables conservative secure import handling while simplifying basic export cases. Policy chain evaluation allows multiple policies applied sequentially with later policies potentially modifying earlier decisions through accept with next term or next policy actions. Policy inheritance supports hierarchical policy with group-level policies inherited by members unless overridden. Best practices include explicit accept or reject as policy terminating actions avoiding reliance on defaults, defensive import policies preventing undesired route acceptance, and export policies protecting route confidentiality. Policy testing using test policy command validates policy behavior before production deployment. Performance optimization minimizes policy complexity and leverages hardware acceleration where available.
B is incorrect because Junos does not accept all routes regardless of policy. This would eliminate policy control and security benefits. Default behavior differs between import and export with import rejecting by default.
C is incorrect because while import policy defaults to reject, export policy defaults to accept. Claiming all routes are rejected would prevent BGP from functioning properly by suppressing route advertisements.
D is incorrect because policy evaluation is deterministic not random. Random decision-making would make network behavior unpredictable and impossible to troubleshoot. Consistent policy evaluation enables reliable routing design.
Question 20
An administrator notices that OSPF is not propagating routes between areas. What is the most likely cause if inter-area routes are missing?
A) Area Border Router misconfiguration or missing area 0 backbone connectivity
B) All routers have identical configuration
C) OSPF is inherently broken and cannot route between areas
D) Inter-area routing is impossible with OSPF
Answer: A
Explanation:
OSPF hierarchical architecture divides autonomous system into areas with area 0 serving as backbone area connecting all other areas. Inter-area routing requires proper area design and Area Border Router configuration. ABRs connect multiple areas maintaining separate link-state database for each area, performing route summarization at area boundaries, and advertising inter-area routes using Type-3 LSAs. Critical requirement for inter-area routing is that all ABRs must connect to area 0 backbone either through physical interface or virtual link. Area 0 serves as routing information hub with ABRs advertising routes from non-backbone areas into backbone, and redistributing routes from backbone into non-backbone areas. Common inter-area routing issues include ABR not connected to area 0 preventing route propagation between areas, area 0 partitioned into disconnected segments breaking inter-area path, ABR misconfiguration missing area definitions preventing proper LSA generation, and area range configuration errors blocking route summarization. Virtual links provide logical area 0 connectivity through transit area when physical area 0 connection is impractical configuring tunnel through intermediate area treating it as area 0 extension. Troubleshooting inter-area issues includes verifying ABR configuration with show ospf overview confirming router is ABR, show ospf database summary examining Type-3 LSAs in database, show route protocol ospf checking if inter-area routes are present, and show ospf interface extensive verifying area assignments. ABR optimization includes route summarization reducing LSA flooding and routing table size, stub area configuration preventing external route advertisement, and NSSA allowing external routes while maintaining stub benefits. Area design best practices include limiting area size to manageable LSDB, minimizing area count reducing complexity, and strategic ABR placement providing optimal traffic paths.
B is incorrect because identical configuration on all routers does not explain inter-area routing failure. ABRs require specific configuration differences including belonging to multiple areas which other routers do not have.
C is incorrect because OSPF is proven reliable protocol capable of inter-area routing when properly configured. Protocol itself is not broken; configuration or topology errors prevent proper operation.
D is incorrect because inter-area routing is fundamental OSPF capability explicitly designed into protocol through hierarchical area structure and ABR functionality. Claiming impossibility contradicts OSPF design and widespread successful deployment.
