46 questions · BGP Advanced · All types, answers revealed
Which TWO BGP attributes are classified as well-known mandatory attributes that must be recognized by all BGP implementations? (Choose TWO)
AS_PATH is a well-known mandatory attribute.
Why this answer
The well-known mandatory BGP attributes are AS_Path, NEXT_HOP, and ORIGIN. Among the choices, AS_PATH and NEXT_HOP are well-known mandatory attributes.
An engineer needs to ensure that a Junos router balances BGP traffic across multiple paths with unequal IGP costs. Which feature must be configured to achieve this?
Relaxing strict AS path or IGP cost checks allows multipath load balancing across paths with different metrics.
Why this answer
By default, BGP multipath requires equal cost. To allow paths with different costs, 'as-path-multipath' or variance-based multipath configurations are required depending on specific Junos platform support, but generally standard multipath requires equal IGP cost unless specific multipath relaxation options are set.
Which command allows an administrator to view the active BGP routes along with their associated communities on a Junos device?
The 'extensive' output includes detailed route attributes such as communities, AS path, and origin.
Why this answer
The 'show route protocol bgp' command with the 'extensive' modifier displays detailed path attributes including BGP communities.
You are troubleshooting a BGP route selection issue on a Junos device. Two otherwise identical paths are received from external peers. According to the standard BGP route selection algorithm implemented by Junos, which tie-breaking attribute is evaluated immediately AFTER comparing the Router ID (RID) of the advertising routers?
The Junos BGP decision process evaluates the lowest neighbor IP address immediately after comparing the router ID.
Why this answer
In the Junos BGP route selection algorithm, after comparing local preference, AS path length, origin, MED, eBGP over iBGP, IGP metric to next hop, and router ID (lowest router ID), the next step evaluates the neighbor IP address (lowest neighbor IP address).
An enterprise network uses a BGP route reflector (RR) architecture where client routers reflect routes to each other. A non-client router sends a route with an Originator ID and Cluster List to the RR. How does the route reflector handle these attributes when reflecting the route to other clients?
The RR preserves the Originator ID and adds its local cluster ID to the front of the Cluster List.
Why this answer
A route reflector leaves the Originator ID untouched and prepends its own Cluster ID to the existing Cluster List to prevent routing loops.
An administrator is troubleshooting BGP route reflector operations in a Junos environment. Which TWO statements regarding route reflection behavior and loop prevention are correct? (Choose two)
Cluster Lists prevent routing loops between clustered route reflectors by rejecting paths containing the local cluster ID.
Why this answer
Route reflectors use Originator ID to prevent loops from clients and Cluster Lists to prevent loops between cluster route reflectors.
An administrator configures a BGP routing policy to match routes with community members using regex matching in Junos. Which statement accurately describes how Junos represents standard BGP communities in policy matching?
Junos uses named community lists with AS:number syntax to match BGP communities.
Why this answer
In Junos, standard BGP communities are represented as target communities or community sets in the form of autonomous_system:identifier or named community lists.
Which THREE optional BGP attributes are supported and processed by Junos routing devices? (Choose three)
Community attributes are fully supported and widely used in Junos policies.
Why this answer
Junos supports standard optional attributes such as MED, Communities, and Atomic Aggregate.
Which TWO well-known BGP communities are recognized universally to control route propagation? (Choose two)
no-export prevents advertising outside the local AS or confederation.
Why this answer
NO_EXPORT and NO_ADVERTISE are standard well-known communities.
During BGP route resolution on Junos, an indirect next hop is encountered. What mechanism does Junos use to resolve this indirect next hop to a direct forwarding next hop?
Junos performs recursive lookups in the routing table to resolve BGP next hops via the resolving protocol (IGP).
Why this answer
Junos relies on the underlying IGP (OSPF or IS-IS) or static routes to resolve indirect BGP next hops to direct interface and gateway forwarding entries.
You are configuring a BGP Route Reflector (RR) in a Junos environment. To prevent routing loops within the iBGP cluster, what mechanism does the route reflector use to track the path of reflected routes?
Cluster List and Originator ID are the standard BGP path attributes designed specifically for route reflection loop prevention.
Why this answer
Route reflectors use the 'Originator ID' (a 4-byte attribute containing the router ID of the route originator) and the 'Cluster List' (a sequence of cluster IDs through which the route has passed) to prevent routing loops.
An enterprise is designing a BGP confederation to reduce iBGP mesh complexity across multiple data centers. Which statement is correct regarding how confederation members interact with BGP attributes?
Confederation member AS numbers are encapsulated in confederation-specific path segments internally and hidden from external peers.
Why this answer
Within a BGP confederation, sub-AS numbers are stripped as routes leave the confederation and are replaced by the confederation identifier. However, within the confederation, sub-AS numbers are kept in the AS_CONFED_SEQUENCE and AS_CONFED_SET path segments to prevent loops, and MED/Local Preference are preserved across member sub-AS boundaries.
An administrator needs to ensure that a Junos routing device performs unequal-cost multi-path (UCMP) load balancing across multiple BGP paths with different local preferences or path attributes. Which BGP feature must be enabled to allow multipath load balancing across paths with different AS path lengths?
The as-path-ignore parameter allows Junos to consider paths for BGP multipath even if their AS path lengths do not match exactly.
Why this answer
By default, Junos multipath requires multiple attributes (such as AS path length) to match. To allow BGP multipath even when AS path lengths differ (as long as they have the same length numerically, or using 'as-path-ignore'), the 'as-path-ignore' statement under protocols bgp multipath must be configured.
A BGP routing policy on a Junos router uses an 'as-path' regular expression '^[0-9]+$' in a match condition. Which routes will this regular expression match?
The anchors ^ and $, combined with digits and plus sign, match exactly a single AS number in the path.
Why this answer
The regex '^[0-9]+$' matches an AS path that consists of exactly one autonomous system number (from the beginning to the end of the path string).
Which command should an engineer use on a Junos device to verify whether a BGP next hop is successfully resolved via the routing table?
This command is specifically designed to inspect BGP next-hop resolution state in Junos.
Why this answer
The 'show route protocol bgp next-hop' command displays all BGP next hops and indicates whether they are resolved in the routing table.
Which THREE actions occur during the BGP next-hop resolution process on a Junos device when evaluating an eBGP-learned route? (Choose THREE)
Once resolved, the forwarding plane programs the next hop with specific interface and adjacency details.
Why this answer
When eBGP routes are learned, the next hop is typically the IP address of the external peer. Junos must resolve this next-hop IP in the routing table (RIB-local) via an active IGP or static route, verify that the resolving route is valid and active, and program the forwarding table (FIB) with the resolved next hop.
An enterprise network uses BGP communities to tag routes at regional branch offices. Headquarters needs to strip these specific communities from routes before advertising them to external transit providers. Which Junos routing policy action accomplishes this?
The 'community delete' action removes the matched community string from the route.
Why this answer
Using the 'community delete' action within a routing policy removes specified communities from routes matching the policy.
Which Junos configuration hierarchy is used to define a BGP peer group to simplify neighbor configurations?
The 'protocols bgp group' hierarchy allows shared configuration parameters across multiple BGP neighbors.
Why this answer
BGP peer groups and neighbor settings are configured under the 'protocols bgp group' hierarchy in Junos.
An engineer is troubleshooting a BGP route reflection topology where a non-client IBGP peer sends a route to a route reflector. Will the route reflector reflect this route to other non-client IBGP peers?
Non-client routes are reflected only to clients, enforcing partial mesh requirements for non-clients.
Why this answer
A route reflector reflects routes from clients to clients, clients to non-clients, and non-clients to clients, but it does NOT reflect routes received from a non-client to other non-client peers.
You want to tag specific BGP routes with a community value using a Junos policy framework. Which configuration snippet correctly defines a community named 'CUSTOM-COMM' with value 65000:100?
This is the correct syntax to define a BGP community member in Junos policy options.
Why this answer
In Junos, BGP communities are defined under 'policy-options community' with a name and the community value specified in format AS:number or target:AS:number.
When designing a BGP confederation architecture, which TWO rules govern how sub-AS numbers and attributes behave across confederation boundaries? (Choose two)
Confederation member ASNs are encapsulated in AS_CONFED segments and stripped at the outer boundary.
Why this answer
Confederations use AS_CONFED_SEQUENCE and AS_CONFED_SET segments to track internal paths, and strip them when advertising to external peers outside the confederation.
Which TWO actions can be performed using Junos BGP routing policies? (Choose two)
Policies can set or adjust local preference for internal paths.
Why this answer
Junos routing policies can filter routes and modify path attributes such as local preference or MED.
An operator configures a BGP routing policy to modify the Local Preference attribute. Under which BGP neighbor relationship type can Local Preference be advertised and evaluated?
Local Preference is strictly an IBGP attribute used for routing within an autonomous system.
Why this answer
Local Preference is an IBGP attribute and is exchanged only between internal BGP peers within the same autonomous system.
Which TWO statements describe characteristics of BGP route reflectors? (Choose two)
Route reflectors reduce IBGP sessions from n(n-1)/2 to a hub-and-spoke design.
Why this answer
Route reflectors reduce IBGP mesh requirements and reflect routes according to strict client/non-client rules.
Which TWO attributes are considered transitive BGP path attributes? (Choose two)
COMMUNITY is an optional transitive attribute.
Why this answer
AS_PATH and COMMUNITY are standard well-known transitive and optional transitive attributes respectively, whereas LOCAL_PREF is well-known discretionary (non-transitive across AS).
An engineer notices that a Junos device is not installing a BGP route into the forwarding table because the BGP next hop is unresolved. Which tool or command can be used to troubleshoot how the BGP next hop is being resolved in the Junos routing table?
The show route resolution command specifically details how Junos resolves next hops through active routing table entries.
Why this answer
To check BGP next-hop resolution, the 'show route resolution' command displays how next hops are resolved via the routing table (RIB-local), showing whether an active IGP or static route covers the BGP next-hop IP address.
An administrator applies a policy containing 'as-path-prepend "65001 65001"' to outbound BGP updates sent to an external provider. What is the primary objective of this configuration?
AS path prepending is a standard traffic engineering technique to make a route less attractive by lengthening the AS path.
Why this answer
Prepending the local AS number multiple times increases the AS path length attribute, making the path less desirable to external peers and manipulating inbound traffic engineering.
When configuring a BGP confederation, how are external peers outside the confederation affected by the internal sub-AS structure?
Confederation encapsulation hides internal sub-AS routing details from external ASNs.
Why this answer
External peers outside the confederation see only the top-level confederation AS number, as internal member AS numbers are stripped when exiting the confederation boundary.
A Junos router receives a BGP route containing multiple extended and large communities. The administrator writes a routing policy to filter routes based on large communities. Which format is used to define a large community in Junos policy-options?
Junos uses the three-part format for defining large communities.
Why this answer
Large communities in Junos policy-options are defined using the format large-community:global_administrator:local_data1:local_data2.
Which BGP community is a well-known transitive community used to prevent a route from being advertised outside of a local autonomous system?
The NO_EXPORT community prevents routes from being advertised outside the local autonomous system.
Why this answer
The NO_EXPORT well-known community instructs a BGP peer not to advertise the affected route to any external BGP peers.
An administrator implements BGP entropy labels and path load balancing on a Junos routing platform. During packet forwarding for a BGP-learned IPv4 prefix with multiple equal-cost paths, how does Junos determine the output interface when hash-based load balancing is active?
Hash-based load balancing uses packet header fields to index into the BGP multipath next-hop array.
Why this answer
Junos hashes packet header fields (such as source/destination IP, ports, and optional hash keys or entropy labels) to select a specific path from the available BGP multipaths.
In a BGP confederation setup, two member ASNs (65001 and 65002) belong to a larger confederation AS (65100). When routes are advertised between these two member ASs, how is the AS path attribute handled?
AS_CONFED_SEQUENCE attributes track paths inside the confederation without affecting external AS path length calculations.
Why this answer
Confederation member AS numbers are included in the AS_CONFED_SEQUENCE attribute and are stripped when the route exits the confederation boundary.
An engineer needs to ensure that routes received from an external BGP peer are evaluated using multipath load balancing across different next hops. By default, Junos requires several conditions to be met for multipath. Which condition is mandatory for BGP multipath to consider two paths as valid candidates?
Equal IGP metric to the next hop is one of the strict default requirements for Junos BGP multipath unless explicitly relaxed.
Why this answer
To qualify for BGP multipath in Junos, multiple attributes must match by default, including AS path length, local preference, origin, MED, and the IGP metric to the respective next hops must be equal (or within an acceptable variance if configured).
An operator configures BGP attribute value manipulation using a Junos routing policy. Specifically, the policy uses 'as-path-action prepend' combined with community matching. When is this outbound policy evaluated relative to BGP route selection?
Export policies apply to outgoing updates after the local router has determined its best path.
Why this answer
Outbound export policies are evaluated when advertising routes to peers, after the best path has been selected locally.
A network engineer is configuring a Junos routing device to use BGP Prefix-Independent Convergence (PIC) edge for faster failover. What is the fundamental mechanism that enables BGP PIC to achieve sub-second convergence upon primary path failure?
PIC pre-computes backup paths in the forwarding table, decoupling convergence time from the number of prefixes.
Why this answer
BGP PIC pre-computes a backup loop-free alternate next hop and installs it in the forwarding table alongside the primary next hop, allowing hardware-based failover.
An engineer is configuring BGP route resolution and next-hop tracking in Junos. Which TWO circumstances will cause a BGP next hop to be marked as unresolved or unusable? (Choose two)
Discard or reject resolving routes invalidate BGP next-hop resolution.
Why this answer
BGP next hops become unusable if the resolving route in the IGP table is missing, points to a discard/reject route, or lacks reachability.
When configuring a BGP route reflector cluster in Junos, which THREE guidelines or operational behaviors should an engineer keep in mind? (Choose THREE)
Standard iBGP full-mesh rules apply between non-client peers; a route reflector reflects routes from clients to all clients and non-clients, and from non-clients to clients only.
Why this answer
Route reflectors reflect routes between clients and non-clients. Loop prevention uses Originator ID and Cluster List. Redundant RRs in the same cluster must share the same cluster ID to prevent routing loops and oscillation.
A Junos router is configured with BGP path selection, and multiple paths arrive with different MED values. Under what specific condition does Junos compare the MED values of paths from different autonomous systems during route selection?
The 'always-compare-med' command forces MED comparison across different autonomous systems.
Why this answer
By default, Junos (like standard BGP implementations) only compares MEDs from different ASs if the 'always-compare-med' configuration is explicitly enabled.
Which THREE methods can be used on Junos devices to influence inbound BGP traffic selection from external ASNs? (Choose three)
Prepending lengthens the AS path, making the path less desirable to external peers.
Why this answer
Inbound traffic from external peers can be influenced by AS path prepending, modifying MED, or utilizing specific BGP communities accepted by transit providers.
An administrator is reviewing the BGP route selection process on a Junos device and notices that two paths have identical weights, local preferences, AS path lengths, origins, and MEDs. Which tie-breaking criterion does Junos evaluate next in the BGP route selection algorithm?
Junos prefers IBGP paths over EBGP paths during the standard BGP route selection tie-breaking steps when earlier attributes are equal.
Why this answer
After comparing weight, local preference, AS path length, origin, and MED, Junos checks the routing table to prefer routes learned via internal peer (IBGP) over external peer (EBGP) before moving to router ID and IGP cost.
A network engineer has configured BGP multipath on a Junos routing device using the 'multipath' statement under the routing-options bgp hierarchy. However, traffic is still only utilizing a single path. Which condition must be met for BGP multipath to become active for a destination prefix?
Junos requires key attributes including AS path length, local preference, origin, and MED to match for multipath consideration.
Why this answer
For BGP multipath to be valid in Junos, the paths must have matching AS path lengths, local preferences, origins, and MEDs, along with valid next-hop resolution through the IGP.
Which THREE conditions must be met for a Junos routing device to consider two BGP paths as valid candidates for multipath load balancing when standard multipath is configured? (Choose three)
Next-hop reachability and resolution via IGP or direct connection are mandatory.
Why this answer
Standard BGP multipath in Junos requires matching AS path lengths, local preferences, and valid IGP next-hop resolution, among other consistent attributes.
A network administrator wants to advertise an aggregate BGP route while suppressing the more specific component routes from being sent to external peers. Which Junos routing-options configuration achieves this?
The 'summary-only' parameter suppresses specific component routes when advertising the aggregate prefix.
Why this answer
Using the 'aggregate' statement with the 'summary-only' option generates the aggregate prefix and suppresses all component specific routes.
Which THREE parameters are part of the standard Junos BGP route selection algorithm prior to evaluating the router ID? (Choose three)
Weight is evaluated first in Junos BGP path selection.
Why this answer
Before router ID, Junos evaluates Weight, Local Preference, AS path length, Origin, MED, and IBGP vs EBGP preference.
A network designer needs to implement BGP route reflection where multiple route reflectors are deployed in the same cluster to provide redundancy. What parameter must be identical across all route reflectors in that same cluster to prevent routing loops?
Route reflectors in the same cluster must be configured with the identical Cluster ID.
Why this answer
Route reflectors within the same cluster must share the same Cluster ID so they can recognize and ignore routes reflected by their peer route reflector.
An administrator configures BGP dampening on a Junos router to mitigate the impact of flapping routes. When a route exceeds the reuse threshold after being suppressed, what happens to its penalty value?
Penalties decay gradually over time based on the configured half-life.
Why this answer
When a dampened route's penalty decays below the reuse threshold, the route becomes eligible again, and its penalty continues to decay exponentially toward zero.
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