JNCIS-ENT BGP Practice Test 2 — 15 Questions

Question 1

An enterprise customer connects to two different service providers via eBGP. They want to preferentially use Service Provider A, but fail over to Service Provider B if Provider A fails. How can they influence the inbound traffic from the providers?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Use AS-PATH prepending in an outbound policy toward Provider B. To influence inbound traffic from service providers, the enterprise can use AS-PATH prepending in an outbound policy toward Provider B. By prepending additional AS numbers to the AS_PATH of routes advertised to Provider B, the path appears longer, making Provider A's path more attractive to external networks. This causes inbound traffic to prefer Provider A, with failover to Provider B if A fails.

Answer

Modify Local Preference in an inbound policy

Answer

Change the MED value in an inbound policy

Answer

Configure a lower router ID

Question 2

Which BGP attribute is classified as 'well-known discretionary'?

Accepted Answer

LOCAL_PREF. Local Preference is a well-known discretionary attribute, meaning all BGP implementations must recognize it, but it does not have to be included in every update.

Answer

AS_PATH

Answer

ORIGIN

Answer

MED

Question 3

An engineer configures a BGP policy to set the MED attribute on routes advertised to an eBGP peer. What is the primary purpose of the MED (Multi-Exit Discriminator) attribute?

Accepted Answer

To indicate to an external AS which entry point is preferred for traffic entering the local AS.. MED is an optional non-transitive attribute used to inform external peers about the preferred entry point into an AS.

Answer

To dictate the preferred path for traffic leaving the local AS.

Answer

To prevent routing loops across multiple autonomous systems.

Answer

To assign a priority score for internal route selection within the same AS.

Question 4

When evaluating the BGP path selection algorithm on a Junos device, which THREE attributes are evaluated BEFORE the router considers the IGP metric to the BGP next hop? (Choose three)

Accepted Answer

Multi-Exit Discriminator (MED). In the Junos BGP path selection algorithm, the sequence begins with Local Preference (E), which is evaluated first among these attributes and is used to prefer a route with a higher value before any AS-path comparison occurs. Next, AS path length (D) is compared, where the route with the shorter AS path is preferred, and this step occurs before MED and before the IGP metric to the next hop. Multi-Exit Discriminator (C) is then evaluated, with the lower MED preferred, and this also happens before the router considers the IGP metric to the BGP next hop. The IGP metric to next hop (A) is evaluated later in the algorithm, after MED, so it is not one of the attributes considered before it. Router ID (B) is evaluated even later, after the IGP metric and other tie-breakers, so it also does not belong among the attributes evaluated before the IGP metric to the next hop.

Answer

IGP metric to next hop

Answer

Router ID

Question 5

Which THREE path attributes are classified as 'well-known mandatory'? (Choose three)

Accepted Answer

NEXT_HOP. The three well-known mandatory BGP path attributes are NEXT_HOP (B), ORIGIN (C), and AS_PATH (D). NEXT_HOP is well-known mandatory because every BGP UPDATE must include the next-hop IP address used to reach the destination, and it is recognized by all BGP implementations. ORIGIN is well-known mandatory because it indicates how the route was originally learned (IGP, EGP, or incomplete) and must appear in every UPDATE. AS_PATH is well-known mandatory because it lists the autonomous systems the route has traversed and is essential for loop prevention and best-path selection. MED (A) and LOCAL_PREF (E) are optional attributes, not well-known mandatory, so they do not belong in this set.

Answer

MED

Answer

LOCAL_PREF

Question 6

An administrator is troubleshooting a BGP peering session that is stuck in the Connect or Active state. Which THREE common causes could explain this issue? (Choose three)

Accepted Answer

TCP port 179 is blocked by a firewall filter.. Option B is correct because BGP peers establish their session over TCP port 179, so a firewall filter blocking that port prevents the TCP three-way handshake from completing, leaving the session stuck in Connect or Active. Option C is correct because if the neighbor IP address configured under protocols bgp does not match the actual reachable address of the peer, the router cannot open the TCP session and the state remains in Connect/Active. Option E is correct because eBGP by default requires directly connected peers (TTL 1); for a non-directly connected eBGP peer, multihop must be configured, otherwise the TCP connection never establishes and the session stays in Connect/Active. Option A is not a cause of Connect/Active because route reflection client parameters only affect route propagation after the session is Established. Option D is also not a cause of Connect/Active because an incorrect peer AS number is detected during the OPEN message exchange, which occurs after TCP is established, resulting in an Active/Idle or OpenSent/OpenConfirm failure rather than a stuck Connect/Active state.

Answer

Route reflection client parameters are misconfigured.

Answer

Incorrect peer AS number configured in the OPEN message parameters.

Question 7

When troubleshooting a BGP routing loop or unexpected route suppression, which THREE Junos operational commands provide visibility into BGP path attributes and decision processes? (Choose three)

Accepted Answer

show route extensive. Option B, 'show route extensive', is correct because it displays detailed route information including all BGP path attributes (AS path, local preference, MED, origin, next-hop) and the route's preference/selection status, which is essential for diagnosing why a route was suppressed or chosen. Option C, 'show bgp neighbor', is correct because it reveals per-neighbor BGP session state, negotiated capabilities, and, critically, the Adj-RIB-In/Adj-RIB-Out and route flap damping information that shows whether routes were suppressed by damping or policy before entering the decision process. Option D, 'show route protocol bgp', is correct because it filters the routing table to only BGP-learned routes, letting you inspect the BGP path attributes and the active/hidden status of each prefix to identify loops or suppressed paths. Option A, 'show ldp database', is incorrect because LDP is an MPLS label distribution protocol unrelated to BGP path attribute or decision-process visibility. Option E, 'show ospf interface', is incorrect because OSPF is a link-state IGP and this command shows OSPF interface parameters, not BGP attributes or route selection.

Answer

show ldp database

Answer

show ospf interface

Question 8

Which TWO actions can be performed using Junos routing policies applied to BGP? (Choose two)

Accepted Answer

Accepting or rejecting routes based on AS_PATH regular expressions.. Option A is correct because Junos routing policies can match on the AS_PATH attribute using regular expressions (e.g., "from as-path <regex>") to accept or reject BGP routes based on the autonomous systems in the path. Option E is correct because a routing policy applied as an import policy on an eBGP session can set the local-preference attribute (e.g., "then local-preference 200"), which influences outbound path selection for routes learned from that neighbor. Option B is not valid because the BGP TCP port 179 is fixed by the protocol and cannot be changed by a routing policy. Option C is not valid because routing policies operate on routing information, not on physical interface speed, and BGP flaps do not trigger interface speed changes. Option D is not valid because Junos routing policies do not perform IPsec encryption of BGP payloads; BGP security is handled by TCP MD5, IPsec at the transport layer, or other mechanisms, not by policy-based key management.

Answer

Changing the TCP port used for BGP session establishment from 179 to another port.

Answer

Directly modifying the physical interface speed when BGP flaps.

Answer

Encrypting BGP payload data using policy-based IPsec keys.

Question 9

Which TWO BGP attributes are strictly internal to an Autonomous System and are never advertised to external eBGP peers? (Choose two)

Accepted Answer

ORIGINATOR_ID. LOCAL_PREF (E) is correct because it is a well-known discretionary attribute used only within an AS to influence outbound path selection; it is not propagated to eBGP peers, and any LOCAL_PREF received from an eBGP neighbor is stripped. ORIGINATOR_ID (C) is correct because it is an optional non-transitive attribute created by a route reflector to prevent routing loops within the AS, and it is never sent outside the local AS. NEXT_HOP (A) is not correct because it is a well-known mandatory attribute that is advertised in eBGP updates (with the next-hop typically set to the eBGP peer's address). ORIGIN (B) is not correct because it is a well-known mandatory attribute carried in eBGP updates to indicate the route's origin (IGP, EGP, or INCOMPLETE). AS_PATH (D) is not correct because it is a well-known mandatory attribute that is included in every eBGP update and is central to inter-AS loop prevention.

Answer

NEXT_HOP

Answer

ORIGIN

Answer

AS_PATH

Question 10

An administrator configures BGP multipath on a Junos router. Which THREE conditions must be met for BGP paths to be considered equal and installed as multipaths? (Choose three)

Accepted Answer

The paths must have the same Local Preference value.. For Junos BGP multipath, paths must be equal under the BGP route-selection tie-breakers before the final path-selection step, so option A is correct: the paths must share the same Local Preference value, since Local Preference is evaluated early in the decision process and differing values would make one path preferred. Option B is correct because MED must match for paths to be considered equal; Junos only relaxes this with configuration such as 'multipath multiple-as' or when MED comparison is disabled, and the parenthetical reflects that MED is normally a required equalizer. Option D is correct because identical AS path length is required for multipath eligibility, unless AS path relaxation (e.g., 'multipath multiple-as' or 'as-path-relax') is enabled to allow paths with different AS path lengths. Option C is not required and is actually counter to typical multipath behavior, since paths from different neighboring ASes are not automatically equal and require additional configuration. Option E is incorrect because identical router IDs are never a multipath requirement; in fact, differing router IDs are normal for paths from different BGP speakers, and router ID is used only as a late tie-breaker in best-path selection.

Answer

The paths must originate from different Autonomous Systems entirely.

Answer

The router IDs of the advertising routers must be identical.

Question 11

A network engineer configures an eBGP session on a Junos device and notices that routes learned from the external peer are not being installed in the routing table or advertised to other peers. What is the default behavior regarding eBGP route propagation and next-hop resolution on Junos OS?

Accepted Answer

The local router does not set the next-hop to itself for eBGP peers, but for iBGP peers the next-hop must be resolvable via an IGP.. By default, eBGP does not change the next-hop address when advertising routes to external peers, but when advertising to iBGP peers, it preserves the next-hop unless 'next-hop-self' is configured. However, for routes to be active, the next-hop must be resolvable in the routing table.

Answer

Junos automatically drops all eBGP routes unless an explicit import policy is applied allowing them.

Answer

eBGP routes require the 'as-override' statement to be active in the forwarding table.

Answer

eBGP automatically changes the next-hop to the local router interface for all advertised routes.

Question 12

An engineer configures iBGP peering between two routers in the same AS. Route reflection is not used. The engineer notices that routes learned from an eBGP peer by Router A are not being advertised by Router A to Router B (its iBGP peer). Why is this happening?

Accepted Answer

BGP split-horizon prevents a router from advertising routes learned from one iBGP peer to another iBGP peer.. BGP split-horizon rule dictates that routes learned from an iBGP peer cannot be advertised to another iBGP peer to prevent routing loops. (Note: standard iBGP to iBGP split horizon applies; to share routes, full mesh, route reflectors, or confederations must be used).

Answer

External routes cannot be carried over iBGP without encapsulation.

Answer

The Local Preference attribute defaults to 0 for iBGP routes, causing them to be suppressed.

Answer

iBGP peers require the 'multipath' statement to be configured before any route exchange can occur.

Question 13

During BGP path selection on a Junos device, two paths are received for the same destination prefix. Path A has a MED of 50 received from a peer in AS 65001, and Path B has a MED of 10. Both AS paths have a length of 2. The router is not configured with 'always-compare-med'. Which path will be preferred by default?

Accepted Answer

MED will be ignored during path selection because the paths originate from different autonomous systems, and the selection will proceed to subsequent tie-breakers.. By default, Junos only compares MED between paths received from the same neighboring AS. Path A (MED 50) came from AS 65001, while Path B (MED 10) came from a different AS, so MED is not a valid comparison point and is skipped. The router proceeds to the next tie-breakers (e.g., eBGP over iBGP, IGP metric, router ID) rather than choosing Path B on MED alone.

Answer

Path B, because it has a lower MED value of 10 compared to Path A's MED of 50.

Answer

Path A, because lower MED is only preferred if 'compare-router-id' is enabled.

Answer

Path A will be preferred because Junos always favors higher MED values for eBGP paths.

Question 14

Which TWO statements correctly differentiate eBGP from iBGP? (Choose two.)

Accepted Answer

eBGP routers prepend their own AS number to the AS_PATH attribute when advertising routes.. Option B is correct because eBGP speakers, when advertising a route to an external peer, prepend their own autonomous system number to the AS_PATH attribute, which is the fundamental loop-prevention and path-vector mechanism between autonomous systems. Option C is correct because eBGP sessions default to a TTL of 1 (multihop disabled), so eBGP peers must be directly connected unless multihop is explicitly configured. Option A is incorrect because iBGP has an administrative distance of 200 in Cisco (and preference 170 in Junos), while 20 is the eBGP administrative distance. Option D is incorrect because it is eBGP, not iBGP, that defaults to TTL 1; iBGP packets default to TTL 255. Option E is incorrect because iBGP does not change the next-hop by default when advertising to iBGP peers (that behavior applies to eBGP), and iBGP relies on the IGP to resolve the next hop.

Answer

iBGP routes have a default administrative distance (preference in Junos) of 20.

Answer

The default TTL for iBGP packets is 1, requiring explicit multihop configurations for non-directly connected peers.

Answer

iBGP routers modify the next-hop attribute to their own interface address by default when advertising routes to iBGP peers.

Question 15

You are the network engineer for a retail company running Juniper MX Series routers. R1 in AS 65001 peers with an ISP router R2 in AS 65002. The eBGP session is established, and R1 receives a route to 203.0.113.0/24 with AS path "65002 65003 65004". R1's BGP configuration includes 'peer 198.51.100.2 as-number 65002' and 'local-as 65010' under the group. What AS path will R1 advertise to its iBGP peer R3 for this route?

Accepted Answer

65010 65002 65003 65004. When local-as is configured under a BGP group in Junos, the router prepends the specified AS number to the AS path when advertising routes learned from that group to other BGP peers. The primary AS number is not prepended for those routes. Therefore, the route advertised to the iBGP peer will have the local AS 65010 added before the existing AS path.

Answer

65001 65002 65003 65004

Answer

65002 65003 65004

Answer

65010 65001 65002 65003 65004