JNCIS-ENT BGP — Timed 25 Questions

Question 1

You need to ensure that an iBGP-learned route is properly advertised to other eBGP peers without triggering routing loops. Which mechanism does BGP use automatically to prevent routing loops within an Autonomous System?

25 minute time limit — choose an answer to begin.

Accepted Answer

AS-path loop prevention on eBGP export. When a BGP router advertises a route to an eBGP peer, it prepends its own AS number to the AS_PATH attribute. If a router receives a route whose AS_PATH already contains its own AS number, it rejects the route, preventing loops between autonomous systems. Within an iBGP context, this mechanism is not used for loop prevention; instead, iBGP relies on the BGP split horizon rule (routes learned via iBGP are not re-advertised to other iBGP peers) and, in route reflectors, the CLUSTER_LIST attribute. However, the question specifically asks about advertising an iBGP-learned route to eBGP peers without triggering routing loops. When that route is exported to an eBGP peer, the local AS number is prepended to the AS_PATH. If the route somehow loops back to the same AS, the AS_PATH will contain that AS number, and the router will detect the loop and discard the route. Thus, the automatic mechanism that prevents loops in this scenario is AS-path loop prevention on eBGP export.

Answer

MED attribute comparison

Answer

Local Preference check

Answer

Cluster list checking

Question 2

An MX Series router is configured with BGP Multipath. Two paths have identical Local Preference, AS path length, origin, and MED. Which additional configuration statement is required under protocols bgp to enable multipath load balancing across these iBGP paths?

Accepted Answer

set protocols bgp multipath relaxation. By default, multipath is only for eBGP. To enable iBGP multipath, the 'as-path-multipath relaxation' or 'ibgp multipath' statement must be used.

Answer

set protocols bgp group internal type external

Answer

set routing-options forwarding-table export load-balance

Answer

set protocols bgp group internal multihop

Question 3

A network engineer configures BGP confederations to scale an enterprise network. Which BGP path attribute is stripped or modified when an update leaves a sub-AS within the confederation and enters another sub-AS?

Accepted Answer

CONFED_SEQUENCE. When a BGP update traverses between sub-ASes within a confederation, the CONFED_SEQUENCE attribute is stripped or modified — specifically, the local sub-AS number is added to CONFED_SEQUENCE when the route leaves a sub-AS, and the entire CONFED_SEQUENCE is removed when the route exits the confederation to an external peer. This attribute is used only inside the confederation to prevent loops between sub-ASes.

Answer

AS_PATH

Answer

LOCAL_PREF

Answer

MED

Question 4

A Junos router receives four identical routes via BGP. Reviewing the routing table, path selection picked route 3. Route 1 has a lower MED, Route 2 has a higher Local Preference, Route 4 is from an eBGP peer while Route 3 is iBGP. Assuming standard default path selection, why was Route 3 chosen over Route 4?

Accepted Answer

Junos prefers iBGP paths over eBGP paths during the tie-breaking steps.. In Junos BGP path selection, after comparing local preference, AS path length, origin type, and MED, Junos prefers internal (iBGP) paths over external (eBGP) paths, provided all other tie-breakers are equal.

Answer

Route 3 had a lower router ID than Route 4.

Answer

Route 3 had a lower MED value.

Answer

Route 3 was received from a peer with a lower peer IP address.

Question 5

An administrator is configuring eBGP between two Junos devices and notices the session is stuck in the Active state. The direct connection is up, and ping succeeds. Which Junos configuration parameter is most likely missing to resolve this issue?

Accepted Answer

set protocols bgp group ebgp multihop. A BGP session stuck in Active state with a reachable directly connected peer typically indicates a TCP/179 reachability or configuration mismatch. If the peer's IP is not on a directly connected subnet (or the session must traverse multiple hops), 'multihop' must be configured so BGP can establish the TCP session beyond the directly connected link. Without it, Junos defaults to TTL 1 for eBGP, and the SYN never reaches the peer.

Answer

set protocols bgp group ebgp type internal

Answer

set protocols bgp group ebgp peer-as 65002 multipath

Answer

set protocols bgp group ebgp local-address 192.168.1.1

Question 6

A network operator needs to peer two routers in different Autonomous Systems that are not directly connected. Security policy mandates that TTL security checks be bypassed. Which command satisfies this requirement for the BGP session?

Accepted Answer

set protocols bgp group ebgp multihop ttl 255. TTL security can be bypassed or adjusted using multihop or adjusting the ttl value directly.

Answer

set protocols bgp group ebgp hold-time 180

Answer

set protocols bgp group ebgp passive

Answer

set protocols bgp group ebgp disable-ttl-drop

Question 7

You are troubleshooting a route leak between two eBGP peers. You notice that an incoming route contains your own ASN in the AS_PATH attribute, yet the route was installed. Which Junos feature allowed this behavior, and where is it configured?

Accepted Answer

allow-as-in. The 'allow-as-in' feature in Junos allows BGP to accept routes that contain the local AS number in the AS_PATH attribute, which would normally be rejected as a loop. It is configured as an import policy action or under the BGP group with 'allow-as-in'. This is useful in scenarios like dual-homed connections to the same provider where the provider uses AS override or in complex AS path manipulation.

Answer

as-override

Answer

as-path-prepend

Answer

loop-detect disable

Question 8

You configure a BGP route reflector in your iBGP topology. Which parameter must be configured on the route reflector client to prevent routing loops caused by reflecting routes?

Accepted Answer

set protocols bgp group ibgp cluster 10.0.0.1. Route reflectors use the Originator ID and Cluster List attributes to prevent loops, which are automatically added by the route reflector. However, clients do not require special client commands other than pointing to the reflector, but on the reflector, cluster-id is mandatory.

Answer

set protocols bgp group ibgp split-horizon disable

Answer

set protocols bgp group ibgp client-to-client reflection disable

Answer

set protocols bgp group ibgp route-reflector-client

Question 9

An enterprise network uses BGP communities to tag routes for regional filtering. A router receives routes with community 65001:100. You need an export policy to match this community. How is the community defined in Junos configuration?

Accepted Answer

set protocols bgp community 65001:100. Communities are defined under policy-options with a specific regex or exact match string.

Answer

set routing-options community-list 65001:100

Answer

set firewall family inet filter COMM term 1 then community 65001:100

Question 10

When a Junos router establishes an eBGP session, what is the default value of the Local Preference attribute assigned to incoming routes if not modified by a policy?

Accepted Answer

100. In Junos, when an eBGP session receives routes, the default Local Preference attribute is set to 100 if not modified by an import policy. Local Preference is a well-known discretionary attribute used within an AS to influence outbound traffic; a higher value is preferred. Since eBGP does not typically carry Local Preference between autonomous systems, the receiving router assigns the default value of 100.

Answer

Not set / Null

Answer

0

Answer

200

Question 11

You need to configure an explicit export policy in Junos so that your eBGP router only advertises locally originated routes and refuses to transit third-party routes. Which AS_PATH regular expression accomplishes this?

Accepted Answer

as-path LOCAL-ONLY "^$";. Locally originated routes have an empty AS_PATH (represented as '^$'), meaning no ASNs have been prepended yet.

Answer

as-path LOCAL-ONLY ".*";

Answer

as-path LOCAL-ONLY "[0-9]+";

Answer

as-path LOCAL-ONLY "^65001$";

Question 12

An operator troubleshooting BGP path selection notices two routes have identical Local Preference, AS path length, origin, MED, and both are eBGP routes. Which attribute does the BGP path selection algorithm evaluate next?

Accepted Answer

Lowest IGP cost to the BGP next hop. According to the BGP best path selection algorithm, after comparing local pref, AS path, origin, MED, and iBGP vs eBGP, the router prefers the path with the lowest IGP metric to the BGP next hop.

Answer

Lowest Router ID

Answer

Oldest route

Answer

Lowest peer IP address

Question 13

Which command allows an administrator to view real-time BGP routing updates being sent and received on a Junos device?

Accepted Answer

monitor traffic protocol bgp. The 'monitor traffic protocol bgp' command captures live packets on the wire matching BGP (TCP port 179) and displays them in real time, similar to tcpdump. It shows actual BGP UPDATE, KEEPALIVE, OPEN, and NOTIFICATION messages as they are sent and received, making it the correct tool for observing real-time BGP updates. This is distinct from log-based or state-based commands that show summarized or historical information.

Answer

traceroute bgp

Answer

show log bgp-log

Answer

show bgp updates

Question 14

An administrator implements an inbound policy to discard BGP routes matching a specific community. Which action statement is required in the policy term?

Accepted Answer

then reject;. To discard routes in a Junos routing policy, the 'reject' action is used.

Answer

then drop;

Answer

then next term;

Answer

then discard;

Question 15

You notice that an iBGP peer is not advertising routes it learned from an eBGP peer to other iBGP peers. What architectural rule explains this behavior?

Accepted Answer

iBGP split-horizon rule. The iBGP split-horizon rule states that routes learned via iBGP cannot be advertised to other iBGP peers to prevent routing loops.

Answer

eBGP TTL security check

Answer

MED restriction across AS boundaries

Answer

AS_PATH loop detection

Question 16

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?

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 17

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 18

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 19

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 20

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