JNCIS-ENT BGP — 30 Questions

Question 1

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 2

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 3

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?

Accepted Answer

AS-path loop prevention on eBGP export. iBGP split horizon rule specifies that routes learned from an iBGP peer are not advertised to other iBGP peers, but it requires full mesh, route reflectors, or confederations. For eBGP advertising, AS path is used.

Answer

MED attribute comparison

Answer

Local Preference check

Answer

Cluster list checking

Question 4

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 5

An administrator wants to modify the Local Preference attribute for all routes received from a specific external BGP peer. Where must this policy be applied in Junos?

Accepted Answer

Import policy applied to the eBGP neighbor or group. Inbound routing policies on Junos affect attributes like Local Preference before path selection occurs.

Answer

Export policy applied to the eBGP group

Answer

Forwarding plane firewall filter

Answer

Global system-level static route policy

Question 6

During troubleshooting of a flapping BGP session, you notice BGP Notification messages being sent. Which BGP message type is responsible for tearing down a session due to an error condition?

Accepted Answer

NOTIFICATION. The Notification message is sent when an error is detected, causing the BGP connection to close immediately.

Answer

KEEPALIVE

Answer

UPDATE

Answer

OPEN

Question 7

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. CONFED_SEQUENCE and CONFED_SET attributes are used within a confederation and are stripped when the update leaves the confederation boundary to a normal eBGP peer.

Answer

AS_PATH

Answer

LOCAL_PREF

Answer

MED

Question 8

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 9

An engineer must inspect the BGP state and verify the received prefix counts from a specific neighbor on an MX Series router. Which operational command should be used?

Accepted Answer

show bgp neighbor 10.1.1.1. The 'show bgp neighbor' command provides detailed information about BGP peer states, prefix counts, and timers.

Answer

show ospf neighbor

Answer

show route protocol bgp

Answer

show bgp summary

Question 10

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. When establishing eBGP, Junos defaults to expecting the peer to be directly connected with a TTL of 1. If connecting via loopback interfaces or a multi-hop path, the multihop statement is required.

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 11

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 12

While reviewing BGP updates, you notice an attribute type code of 3 with a value representing an IP address. Which well-known mandatory path attribute is this?

Accepted Answer

NEXT_HOP. Attribute type code 3 corresponds to NEXT_HOP, which is a well-known mandatory attribute indicating the IP address of the router used to reach the destination.

Answer

AS_PATH

Answer

ORIGIN

Answer

LOCAL_PREF

Question 13

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' command under BGP configuration allows a router to accept BGP updates that contain its own ASN in the AS_PATH, typically used in VPN or hub-and-spoke scenarios.

Answer

as-override

Answer

as-path-prepend

Answer

loop-detect disable

Question 14

An administrator configures a BGP policy to reject routes with an AS_PATH containing a specific AS number. Which match condition in a Junos routing policy is used to evaluate the AS_PATH?

Accepted Answer

as-path. In Junos, the as-path match condition evaluates routes against a defined as-path regular expression.

Answer

pref

Answer

metric

Answer

community

Question 15

What is the primary function of the BGP OPEN message?

Accepted Answer

To establish a peering session and negotiate capabilities such as ASNs and timers.. The OPEN message is the first message sent after a TCP connection is established, used to negotiate BGP parameters such as ASN, hold time, and router ID.

Answer

To advertise reachability and path attributes for network prefixes.

Answer

To maintain the connection state and verify liveliness at regular intervals.

Answer

To signal fatal errors and immediately terminate the BGP session.

Question 16

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. The default Local Preference in Junos and standard BGP implementations is 100.

Answer

Not set / Null

Answer

0

Answer

200

Question 17

Which TCP port is used by BGP to establish communication between peers?

Accepted Answer

TCP 179. BGP operates over TCP port 179.

Answer

UDP 520

Answer

TCP 80

Answer

TCP 500

Question 18

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 19

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 20

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. Tracing options and packet/update monitoring can be performed using 'monitor traffic' or BGP tracing, but for dynamic operational logging of updates, tracing is enabled. For quick viewing of summary updates, monitor traffic protocol bgp is used.

Answer

traceroute bgp

Answer

show log bgp-log

Answer

show bgp updates