JNCIS-ENT BGP — 10 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?

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

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