JNCIA Routing Protocols 2 — Questions and Answers

Question 1: What is the Junos default BGP hold time?

90 seconds (Correct answer)
30 seconds
60 seconds
180 seconds

Correct answer: 90 seconds

The default BGP hold timer in Junos is 90 seconds. If no KEEPALIVE is received within the hold time, the BGP session is considered down and routes are withdrawn.

BGP uses hold timers to detect dead sessions. The hold time is negotiated as the minimum of the two peers' configured values. Default in Junos: hold-time 90 seconds, keepalive 30 seconds (1/3 of hold time). If 3 consecutive keepalives are missed, the session drops. Configured in Junos with 'set protocols bgp hold-time [seconds]'. A hold time of 0 disables keepalives (never drops for missing keepalives). Short hold times (e.g., 9 seconds) improve failure detection but increase sensitivity to transient packet loss.

Question 2: In OSPF, what is the role of the Area Border Router (ABR)?

Connects two or more OSPF areas and summarizes LSAs between them, reducing LSDB size in non-backbone areas (Correct answer)
Connects the OSPF domain to external routing domains
Forwards data traffic between areas
Performs DR/BDR election for multiple areas

Correct answer: Connects two or more OSPF areas and summarizes LSAs between them, reducing LSDB size in non-backbone areas

An ABR sits at the boundary between OSPF areas. It maintains separate LSDBs for each connected area and generates Summary LSAs (Type 3) to advertise inter-area routes between areas.

OSPF ABRs connect non-backbone areas to Area 0. An ABR maintains a full LSDB for each area it belongs to and generates Type 3 Summary LSAs to advertise inter-area prefixes. ABRs also perform inter-area route summarization (configured with 'set protocols ospf area N area-range prefix/len'). The Autonomous System Boundary Router (ASBR) is different — it connects OSPF to external routing domains (imports external routes as Type 5 External LSAs). A single router can be both ABR and ASBR.

Question 3: What is the purpose of OSPF 'passive' interface configuration in Junos?

Advertises the connected subnet into OSPF without sending or receiving OSPF hellos, preventing neighbor formation (Correct answer)
Disables OSPF completely on the interface
Forces the interface to be elected DR
Enables OSPF authentication on the interface

Correct answer: Advertises the connected subnet into OSPF without sending or receiving OSPF hellos, preventing neighbor formation

A passive OSPF interface advertises its connected subnet into the OSPF domain (via a stub link LSA) but does not send or receive OSPF Hello packets, preventing the formation of OSPF neighbors on that segment.

OSPF passive interfaces are used for subnets that should be in the routing domain but have no OSPF-capable neighbors (e.g., server-facing interfaces, loopbacks). Configured with 'set protocols ospf area N interface ge-x/x/x passive'. Best practice: configure loopback (lo0.0) as passive in OSPF to advertise the router-ID/loopback address. Passive interfaces save CPU (no hello processing) and prevent accidental neighbor formation with hosts. On point-to-point links to non-OSPF devices, use passive to advertise the prefix.

Question 4: In Junos BGP, what does the 'export' policy control?

Which routes from the local routing table are advertised to BGP peers (Correct answer)
Which BGP routes are accepted from peers
The BGP local preference value
The BGP session establishment parameters

Correct answer: Which routes from the local routing table are advertised to BGP peers

The BGP 'export' policy in Junos controls which routes from the local RIB are redistributed into BGP and advertised to peers. Without an export policy, only BGP-learned routes are re-advertised.

In Junos, routing policy works bidirectionally: 'import' policy filters routes received from peers before installing in the RIB; 'export' policy filters routes from the RIB before advertising to peers. BGP export policy is configured with 'set protocols bgp group GROUP export POLICY-NAME'. To redistribute OSPF into BGP: create a policy that accepts routes with protocol OSPF and apply as export. Without export policy, Junos BGP only re-advertises routes whose protocol is BGP (not static, OSPF, etc.).

Question 5: What is IS-IS and how does it differ from OSPF in a Junos context?

IS-IS is a link-state IGP that runs over the data-link layer (Layer 2) rather than IP, making it inherently more resilient to IP routing issues (Correct answer)
IS-IS uses distance-vector algorithms while OSPF uses link-state
IS-IS is only for IPv6 while OSPF is IPv4-only
IS-IS uses hop count as its metric while OSPF uses cost

Correct answer: IS-IS is a link-state IGP that runs over the data-link layer (Layer 2) rather than IP, making it inherently more resilient to IP routing issues

IS-IS (Intermediate System to Intermediate System) is a link-state IGP like OSPF, but it runs directly over Layer 2 using ISO CLNP, not IP. This makes it immune to IP routing loops and preferred by many ISPs.

IS-IS (ISO 10589, RFC 1195 for IP) is the IGP of choice for many large ISPs and hyperscale networks (Juniper, Cisco, Google backbone). Key differences from OSPF: runs over L2 (not IP, so no IP needed for protocol exchange); uses TLVs (extensible); single LSP type vs. OSPF's multiple LSA types; area structure differs (Level 1/Level 2 vs. OSPF area 0/areas); natively supports both IPv4 and IPv6 via multi-topology IS-IS. In Junos: 'set protocols isis interface ge-x/x/x level 2'.

Question 6: What is the Junos command to view detailed BGP route information including all path attributes?

show route [prefix] detail (Correct answer)
show bgp route [prefix] attributes
show route bgp detail [prefix]
display bgp path [prefix]

Correct answer: show route [prefix] detail

'show route [prefix] detail' in Junos displays detailed route information including all BGP attributes: AS path, communities, LOCAL_PREF, MED, ORIGIN, and next-hop.

'show route [prefix] detail' shows all protocol attributes for matching routes. For BGP routes: AS path, communities (well-known and extended), LOCAL_PREF, MED/metric, ORIGIN (IGP/EGP/Incomplete), next-hop IP, BGP router-ID of advertising peer, cluster list (for route reflectors), and originator-ID. 'show route receive-protocol bgp [peer] [prefix] detail' shows pre-policy received routes. 'show bgp neighbor [peer] received' shows received updates. Understanding path attributes is essential for BGP troubleshooting.

JNCIA Routing Protocols 2

What is the Junos default BGP hold time?