Interior Gateway Protocols (IGPs)
What the IGP domain tests
JNCIP-ENT tests Junos-level OSPF and IS-IS design and troubleshooting: adjacency formation, area and level boundaries, summarization, authentication, and route selection. You must be able to explain why a neighbor stays down and how a summary changes what other routers learn.
Core rules
- OSPF neighbors on a broadcast segment must agree on area, hello/dead timers, authentication, and MTU; Junos default hello is 10 seconds and dead is 40 seconds on broadcast.
- An ABR generates Type 3 summaries; an ASBR injects external routes as Type 5, or Type 7 in an NSSA.
- IS-IS uses Level 1 inside an area and Level 2 between areas; a Level 1-2 router sets the overload bit or attaches with proper metrics.
- Junos route preference favors OSPF internal over IS-IS and BGP, so redistribution must include a policy and a metric.
Worked example
Two Junos routers share a Gigabit Ethernet subnet. OSPF stays in ExStart and never reaches Full. The interface MTUs are 1500 and 9000. OSPF database-description packets carry the MTU; the mismatch prevents adjacency. The fix is to make MTUs consistent, for example set both interfaces to 9000, or lower the jumbo side to 1500. After the change, the routers exchange DBDs, load LSAs, and reach Full.
Traps
Do not assume an area range suppresses every more-specific route in every direction; verify the ABR Type 3 LSAs. Do not mix IS-IS narrow and wide metrics without planning. Authentication mismatch produces silent adjacency failures, so check both hello and LSP authentication.