JNCIP-SP cheat sheet

IS-IS levels: Level 1 is intra-area only; Level 2 is the backbone. An L1/L2 router sets the ATT bit in L1 LSPs to advertise a default route toward L2; it never leaks L1 routes into L2 by default.

OSPF in SP cores: use area 0 as the backbone, keep area ranges contiguous, and prefer point-to-point or point-to-multipoint network types on unnumbered core links to avoid DR/BDR election overhead.

BGP path selection order (Junos, simplified): highest Local Preference, then shortest AS path, then lowest Origin (IGP < EGP < Incomplete), then lowest MED, then eBGP over iBGP, then lowest IGP metric to next hop, then lowest router ID.

Next-hop reachability: an iBGP route is usable only if the next hop resolves in the IGP. Use 'next-hop self' on route reflectors or set an export policy on the border router to rewrite next hop.

Route reflection: a reflector reflects client routes to other clients and non-clients, but does not reflect non-client routes to clients. Cluster ID must be unique per cluster to prevent loops.

MPLS LDP: labels are bound to FECs from the IGP route table; LDP needs a loopback reachable via IGP. RSVP-TE adds bandwidth reservation and explicit paths, signaled with PATH and RESV messages.

Convergence knobs: BFD for sub-second failure detection, IS-IS/OSPF LFA or TI-LFA for pre-computed backup paths, and BGP additional-paths or PIC for fast eBGP failover.

Policy order in Junos: import policy is evaluated before route selection on inbound updates; export policy is evaluated on outbound advertisements. A term without 'then accept' falls through to the next term.

Junos commit model: 'commit check' validates syntax, 'commit confirmed ' auto-rolls back unless confirmed, and 'rollback 1' restores the previous candidate configuration.

show commands: 'show route protocol bgp hidden' lists routes rejected by policy; 'show route receive-protocol bgp ' shows pre-policy inbound routes; 'show route advertising-protocol bgp ' shows what is actually sent.

MPLS label stack: the outer label is swapped at each LSR; the bottom label identifies the FEC. Penultimate-hop popping removes the outer label so the egress router receives an unlabeled IP packet.

Segment routing: SR-MPLS uses prefix-SID labels from the IGP (IS-IS/OSPF extensions). SRv6 uses 128-bit SIDs in the IPv6 header; TI-LFA provides sub-50ms protection with explicit segment lists.

VPN routing: VRF route targets control import/export of routes into VRFs; the route distinguisher only makes prefixes unique and does not control import. A route is imported if its RT matches a VRF import target.

BGP communities: well-known ones include no-export (0xFFFFFF01), no-advertise (0xFFFFFF02), and no-export-subconfed. Large communities are 3x 32-bit values, useful for 4-byte AS environments.

RPF check: multicast requires the incoming interface to match the unicast route to the source. PIM-SM uses a shared tree rooted at the RP, then switches to a shortest-path tree after the first packet.

Troubleshooting order: verify physical/link state, then IGP adjacency, then label bindings, then BGP session state ('show bgp summary'), then policy and route selection.