JNCIS-SP flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

OSPF reference bandwidth default

Junos defaults to 100 Mbps. Interface cost equals reference bandwidth divided by interface bandwidth, rounded to an integer with a minimum of 1, so a 1 Gbps interface costs 1 unless the reference is raised.

OSPF stub area behavior

A stub area blocks Type 5 external LSAs and the ABR injects a default route as a Type 3 summary, letting internal routers reach external destinations without carrying external LSAs.

OSPF NSSA Type 7

An NSSA blocks Type 5 LSAs but allows external routes originated inside the area as Type 7 LSAs; the ABR translates Type 7 to Type 5 when flooding into other areas.

OSPF E1 versus E2 metrics

Type 1 external metrics add the internal cost to reach the ASBR to the redistributed metric, while Type 2 (the default) uses only the redistributed metric regardless of internal distance.

OSPF virtual link rules

A virtual link is configured inside a transit area, both endpoints must be in that same area, and at least one endpoint must belong to area 0; it is used to repair a partitioned backbone.

OSPF LSA type numbers

Type 1 router, Type 2 network, Type 3 summary, Type 4 ASBR summary, Type 5 external, and Type 7 NSSA external. Types 1 and 2 stay within an area.

IS-IS NET address format

A NET consists of a 1-13 byte area ID, a fixed 6-byte system ID, and a 1-byte NSEL always set to 00, for example 49.0001.1921.6800.1001.00.

IS-IS Level 1 reachability

Level 1 routers build only an intra-area topology and reach other areas through the nearest L1/L2 router, which sets the attached bit to signal a default route.

IS-IS narrow versus wide metrics

Narrow metrics use 6 bits per interface (max 63) and a 10-bit total (max 1023); wide metrics use 24 bits (max 16,777,215) and are required for MPLS traffic engineering.

IS-IS DIS election

On broadcast networks the Designated Intermediate System is chosen by highest interface priority then highest MAC/SNPA. Unlike an OSPF DR, the DIS is preemptive.

IS-IS adjacency requirements

Neighbors must match level, have sufficient MTU with padding support, and for Level 1 must share an area address; these values are carried in IIH PDUs.

BGP best-path order

Highest local preference, then locally originated, shortest AS path, lowest origin, lowest MED, eBGP over iBGP, lowest IGP metric to next hop, lowest router ID, shortest cluster list, lowest peer address.

Route reflector loop prevention

An RR reflects client routes to clients and non-clients but never back to the originator, adding ORIGINATOR_ID and CLUSTER_LIST attributes so receiving routers can detect and drop loops.

BGP confederation behavior

A confederation splits one AS into sub-ASes that peer with eBGP-like rules internally while advertising a single AS number externally; the sub-AS path appears in parentheses and is stripped at the border.

iBGP next-hop handling

eBGP rewrites the next hop to the sending router, but iBGP preserves it, so the border router usually needs next-hop-self or a multihop session with an increased TTL.

BGP well-known communities

no-export keeps a route inside the local AS, no-advertise prevents sending it to any peer, and no-export-subconfed keeps it within the confederation sub-AS.

BGP finite state machine order

Idle, Connect, Active, OpenSent, OpenConfirm, Established. The Active state means the router is listening for an inbound TCP port 179 connection and retrying.

MPLS shim header layout

The 32-bit label stack entry contains a 20-bit label, 3-bit EXP field for class of service, 1-bit bottom-of-stack flag, and an 8-bit TTL.

Penultimate hop popping

The penultimate router pops the transport label so the egress PE receives the packet with only the service label. Explicit-null preserves a label for egress QoS.

Route distinguisher versus target

The route distinguisher makes otherwise identical customer prefixes unique in VPNv4 BGP, while the route target extended community decides which VRFs import and export the route.

RSVP-TE signaled LSP

RSVP-TE uses PATH and RESV messages to establish explicit-path LSPs with bandwidth reservation, enabling traffic engineering and fast reroute in the MPLS core.

LDP label distribution

LDP advertises a label for each IGP-learned prefix using downstream-unsolicited mode, so the resulting LSP follows the IGP shortest path and bindings populate the LFIB.

802.1Q trunk tagging

A 4-byte tag after the source MAC carries a 12-bit VLAN ID in the range 1 to 4094. The native VLAN is transmitted untagged across the trunk.

STP root bridge election

The root bridge has the lowest bridge ID, made of a 2-byte priority (default 32768, adjustable in steps of 4096) followed by the switch MAC address.

QinQ double tagging

802.1ad stacks a provider S-VLAN tag outside the customer C-VLAN tag, allowing the provider to carry many customers whose VLAN IDs overlap.

LACP LAG requirements

802.3ad LACP bundles links in active or passive mode; member links must match in speed, duplex and MTU, and frames are hashed across members by L2, L3 or L4 fields.

RSTP edge port

An RSTP edge port moves directly to forwarding and does not cause topology changes. BPDU guard disables the port if a BPDU is unexpectedly received.

Junos policy default actions

The Junos default BGP import policy is accept and the default export policy is reject. A policy term with no terminating action falls through to the next term.

Graceful restart helper mode

The helper keeps forwarding traffic for a restarting neighbor during the restart interval and waits for the End-of-RIB marker before recomputing and installing new best paths.

Nonstop routing operation

NSR synchronizes protocol state between primary and backup Routing Engines so a failover does not reset BGP, OSPF or IS-IS adjacencies, and it needs no neighbor support.

BFD failure detection time

BFD negotiates transmit and receive intervals plus a multiplier; the session is declared down after multiplier times the interval of missed control packets, far faster than IGP hellos.

BGP maximum-prefix limit

A maximum-prefix limit configured on a BGP group or neighbor caps the accepted routes and can warn, log, or tear down the session to protect the routing table.