JNCIA-DC cheat sheet

Spine-leaf (Clos) topology: every leaf connects to every spine; leaves never connect to other leaves, spines never connect to other spines. Server-to-server path is always leaf-spine-leaf, giving a constant, predictable hop count.

Oversubscription ratio = total downlink bandwidth / total uplink bandwidth on a switch tier. A 1:1 ratio is non-blocking; 3:1 means the uplink can carry one third of the aggregate host demand.

East-west traffic stays inside the data center (server-to-server, storage, VM migration); north-south traffic crosses the data center boundary to clients or the internet.

Layer 2 scaling limits: a single VLAN is one broadcast domain, and STP blocks redundant links. Leaf-spine designs replace STP with L3 routing (or EVPN-VXLAN) to use all links actively.

VXLAN encapsulates original L2 frames in UDP (destination port 4789) with a 24-bit VNI, supporting about 16 million segments versus 4094 VLAN IDs.

EVPN is a BGP control plane for VXLAN: MP-BGP carries MAC and IP reachability (EVPN route types 2, 3, 5), replacing flood-and-learn with a controller-free control plane.

Anycast gateway: the same distributed IRB gateway IP and MAC exist on every leaf, so a host keeps its gateway when it migrates and traffic is routed at the first leaf.

Junos Fusion Provider Edge and MC-LAG provide multi-chassis link aggregation; MC-LAG lets a server dual-home to two independent devices without STP blocking one link.

Redundancy vocabulary: a single failure that does not interrupt service is N+1; 2N means fully duplicated capacity; a device failure causing no traffic loss requires redundant paths plus fast convergence.

Junos separates the control plane (routing protocol daemons, RE) from the forwarding plane (PFEs); the RE builds the forwarding table and pushes it to the PFE, so a control-plane restart need not stop forwarding.

Configuration model: candidate configuration is edited, then committed. 'commit check' validates syntax without applying; 'commit confirmed ' auto-rolls back unless confirmed with 'commit'.

Hierarchy levels: 'edit' enters a hierarchy, 'set' edits from anywhere, 'show | display set' renders the config as set commands, 'show | compare' shows candidate versus active differences.

Operational mode commands start with 'show', 'clear', 'monitor', or 'request'; configuration mode is entered with 'configure' and exited with 'exit configuration-mode' or 'commit' then 'exit'.

Rollback: 'rollback 0' reloads the active config into the candidate; 'rollback 1' loads the previous commit; 'show system rollback compare 1 0' diffs two saved versions (up to 50 stored).

User classes: super-user, operator, read-only, unauthorized. 'set system login user class ' assigns privileges; 'authentication-order' sets the login method sequence.

Rescue configuration: 'request system configuration rescue save' stores a known-good config; 'rollback rescue' restores it at the [edit] prompt.

Junos CLI help: '?' lists commands or options in context, 'help topic ' and 'help reference ' give syntax; Tab completes a partial command.

Interface config: 'set interfaces xe-0/0/0 unit 0 family ethernet-switching interface-mode access vlan-id 10' for L2 access; 'family inet address 10.0.0.1/24' for L3.

VLAN and IRB: define 'set vlans V10 vlan-id 10', then 'set interfaces irb unit 10 family inet address 10.10.10.1/24' and 'set vlans V10 l3-interface irb.10' to route between VLANs.

Trunk ports use 'interface-mode trunk' plus 'vlan members [ 10 20 ]'; native VLAN is set with 'native-vlan-id'.

LAG/LACP: 'set interfaces ae0 aggregated-ether-options lacp active' and add member links with 'set interfaces xe-0/0/0 ether-options 802.3ad ae0'.

Monitoring: 'show interfaces xe-0/0/0 extensive' shows errors and stats; 'show ethernet-switching table' shows MAC learning; 'show arp' and 'show route' verify L2/L3 reachability.

Troubleshooting flow: 'show log messages' for events, 'show chassis alarms' and 'show system alarms' for hardware, 'monitor traffic interface ' for packet capture (tcpdump-style).

Commit workflow for changes: 'commit check' then 'commit comment "text"'; 'show system commit' lists commit history with user and time.

Zero Touch Provisioning (ZTP) bootstraps a factory-default device from DHCP options plus a config file or script; 'request system zeroize' returns a device to factory default.

Junos routing preference (lower wins): connected 0, static 5, OSPF internal 10, RIP 100, OSPF external 150, EBGP 170, IBGP 200. Use 'preference' under protocols or static routes to override.

Routing policy is a chain of terms evaluated top-down; each term has 'from' conditions, 'then' actions (accept, reject, next term, next policy), and the first matching term with a terminating action decides.

Import policy filters routes before they enter the routing table; export policy filters routes advertised to neighbors. A route rejected by policy is not installed/advertised.

BGP: 'set protocols bgp group type external peer-as neighbor '; IBGP requires 'type internal' and usually a full mesh or route reflector. 'show bgp summary' shows peer state and prefix counts.

OSPF: 'set protocols ospf area 0 interface ge-0/0/0.0' enables OSPF on that interface; 'show ospf neighbor' verifies adjacency state (Full is converged).

Stateless firewall filter: 'set firewall family inet filter F term T from source-address 10.0.0.0/8' plus 'then accept|discard|reject'; apply with 'set interfaces xe-0/0/0 unit 0 family inet filter input F'.

A firewall filter with no matching term applies the implicit default, which is discard; add an explicit 'then accept' term for permitted traffic.

Prefix lists and route filters: 'set policy-options prefix-list P 10.0.0.0/8', then reference in a policy term with 'from prefix-list P'.

Verification: 'show route detail' shows preference, next hop, and policy; 'show firewall filter F' displays per-term counters to confirm matches.