CCNA Last-Minute Revision

30 quick revision notes for CCNA

1. OSI layers from bottom to top are Physical, Data Link, Network, Transport, Session, Presentation and Application; troubleshoot bottom-up.
2. TCP is connection-oriented with a three-way handshake of SYN, SYN-ACK and ACK, while UDP is connectionless and faster.
3. Each switch port is its own collision domain, and each VLAN or router interface is its own broadcast domain.
4. Block size equals 256 minus the interesting mask octet; a /26 has a block of 64 and 62 usable hosts.
5. Private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, defined in RFC 1918.
6. A 169.254.x.x address means APIPA: the host did not get a reply from any DHCP server.
7. IPv6 global unicast addresses start with 2000::/3, link-local with FE80::/10 and unique local with FC00::/7.
8. EUI-64 splits the 48-bit MAC, inserts FFFE in the middle and flips the seventh bit to build a 64-bit interface ID.
9. An access port carries one untagged VLAN, while an 802.1Q trunk tags frames for many VLANs except the native VLAN.
10. Disable DTP with switchport nonegotiate and set the native VLAN to an unused VLAN to reduce VLAN hopping risk.
11. The root bridge has the lowest bridge ID: priority in steps of 4096, plus VLAN ID, then the lowest MAC as tie-breaker.
12. Rapid PVST+ port roles are root, designated, alternate and backup; states are discarding, learning and forwarding.
13. BPDU Guard err-disables a PortFast port that receives a BPDU, which stops users from creating loops with rogue switches.
14. LACP modes are active and passive; PAgP modes are desirable and auto; on mode forces a bundle with no negotiation.
15. EtherChannel members must match speed, duplex, VLANs, trunk mode and native VLAN, or they become suspended.
16. Routers choose routes by longest prefix match first, then lowest administrative distance, then lowest metric.
17. Default administrative distances: connected 0, static 1, eBGP 20, EIGRP 90, OSPF 110, RIP 120, iBGP 200.
18. OSPF cost equals reference bandwidth divided by interface bandwidth; the default reference is 100 Mbps, so raise it.
19. OSPF neighbours must match area, subnet, hello and dead timers, authentication, stub flags and MTU to reach FULL.
20. OSPF router ID is chosen from the manual router-id, then the highest loopback IP, then the highest active interface IP.
21. HSRP uses a virtual IP and MAC; the highest priority wins, and preempt lets a recovered router take over again.
22. DHCP uses DORA: Discover, Offer, Request and Acknowledge; ip helper-address relays requests to a remote server.
23. PAT maps many inside local addresses to one inside global address using unique source port numbers.
24. Syslog levels run from 0 Emergency to 7 Debugging; remember Every Awesome Computer Engineer Will Need Ice cream Daily.
25. SNMPv3 adds authentication and encryption, while SNMPv2c sends community strings in clear text.
26. Standard ACLs filter only on source and go near the destination; extended ACLs go near the source.
27. Every ACL ends with an implicit deny any, so a list with only deny lines blocks all traffic.
28. TACACS+ uses TCP 49 and encrypts the whole payload; RADIUS uses UDP 1812 and 1813 and encrypts only the password.
29. Use non-overlapping channels 1, 6 and 11 in 2.4 GHz; WPA3 replaces the PSK handshake with SAE.
30. REST APIs use GET to read, POST to create, PUT or PATCH to update and DELETE to remove, usually with JSON.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.