01
3.1 Read the routing table
- Codes: C = connected, L = local (/32), S = static, O = OSPF, D = EIGRP, R = RIP, * = candidate default.
- Entry format: prefix [AD/metric] via next-hop. Gateway of last resort = the 0.0.0.0/0 default route.
M3 25% of exam domain 3.0 (25%)
IP Connectivity - Routing & OSPF
The biggest domain. Reading the routing table, the forwarding decision, static routes, and single-area OSPFv2 (the only routing protocol you configure).
- 1) Longest prefix match ALWAYS wins first (most specific route).
- 2) Among same-prefix routes from different sources, lowest Administrative Distance wins.
- 3) Within the same protocol, lowest metric wins; equal-cost paths load-balance (ECMP).
| Source | AD |
|---|---|
| Connected | 0 |
| Static | 1 |
| eBGP | 20 |
| EIGRP (internal) | 90 |
| OSPF | 110 |
| RIP | 120 |
| iBGP | 200 |
| Unusable | 255 |
03
3.3 Static & default routing
- Default: ip route 0.0.0.0 0.0.0.0 <next-hop>. Network: ip route <net> <mask> <next-hop|exit-int>.
- Floating static = a backup static with a HIGHER AD than the primary route it backs up.
- IPv6 needs "ipv6 unicast-routing" enabled; ipv6 route ::/0 <next-hop> for default.
04
3.4 Single-area OSPFv2
- Link-state protocol, AD 110, cost = 10^8 / bandwidth (default reference 100 Mbps - Gig and 10-Gig both cost 1 unless you raise the reference).
- Router-ID selection order: manual "router-id" > highest loopback IP > highest active interface IP.
- Neighbors must match: area, hello/dead timers, subnet, MTU, and authentication - a mismatch stalls at 2-WAY or EXSTART.
- Broadcast networks elect a DR/BDR (highest priority, then highest router-ID; non-preemptive). Point-to-point links do NOT elect a DR.
- config: router ospf 1 ; network <ip> <wildcard> area 0.
05
3.5 First-Hop Redundancy (concepts only)
- FHRP gives hosts a redundant default gateway via a shared VIRTUAL IP. Hosts point at the virtual IP.
- HSRP (Cisco): Active/Standby. VRRP (open standard): Master/Backup. GLBP (Cisco): load-balances across gateways.
- "preempt" lets a higher-priority router take back the active role. Describe-only on the exam - no config required.