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03 - IP Connectivity / Routing (Domain 3, 25%)

The largest exam domain. Master routing concepts, static routes, OSPF single-area, and FHRP concepts.

Routing fundamentals

How routers decide

  1. Look up destination IP in routing table.
  2. Pick the longest prefix match that covers the destination.
  3. Forward out the matching interface (or to the next-hop IP).

If multiple routes are equal-prefix-length, administrative distance (AD) is the tiebreaker:

Source AD
Connected 0
Static 1
EIGRP summary 5
External BGP 20
Internal EIGRP 90
OSPF 110
RIPv2 120
External EIGRP 170
Internal BGP 200
Unknown / Untrusted 255

If still tied (same AD), the routing protocol's metric is the tiebreaker.

show commands

show ip route                    # full routing table
show ip route 192.168.1.0       # specific
show ip route ospf
show ip route static
show ip route connected
show ip protocols                # active routing protocols
show ip cef                      # FIB (forwarding info base)
show ip arp

Symbols in show ip route:

  • C - connected
  • L - local (router's own IP)
  • S - static
  • S* - candidate default
  • O - OSPF intra-area
  • O IA - OSPF inter-area
  • O E1, O E2 - OSPF external
  • R - RIP
  • D - EIGRP
  • B - BGP

Static routing

Syntax

ip route <destination> <mask> <next-hop-ip-or-interface>

ip route 0.0.0.0 0.0.0.0 192.168.1.1                # default route
ip route 10.0.0.0 255.0.0.0 192.168.1.1
ip route 10.0.0.0 255.0.0.0 GigabitEthernet0/1
ip route 10.0.0.0 255.0.0.0 192.168.1.1 5            # AD 5 (floating static)

Floating static routes

A static route with higher AD than the dynamic route. It's installed only if the dynamic route fails.

router ospf 1
 network 10.0.0.0 0.255.255.255 area 0
 ! AD 110

ip route 10.0.0.0 255.0.0.0 192.168.99.1 200      ! AD 200; only used if OSPF dies

Default route

ip route 0.0.0.0 0.0.0.0 192.168.1.1
ip default-network 192.168.1.0          ! older syntax

For OSPF advertising default:

router ospf 1
 default-information originate

IPv6 static

ipv6 unicast-routing                     ! enable IPv6 routing
ipv6 route 2001:db8:abcd::/48 2001:db8::1
ipv6 route ::/0 2001:db8::1              ! default

OSPF single-area

Concepts

  • Type: Link-state
  • Algorithm: Dijkstra (SPF)
  • Metric: Cost = reference_bandwidth / interface_bandwidth (default ref = 100 Mbps; bump to 100 Gbps with auto-cost reference-bandwidth 100000)
  • Hello / Dead intervals: 10/40s on broadcast LANs; 30/120s on NBMA
  • Multicast: 224.0.0.5 (all OSPF), 224.0.0.6 (DR/BDR)
  • Areas: All areas connect to area 0 (backbone). CCNA covers single-area.
  • Router ID: Highest configured loopback IP, else highest active interface IP, or router-id command.

Configure

router ospf 1
 router-id 1.1.1.1
 network 10.10.10.0 0.0.0.255 area 0
 network 192.168.1.0 0.0.0.255 area 0
 passive-interface default
 no passive-interface gi0/1
 no passive-interface gi0/2

network uses wildcard mask (inverted subnet mask).

passive-interface default then no passive-interface <intf> is the modern best practice (passive everything by default, enable on uplinks).

Neighbor states

Down → Init → 2-Way → Exstart → Exchange → Loading → Full

  • 2-Way is normal on multi-access (LAN) for non-DR/BDR neighbors.
  • Full means full LSDB sync.
  • Stuck in Init or Exstart usually means MTU mismatch, hello/dead mismatch, or area mismatch.

DR / BDR

On a multi-access (broadcast / Ethernet) network, OSPF elects a Designated Router (DR) and Backup DR (BDR) to avoid full mesh adjacencies.

  • DR has the highest OSPF priority (default 1; tiebreaker is highest router-id).
  • Priority 0 = will never become DR.
  • DR election is non-preemptive - changing priorities doesn't trigger a re-election.
interface gi0/1
 ip ospf priority 100                ! make this router preferred DR

To force re-election: clear ip ospf process on the affected router.

Common adjacency requirements

For two OSPF routers to become Full:

  • Same area
  • Same hello / dead intervals
  • Same subnet (same OSPF interface)
  • Same MTU (huge gotcha)
  • Matching authentication (if configured)
  • Compatible network type
  • Matching stub area flags

show commands

show ip ospf
show ip ospf neighbor
show ip ospf interface
show ip ospf interface gi0/1
show ip ospf database
show ip protocols
debug ip ospf events                ! use sparingly
debug ip ospf adj

OSPFv3 (IPv6)

ipv6 unicast-routing
interface gi0/1
 ipv6 ospf 1 area 0
router ospfv3 1
 router-id 1.1.1.1
 address-family ipv6 unicast
  passive-interface default
  no passive-interface gi0/1
 exit-address-family

First-Hop Redundancy (FHRP) - concept-level

CCNA tests recognition; full config is CCNP territory.

HSRP (Hot Standby Router Protocol)

  • Cisco proprietary
  • One active, one standby
  • Virtual IP and virtual MAC (0000.0c07.acXX where XX is group number)
  • Config snippet:
interface vlan 10
 ip address 10.10.10.2 255.255.255.0
 standby 1 ip 10.10.10.1
 standby 1 priority 110
 standby 1 preempt

VRRP (Virtual Router Redundancy Protocol)

  • Open standard (RFC 5798)
  • Master / backup
  • Multicast 224.0.0.18
  • Similar concept to HSRP

GLBP (Gateway Load Balancing Protocol)

  • Cisco proprietary
  • Active / active load balancing
  • Single virtual IP, multiple virtual MACs

What you need to know on CCNA

  • HSRP and VRRP are active/standby
  • GLBP is active/active load-balanced
  • HSRP and GLBP are Cisco-only; VRRP is open standard
  • HSRP virtual MAC pattern (0000.0c07.acXX) is high-yield trivia

Common exam triggers

  • "Add a default route via 192.168.1.1" → ip route 0.0.0.0 0.0.0.0 192.168.1.1
  • "Backup route used only if OSPF fails" → floating static with AD > 110
  • "OSPF neighbors stuck in EXSTART" → MTU mismatch, check show ip ospf interface
  • "Make this router the OSPF DR" → ip ospf priority 255 (and then clear ip ospf process on the segment)
  • "Don't form OSPF adjacency on user-facing port" → passive-interface gi0/1
  • "OSPF default route announcement" → default-information originate
  • "Two OSPF neighbors won't form" → check area, hello/dead, MTU, subnet, auth
  • "Active/active gateway redundancy" → GLBP

Troubleshooting workflow

When a route is missing or traffic doesn't flow:

  1. show ip interface brief - interface up/up?
  2. ping <directly-connected> - L2/L1 working?
  3. show ip route <destination> - is the route there?
  4. If not, show ip route - look for the parent supernet
  5. show ip protocols - is the routing process running?
  6. show ip ospf neighbor - did neighbor form?
  7. show ip ospf interface - did the interface get included? Right area? Hello/dead matching?
  8. traceroute <destination> - where does the packet die?
  9. debug ip ospf events (use carefully)