IP Connectivity Study Guide

CCNA 200-301 Exam Domain 3 (25% of exam)

Table of Contents

1. Routing Concepts

Routing is the process of selecting the best path for network traffic to travel from source to destination across multiple networks. Routers operate at Layer 3 (Network Layer) and use IP addresses to make forwarding decisions.

Key Concept: A router's primary job is to receive packets on one interface and forward them out another interface toward the destination. This decision is based on the routing table.

How Routers Work

  1. Receive packet on ingress interface
  2. Examine destination IP in packet header
  3. Lookup in routing table for matching route
  4. Determine exit interface and next-hop
  5. Rewrite Layer 2 header (new source/dest MAC)
  6. Forward packet out egress interface
Routing Decision Process: Packet arrives: Dest IP = 10.2.3.100 | v +-------------------+ | Routing Table | +-------------------+ | 10.1.0.0/16 -> Gi0/0 | 10.2.0.0/16 -> Gi0/1 <-- Match! | 10.3.0.0/16 -> Gi0/2 | 0.0.0.0/0 -> Gi0/3 +-------------------+ | v Forward out Gi0/1 toward 10.2.0.0/16

Packet vs Frame

Characteristic Packet (Layer 3) Frame (Layer 2)
Addressing IP addresses MAC addresses
Scope End-to-end (unchanged) Hop-to-hop (changes at each router)
Device Router Switch
When a router forwards a packet, the IP header (source/destination IP) stays the same, but the Ethernet frame header (source/destination MAC) changes at each hop.

2. The Routing Table

The routing table contains all known networks and how to reach them. Routes can be learned through different methods.

Route Sources

Code Source Description
C Connected Directly connected networks (interfaces with IP)
L Local IP address assigned to router's interface (/32)
S Static Manually configured routes
O OSPF Learned via OSPF protocol
D EIGRP Learned via EIGRP protocol
R RIP Learned via RIP protocol
B BGP Learned via BGP protocol
S* Static Default Default route (0.0.0.0/0) configured statically

Reading the Routing Table

Router# show ip route Codes: C - connected, S - static, O - OSPF, D - EIGRP L - local, B - BGP, * - candidate default Gateway of last resort is 10.0.0.1 to network 0.0.0.0 10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks C 10.1.1.0/24 is directly connected, GigabitEthernet0/0 L 10.1.1.1/32 is directly connected, GigabitEthernet0/0 S 10.2.0.0/16 [1/0] via 10.1.1.254 O 10.3.3.0/24 [110/20] via 10.1.1.2, 00:05:32, GigabitEthernet0/0 S* 0.0.0.0/0 [1/0] via 10.0.0.1

Understanding Route Entries

Route Entry Breakdown: O 10.3.3.0/24 [110/20] via 10.1.1.2, 00:05:32, Gi0/0 | | | | | | | | | | | | | +-- Exit interface | | | | | +----------- Uptime | | | | +---------------------- Next-hop IP | | | +------------------------------ Metric | | +---------------------------------- Administrative Distance | +-------------------------------------------- Network/Prefix +------------------------------------------------------ Route source (OSPF)

Administrative Distance (AD)

AD is the trustworthiness of a routing source. Lower AD = more trusted. When multiple routing sources know a route, the one with lowest AD wins.

Route Source Default AD
Connected 0
Static 1
EIGRP Summary 5
eBGP 20
EIGRP (Internal) 90
OSPF 110
IS-IS 115
RIP 120
EIGRP (External) 170
iBGP 200
Unknown 255 (unusable)
Longest Prefix Match: When multiple routes match a destination, the router uses the route with the longest (most specific) prefix. Example: 10.1.1.0/24 beats 10.1.0.0/16 for destination 10.1.1.50.
Memorize the common AD values: Connected=0, Static=1, EIGRP=90, OSPF=110, RIP=120. These are frequently tested!

3. Static Routing

Static routes are manually configured by an administrator. They don't change unless manually modified, making them simple but not scalable for large networks.

When to Use Static Routes

Static Route Syntax

! Basic static route syntax Router(config)# ip route [destination-network] [subnet-mask] [next-hop-ip | exit-interface] ! Static route using next-hop IP address Router(config)# ip route 192.168.2.0 255.255.255.0 10.1.1.2 ! Static route using exit interface Router(config)# ip route 192.168.2.0 255.255.255.0 GigabitEthernet0/1 ! Static route using both (fully specified) Router(config)# ip route 192.168.2.0 255.255.255.0 GigabitEthernet0/1 10.1.1.2 ! Verify static routes Router# show ip route static

Types of Static Routes

Standard Static Route

! Route to specific network Router(config)# ip route 172.16.0.0 255.255.0.0 10.0.0.2

Default Route (Gateway of Last Resort)

! Default route - matches all destinations Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1 ! Verify gateway of last resort is set Router# show ip route Gateway of last resort is 203.0.113.1 to network 0.0.0.0

Floating Static Route (Backup Route)

! Primary route via OSPF (AD=110) ! Backup static route with higher AD Router(config)# ip route 10.2.0.0 255.255.0.0 10.1.1.254 130 ^^^ AD of 130 (higher than OSPF's 110) Only used if OSPF route is lost

Summary Route

! Instead of 4 separate /24 routes: ! 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24 ! Use one summary route: Router(config)# ip route 10.1.0.0 255.255.252.0 10.0.0.2

IPv6 Static Routes

! Enable IPv6 routing Router(config)# ipv6 unicast-routing ! IPv6 static route to network Router(config)# ipv6 route 2001:db8:2::/64 2001:db8:1::2 ! IPv6 default route Router(config)# ipv6 route ::/0 2001:db8:1::1 ! IPv6 static route using link-local (requires interface) Router(config)# ipv6 route 2001:db8:3::/64 GigabitEthernet0/0 fe80::1
When using link-local addresses as next-hop for IPv6, you must also specify the exit interface because link-local addresses are only unique per interface.

Static Routing Key Points

  • AD = 1 by default (can be changed for floating static)
  • 0.0.0.0/0 is the default route (catches all unmatched traffic)
  • Floating static routes are backups with higher AD
  • Fully specified routes use both next-hop and exit interface
  • IPv6 link-local next-hop requires exit interface

4. Dynamic Routing Protocols

Dynamic routing protocols automatically discover and maintain routes. They adapt to network changes, making them essential for larger networks.

Types of Dynamic Routing Protocols

Dynamic Routing Protocols Classification: Routing Protocols | +-----------------+-----------------+ | | Interior (IGP) Exterior (EGP) Within one AS Between AS's | | +-------+-------+ BGP | | Distance Vector Link State | | RIP, EIGRP OSPF, IS-IS

Distance Vector vs Link State

Feature Distance Vector Link State
Information Shared Routing table (distances) Topology database (links)
Algorithm Bellman-Ford Dijkstra SPF
Network View Neighbor's perspective Complete topology
Convergence Slower Faster
CPU/Memory Lower Higher
Examples RIP, EIGRP* OSPF, IS-IS

*EIGRP is technically an "advanced distance vector" or hybrid protocol

Routing Protocol Comparison

Protocol Type Metric AD CCNA Focus
RIPv2 Distance Vector Hop count (max 15) 120 Basic awareness
OSPF Link State Cost (bandwidth) 110 Primary focus
EIGRP Advanced DV Composite (BW+delay) 90/170 Basic awareness
BGP Path Vector Attributes 20/200 Conceptual
Metric: The value a routing protocol uses to determine the best path when multiple paths exist. Each protocol uses different metrics - OSPF uses cost, RIP uses hop count, EIGRP uses bandwidth and delay.
For CCNA, focus heavily on OSPF. Know the basics of RIP (hop count metric, max 15 hops) and EIGRP (bandwidth + delay metric, Cisco proprietary history) but deep configuration knowledge isn't required.

5. OSPF Single-Area

Open Shortest Path First (OSPF) is an open standard link-state routing protocol. It's the most commonly used IGP in enterprise networks.

OSPF Characteristics

OSPF Cost Calculation

OSPF Cost Formula: Cost = Reference Bandwidth / Interface Bandwidth Default Reference Bandwidth = 100 Mbps (10^8) Examples: - 10 Mbps: 100,000,000 / 10,000,000 = 10 - 100 Mbps: 100,000,000 / 100,000,000 = 1 - 1 Gbps: 100,000,000 / 1,000,000,000 = 0.1 → rounded to 1 - 10 Gbps: 100,000,000 / 10,000,000,000 = 0.01 → rounded to 1 Problem: With default reference, 100Mbps, 1Gbps, 10Gbps all have cost = 1!
The default reference bandwidth of 100 Mbps doesn't distinguish between modern high-speed links. Always change it to 10 Gbps (10000) or higher on all OSPF routers in your network.
! Change reference bandwidth to 10 Gbps Router(config)# router ospf 1 Router(config-router)# auto-cost reference-bandwidth 10000 ! Resulting costs: ! 10 Mbps = 1000 ! 100 Mbps = 100 ! 1 Gbps = 10 ! 10 Gbps = 1

OSPF Messages

Type Name Purpose
1 Hello Discover neighbors, maintain adjacencies
2 Database Description (DBD) Summary of LSDB contents during initial sync
3 Link-State Request (LSR) Request specific LSAs from neighbor
4 Link-State Update (LSU) Contains actual LSAs (routing information)
5 Link-State Acknowledgment (LSAck) Confirms receipt of LSU

OSPF Multicast Addresses

Address Name Used By
224.0.0.5 AllSPFRouters All OSPF routers listen on this
224.0.0.6 AllDRouters DR and BDR listen on this

OSPF Timers

Timer Broadcast/P2P NBMA Purpose
Hello 10 seconds 30 seconds Interval between Hello packets
Dead 40 seconds 120 seconds Time before declaring neighbor down
Dead Interval: By default, the dead interval is 4x the hello interval. If no Hello is received within the dead interval, the neighbor is declared down and routes through it are removed.
OSPF neighbors must match: Area ID, Hello/Dead intervals, authentication, stub flags, and MTU. Network type affects Hello/Dead defaults.

6. OSPF Configuration

Basic OSPF Configuration

! Enable OSPF process (process ID is locally significant) Router(config)# router ospf 1 ! Configure router ID (recommended) Router(config-router)# router-id 1.1.1.1 ! Advertise networks using network command Router(config-router)# network 10.1.1.0 0.0.0.255 area 0 Router(config-router)# network 192.168.1.0 0.0.0.255 area 0 ! Or use interface-level configuration (newer method) Router(config)# interface gigabitethernet 0/0 Router(config-if)# ip ospf 1 area 0
Wildcard Mask: OSPF uses wildcard masks (inverse of subnet mask). 0 bits must match, 1 bits are "don't care". Example: 0.0.0.255 matches any IP where first 3 octets match exactly.

Router ID Selection

OSPF Router ID is chosen in this order:

  1. Manually configured router-id command
  2. Highest IP on any loopback interface
  3. Highest IP on any active physical interface
! Best practice: Use loopback for router ID Router(config)# interface loopback 0 Router(config-if)# ip address 1.1.1.1 255.255.255.255 Router(config-if)# exit Router(config)# router ospf 1 Router(config-router)# router-id 1.1.1.1 ! Clear OSPF process to apply new router-id Router# clear ip ospf process

Passive Interfaces

! Stop sending OSPF Hellos on interface (but still advertise network) Router(config-router)# passive-interface gigabitethernet 0/2 ! Make all interfaces passive, then enable specific ones Router(config-router)# passive-interface default Router(config-router)# no passive-interface gigabitethernet 0/0

Default Route Advertisement

! On border router with default route, advertise to OSPF Router(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1 Router(config)# router ospf 1 Router(config-router)# default-information originate ! Always advertise default (even without one in routing table) Router(config-router)# default-information originate always

Modifying OSPF Cost

! Change cost on interface Router(config)# interface gigabitethernet 0/0 Router(config-if)# ip ospf cost 50 ! Or change bandwidth (affects cost calculation) Router(config-if)# bandwidth 100000 ! in Kbps

OSPF Verification Commands

! View OSPF neighbors Router# show ip ospf neighbor ! View OSPF database (LSDB) Router# show ip ospf database ! View OSPF interfaces Router# show ip ospf interface Router# show ip ospf interface brief ! View OSPF routes in routing table Router# show ip route ospf ! View OSPF process information Router# show ip ospf ! View specific interface OSPF settings Router# show ip ospf interface gigabitethernet 0/0

OSPFv3 for IPv6

! Enable IPv6 routing Router(config)# ipv6 unicast-routing ! Configure OSPFv3 on interface Router(config)# interface gigabitethernet 0/0 Router(config-if)# ipv6 ospf 1 area 0 ! Configure router ID (still uses IPv4 format) Router(config)# ipv6 router ospf 1 Router(config-rtr)# router-id 1.1.1.1 ! Verification Router# show ipv6 ospf neighbor Router# show ipv6 route ospf

7. OSPF Neighbor States & DR/BDR

OSPF Neighbor States

State Description
Down No Hellos received from this neighbor
Init Hello received but own router-id not in neighbor's Hello
2-Way Bidirectional communication confirmed; DR/BDR election occurs
ExStart Master/slave negotiation for DBD exchange
Exchange DBD packets exchanged describing LSDB contents
Loading LSR/LSU exchange to get missing LSAs
Full Fully adjacent; databases synchronized
OSPF Neighbor State Progression: Down → Init → 2-Way → ExStart → Exchange → Loading → Full | +-- DR/BDR election happens here | (on multi-access networks) | +-- DROther routers stay at 2-Way with each other (only go Full with DR/BDR)

DR/BDR Election

On multi-access networks (Ethernet), OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce flooding overhead.

Why DR/BDR? Without DR/BDR, every router would need a full adjacency with every other router on the segment. With 10 routers, that's 45 adjacencies! With DR/BDR, each router only needs 2 adjacencies (to DR and BDR).

DR/BDR Election Process

  1. Highest OSPF Priority wins (default = 1, range 0-255)
  2. Priority 0 = cannot be DR/BDR
  3. Highest Router ID is tiebreaker
  4. Election is non-preemptive - existing DR stays even if higher priority router joins
! View DR/BDR status Router# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/DR 00:00:32 10.1.1.2 Gi0/0 3.3.3.3 1 FULL/BDR 00:00:35 10.1.1.3 Gi0/0 4.4.4.4 1 2WAY/DROTHER 00:00:38 10.1.1.4 Gi0/0 ! Change OSPF priority (to influence DR election) Router(config)# interface gigabitethernet 0/0 Router(config-if)# ip ospf priority 100 ! Prevent becoming DR/BDR Router(config-if)# ip ospf priority 0

OSPF Network Types

Network Type DR/BDR? Hello Dead Example
Broadcast Yes 10s 40s Ethernet (default)
Point-to-Point No 10s 40s Serial links, P2P subinterfaces
NBMA Yes 30s 120s Frame Relay (legacy)
Point-to-Multipoint No 30s 120s Hub-and-spoke WAN
! Change network type (useful for efficiency) Router(config-if)# ip ospf network point-to-point
On point-to-point links (like between two routers), changing network type to point-to-point skips DR/BDR election and reaches Full state faster.

8. First Hop Redundancy Protocols (FHRP)

FHRPs provide default gateway redundancy for hosts. If the primary router fails, another router takes over the virtual gateway IP seamlessly.

FHRP Concept: Internet | +------+------+ | | [Router A] [Router B] 10.1.1.2 10.1.1.3 Active Standby | | +------+------+ | Virtual IP: 10.1.1.1 <-- Hosts use this as gateway | [Hosts] If Router A fails, Router B takes over 10.1.1.1 Hosts don't need to change anything!

FHRP Comparison

Feature HSRP VRRP GLBP
Standard Cisco proprietary IEEE (open) Cisco proprietary
Active Router Active Master AVG (Active Virtual Gateway)
Standby Router Standby Backup AVF (Active Virtual Forwarder)
Virtual MAC 0000.0c07.acXX 0000.5e00.01XX 0007.b400.XXYY
Load Balancing Per-group only Per-group only Per-host (built-in)
Multicast 224.0.0.2 (v1) / 224.0.0.102 (v2) 224.0.0.18 224.0.0.102

HSRP Configuration

! Router A (Active) RouterA(config)# interface gigabitethernet 0/0 RouterA(config-if)# ip address 10.1.1.2 255.255.255.0 RouterA(config-if)# standby version 2 RouterA(config-if)# standby 1 ip 10.1.1.1 RouterA(config-if)# standby 1 priority 110 RouterA(config-if)# standby 1 preempt ! Router B (Standby) RouterB(config)# interface gigabitethernet 0/0 RouterB(config-if)# ip address 10.1.1.3 255.255.255.0 RouterB(config-if)# standby version 2 RouterB(config-if)# standby 1 ip 10.1.1.1 RouterB(config-if)# standby 1 priority 100 ! Default priority RouterB(config-if)# standby 1 preempt ! Verification Router# show standby Router# show standby brief

HSRP States

State Description
Initial Starting state
Learn Waiting to hear from active router
Listen Knows virtual IP, listening for active/standby
Speak Participating in election
Standby Candidate to become next active router
Active Currently forwarding traffic for virtual IP

HSRP Tracking

! Track upstream interface - reduce priority if it fails RouterA(config-if)# standby 1 track gigabitethernet 0/1 20 ^^^ Decrement priority by 20 if Gi0/1 goes down

FHRP Key Points

  • HSRP is Cisco proprietary; VRRP is open standard
  • Higher priority = more likely to be active
  • Preempt allows higher-priority router to take over
  • Use tracking to respond to upstream failures
  • GLBP provides true load balancing per-host
  • Virtual IP is what hosts use as default gateway
Know the difference between HSRP (Cisco) and VRRP (standard). Remember: HSRP uses "Active/Standby" terminology while VRRP uses "Master/Backup". Default HSRP priority is 100.