1. Dynamic Routing Definition: Dynamic routing involves configuring a routing protocol on routers that automatically discovers and maintains routes to destination networks, unlike static routing where each route must be manually configured using the ip route command.

2. Purpose of Dynamic Routing: Dynamic routing protocols allow routers to automatically share routing information, adapt to network topology changes, select optimal paths based on metrics, and provide redundancy through automatic failover to backup routes.

3. IGP vs EGP Classification: Interior Gateway Protocols (IGPs) are used to share routes within a single autonomous system (organization), while Exterior Gateway Protocols (EGPs) are used to share routes between different autonomous systems.

4. Algorithm Types: Routing protocols are categorized by their algorithm type—Distance Vector protocols share routing tables with neighbors ("routing by rumor"), Link State protocols build a complete network topology map, and Path Vector protocols (BGP) track the path of autonomous systems.

5. Metric Purpose: A routing protocol's metric is a numerical value used to determine the best route to a destination when multiple routes exist within the same routing protocol—lower metrics indicate better (preferred) routes.

6. Administrative Distance Purpose: Administrative Distance (AD) is used to determine route preference when routes to the same destination are learned via different routing protocols or methods—lower AD values indicate more trustworthy route sources.

7. ECMP Load Balancing: Equal Cost Multi-Path (ECMP) occurs when multiple routes to the same destination have identical metrics, causing all routes to be added to the routing table with traffic load-balanced across them.

┌─────────────────────────────────────────────────────────────┐

│ DYNAMIC ROUTING PROTOCOLS │

├─────────────────────────────┬───────────────────────────────┤

│ IGP (Interior Gateway │ EGP (Exterior Gateway │

│ Protocol) │ Protocol) │

├─────────────────────────────┼───────────────────────────────┤

│ • Used WITHIN an │ • Used BETWEEN different │

│ Autonomous System (AS) │ Autonomous Systems │

│ │ │

│ Examples: │ Example: │

│ - RIP │ - BGP (Border Gateway │

│ - OSPF │ Protocol) │

│ - EIGRP │ │

│ - IS-IS │ │

└─────────────────────────────┴───────────────────────────────┘

DYNAMIC ROUTING PROTOCOLS

│

┌────────────────────┼────────────────────┐

│ │ │

▼ ▼ ▼

┌──────────┐ ┌──────────┐ ┌──────────────┐

│ DISTANCE │ │ LINK │ │ HYBRID/ │

│ VECTOR │ │ STATE │ │ ADVANCED │

└──────────┘ └──────────┘ └──────────────┘

│ │ │

• RIP • OSPF • EIGRP

• RIPv2 • IS-IS

• RIPng

Protocols:

• RIPv1 - Classful, broadcast updates

• RIPv2 - Classless, multicast updates

• RIPng - For IPv6

Router A ──── Router B ──── Router C

│ │ │

"I can reach "I can reach "I can reach

X in 1 hop" X in 2 hops" X in 3 hops"

Protocols:

• OSPF (Open Shortest Path First)

• IS-IS (Intermediate System to Intermediate System)

┌─────────────────────────────┐

│ LINK STATE DATABASE │

│ (Complete Topology Map) │

│ │

│ A ─── B ─── C │

│ │ │ │ │

│ D ─── E ─── F │

└─────────────────────────────┘

Protocol:

• EIGRP (Enhanced Interior Gateway Routing Protocol)

┌─────────────────────┐

│ ROUTING PROTOCOLS │

└──────────┬──────────┘

│

┌────────────────────┴────────────────────┐

│ │

┌─────┴─────┐ ┌──────┴──────┐

│ IGP │ │ EGP │

└─────┬─────┘ └──────┬──────┘

│ │

┌─────┴─────────────────┐ │

│ │ │ │

┌───┴───┐ ┌────┴────┐ ┌───┴────┐ ┌───┴───┐

│Distance│ │ Link │ │ Hybrid │ │ Path │

│Vector │ │ State │ │ │ │Vector │

└───┬───┘ └────┬────┘ └───┬────┘ └───┬───┘

│ │ │ │

RIP OSPF EIGRP BGP

RIPv2 IS-IS

RIPng

Routing protocols use different metrics and algorithms to determine the best path for data packets. Here's an overview:

Common Path Selection Metrics

How Different Protocols Select Paths

1. RIP (Routing Information Protocol)

• Uses hop count only

• Maximum 15 hops

• Selects path with fewest hops

2. OSPF (Open Shortest Path First)

• Uses cost (based on bandwidth)

• Formula: Cost = Reference Bandwidth / Interface Bandwidth

• Selects lowest cost path

3. EIGRP (Enhanced Interior Gateway Routing Protocol)

• Uses composite metric:

  • Bandwidth

  • Delay

  • (Optionally: Load, Reliability)

• Selects path with lowest metric value

4. BGP (Border Gateway Protocol)

• Uses multiple path attributes:

  1. Highest Weight (Cisco)

  2. Highest Local Preference

  3. Locally originated routes

  4. Shortest AS Path

  5. Lowest Origin type

  6. Lowest MED

  7. eBGP over iBGP

  8. Lowest IGP metric

Administrative Distance (AD)

When multiple protocols suggest different paths, routers use Administrative Distance to choose:

Lower AD = More trusted

Summary Diagram

Packet arrives → Check routing table → Multiple paths?

↓

Compare metrics (within same protocol)

↓

Compare AD (between different protocols)

↓

Forward via best path

Dynamic Routing Fundamentals

• Routers advertise connected routes and learned routes to neighbors

• Routers form adjacencies (neighbor relationships/neighborships) with directly connected routers

• Routes are automatically added/removed based on network changes

• Superior routes are determined by metric (lower = better)

Route Types

Protocol Classifications

Distance Vector vs Link State

Administrative Distance Values (CRITICAL!)

Metric Types by Protocol

Route Table Display Format

[AD/Metric]

Example: [110/3] = AD of 110, Metric of 3

1. Confusing AD and Metric

   • AD compares routes from DIFFERENT routing protocols

   • Metric compares routes from the SAME routing protocol

   • AD is checked FIRST, then metric

2. RIP Metric Limitations

   • RIP counts hops only—a 10 Mbps link = 1 hop, a 10 Gbps link = 1 hop

   • This can result in suboptimal routing decisions

3. ECMP Requirements

   • Routes must be from the SAME routing protocol

   • Routes must have the SAME metric

   • Routes must be to the EXACT same destination (same network AND prefix length)

4. Static Route AD Modification

   • The command shows "distance metric" but this refers to AD, NOT metric

   • Static route metric is always 0

5. AD of 255

   • Routes with AD 255 are NOT installed in the routing table

   • Router considers the source untrustworthy

6. EIGRP vs OSPF Selection

   • When both learn the same route, EIGRP wins (AD 90 < AD 110)

   • Don't compare their metrics—they're incompatible

7. BGP Types

   • eBGP (external) AD = 20

   • iBGP (internal) AD = 200

   • These are very different values!

AD Values Mnemonic

"Direct Static External EIGRP, I'm Old, IS Really Important, External Internal"

• Direct = 0

• Static = 1

• External BGP = 20

• EIGRP = 90

• IGRP = 100 (Old = OSPF = 110)

• IS-IS = 115

• RIP = 120 (Important)

• External EIGRP = 170

• Internal BGP = 200

Protocol Algorithm Memory

• RIP & EIGRP = "REmote Distance" (Distance Vector)

• OSPF & IS-IS = "Open Intermediate Link" (Link State)

• BGP = "Border Path" (Path Vector)

Lower is Better Rule

• Lower AD = More trustworthy

• Lower Metric = Better route

• Lower Root Cost = Better path (STP comparison)

Distance Vector = "Routing by Rumor"

• Router only knows what neighbors TELL it

• Like gossip—information passed along without verification

Example 1: Route Selection with Different Protocols

Routes to 10.1.1.0/24:

- RIP route: metric 5, AD 120

- OSPF route: metric 10, AD 110

Winner: OSPF route (lower AD wins first)

Example 2: ECMP Load Balancing

Router# show ip route

O 192.168.4.0/24 [110/3] via 10.0.13.2

[110/3] via 10.0.12.2

Both routes added because:

- Same protocol (OSPF)

- Same AD (110)

- Same metric (3)

- Same destination

Example 3: Floating Static Route Configuration

Router(config)# ip route 192.168.4.0 255.255.255.0 10.0.12.2 100

This creates a static route with AD 100 (instead of default 1)

- Will be inactive if EIGRP route (AD 90) exists

- Becomes active if EIGRP route is lost

Example 4: Reading Route Table Output

S 10.0.24.0/30 [1/0] via 10.0.12.2

^ ^ ^

| | |-- Metric (0 for static)

| |-- Administrative Distance (1)

|-- Code (S = Static)

A router learns routes to 172.16.0.0/16 from the following sources:

• OSPF with metric 50

• EIGRP with metric 2816000

• RIP with metric 3

  
  

Which route will be installed in the routing table?

A) OSPF route

B) EIGRP route

C) RIP route

D) All routes will be installed

Answer: B

Explanation: When routes are learned from different routing protocols, Administrative Distance determines which route is installed. EIGRP has AD of 90, OSPF has AD of 110, and RIP has AD of 120. EIGRP has the lowest AD, so its route is preferred regardless of metric values.

Which of the following correctly describes the difference between IGP and EGP?

A) IGPs use metrics while EGPs do not

B) IGPs route within an autonomous system; EGPs route between autonomous systems

C) IGPs are faster than EGPs

D) IGPs use link-state algorithms; EGPs use distance-vector algorithms

Answer: B

Explanation: IGPs (Interior Gateway Protocols) like OSPF and EIGRP are designed to route within a single organization (autonomous system). EGPs (Exterior Gateway Protocols) like BGP are designed to route between different autonomous systems, such as between ISPs.

A network administrator configures the following command:

Router(config)# ip route 10.0.0.0 255.0.0.0 192.168.1.1 95

What is the purpose of the value "95" in this command?

A) It sets the metric of the static route

B) It sets the administrative distance of the static route

C) It sets the hop count to the destination

D) It sets the bandwidth calculation for the route

Answer: B

Explanation: The optional number at the end of a static route command sets the administrative distance. By setting AD to 95, this route becomes less preferred than EIGRP routes (AD 90) but more preferred than OSPF routes (AD 110). This creates a "floating static route."

Which routing protocol uses "routing by rumor" to share network information?

A) OSPF

B) IS-IS

C) BGP

D) EIGRP

Answer: D

Explanation: EIGRP is a distance vector protocol, which uses "routing by rumor." Distance vector protocols share their routing tables with neighbors, who then share with their neighbors. Routers only know what their neighbors tell them. OSPF and IS-IS are link-state protocols that build complete network maps.

Examine the following routing table entry:

O 192.168.10.0/24 [110/65] via 10.1.1.2, 00:05:32, GigabitEthernet0/1

What does the value "65" represent?

A) Administrative distance

B) Metric (cost)

C) Hop count

D) Bandwidth in Mbps

Answer: B

Explanation: In the format [AD/Metric], the first number (110) is the administrative distance (OSPF), and the second number (65) is the metric. For OSPF, this metric represents the cost calculated based on interface bandwidth.

R1 has the following routes in its routing table:

O 10.10.10.0/24 [110/20] via 192.168.1.2

O 10.10.10.0/24 [110/20] via 192.168.2.2

What will happen when R1 receives a packet destined for 10.10.10.50?

A) The packet will be dropped

B) The packet will be sent via 192.168.1.2 only

C) The packet will be sent via 192.168.2.2 only

D) Traffic will be load-balanced across both paths

Answer: D

Explanation: Both routes have the same AD (110) and metric (20) to the same destination. This is Equal Cost Multi-Path (ECMP) load balancing. The router will distribute traffic across both paths.

Which of the following administrative distance values is correct?

A) OSPF = 100

B) RIP = 110

C) Static = 0

D) EIGRP = 90

Answer: D

Explanation: EIGRP has an AD of 90. OSPF = 110 (not 100), RIP = 120 (not 110), and Static routes = 1 (not 0). Directly connected routes have AD of 0.

A router is running both OSPF and RIP. The router learns a route to 172.16.0.0/16 via OSPF with a metric of 100. The router also learns the same route via RIP with a metric of 2. Which statement is true?

A) The RIP route is preferred because it has a lower metric

B) The OSPF route is preferred because it has a lower administrative distance

C) Both routes will be installed for load balancing

D) Neither route will be installed because they conflict

Answer: B

Explanation: When comparing routes from different protocols, AD is used first. OSPF AD (110) is lower than RIP AD (120), so the OSPF route is preferred. Metrics cannot be compared between different protocols.

Which metric does RIP use, and what is a significant limitation of this metric?

A) Cost; it doesn't consider delay

B) Hop count; it doesn't consider link bandwidth

C) Bandwidth and delay; it's too complex

D) Administrative distance; it's not configurable

Answer: B

Explanation: RIP uses hop count as its metric, counting each router as one hop. A significant limitation is that all links count equally—a 10 Mbps link is 1 hop, same as a 10 Gbps link. This can result in suboptimal path selection.

What administrative distance value makes a route unusable?

A) 0

B) 1

C) 200

D) 255

Answer: D

Explanation: An AD of 255 indicates the route is not trustworthy and will NOT be installed in the routing table. AD of 0 is for connected routes, 1 is for static routes, and 200 is for iBGP.

Configuring a routing protocol on routers that automatically discovers, learns, and maintains routes to destination networks without manual configuration.

Manually configuring routes to destinations using the ip route command. Routes don't change unless manually modified.

A single organization or administrative domain, such as a company, that uses its own routing policies.

Interior Gateway Protocol - A routing protocol used to share routes WITHIN a single autonomous system. Examples: OSPF, EIGRP, RIP, IS-IS

Exterior Gateway Protocol - A routing protocol used to share routes BETWEEN different autonomous systems. BGP is the only EGP in modern use.

A neighbor relationship formed between routers to exchange routing information. Also called "neighbor relationship" or "neighborship."

A route to a network or subnet with a mask length less than /32. Example: 192.168.1.0/24

A route to a single specific host, using a /32 mask. Example: 192.168.1.1/32

RIP (Routing Information Protocol) and EIGRP (Enhanced Interior Gateway Routing Protocol)

OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System)

BGP (Border Gateway Protocol) - the only EGP in use today

A characteristic of Distance Vector protocols where routers only know routes their neighbors tell them about, without seeing the complete network topology.

They send their known destination networks and their metrics to reach those networks to directly connected neighbors.

Each router advertises information about its interfaces to neighbors. All routers build a complete "connectivity map" of the network and independently calculate best routes.

Link State protocols use more resources because they maintain a complete map of the network, but they converge faster.

A value used to determine the best route to a destination within the SAME routing protocol. Lower metric = better route.

Hop Count - Each router in the path counts as one hop. Limitation: doesn't consider link bandwidth.

Bandwidth and Delay (by default). Uses slowest bandwidth in path + total delay of all links.

Cost - Calculated based on interface bandwidth. Total cost = sum of all link costs in the path.

Cost - Default of 10 per link (not automatically based on bandwidth). Functions like hop count without configuration.

0 - Static routes don't use metric calculations.

A value indicating the trustworthiness of a route source. Used to compare routes from DIFFERENT routing protocols. Lower AD = more preferred.

0

1

20

90

100

110

115

120

170

200

The route is considered untrustworthy and will NOT be installed in the routing table.

EIGRP (90) has lower AD than OSPF (110), so EIGRP routes are preferred.

IS-IS (115) has lower AD than RIP (120), so IS-IS routes are preferred.

Equal Cost Multi-Path - When multiple routes to the same destination have equal metrics, all routes are added to the routing table and traffic is load-balanced.

1) Same routing protocol 2) Same metric 3) Exact same destination (network address AND prefix length)

Yes - Configure multiple static routes to the same destination, and traffic will be load-balanced.

A static route configured with a higher AD than dynamic routes, making it inactive unless the dynamic route is lost.

Add the AD value at the end of the ip route command:ip route 192.168.1.0 255.255.255.0 10.0.0.1 95

To create a backup route that only becomes active when the primary dynamic route fails.

Administrative Distance (110 = OSPF)

Metric (for OSPF, this is the cost)

OSPF-learned route

Static route

Directly Connected route

Local route (host route to the router's own interface IP)

When routes are learned from the SAME routing protocol

When routes are learned from DIFFERENT routing protocols or sources

1) Compare AD first (lower wins) 2) If same AD, compare metric (lower wins) 3) If same metric, ECMP load balancing

Hop count treats all links equally - a 10 Mbps link = 1 hop, same as a 10 Gbps link, leading to potentially suboptimal path selection.

OSPF - but you still need basic understanding of other protocols to compare and contrast them.

25%

1. Distance Vector: RIP, EIGRP

2. Link State: OSPF, IS-IS

3. Path Vector: BGP

OSPF - it's the only dynamic routing protocol specifically listed in exam topics and will be covered in depth.