Routing Protocols
Routing Protocols¶
Overview¶
This document covers the types and characteristics of dynamic routing protocols. You'll learn about the operating principles of major routing protocols such as RIP, OSPF, and BGP, and understand the appropriate environments for each protocol.
Difficulty: βββ Estimated Learning Time: 3-4 hours Prerequisites: 08_Routing_Basics.md
Table of Contents¶
- Classification of Routing Protocols
- Distance Vector vs Link State
- RIP (Routing Information Protocol)
- OSPF (Open Shortest Path First)
- BGP (Border Gateway Protocol)
- AS (Autonomous System)
- Practice Problems
- Next Steps
- References
1. Classification of Routing Protocols¶
1.1 IGP vs EGP¶
Routing protocols are classified into IGP and EGP based on their scope of use.
Routing Protocol Classification
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β Internet β
β β
β ββββββββββββββββββββ ββββββββββββββββββββ β
β β AS 100 β EGP β AS 200 β β
β β βββββββββββΊβ β β
β β (e.g., KT) β BGP β (e.g., SKT) β β
β β β β β β
β β ββββββ ββββββ β β ββββββ ββββββ β β
β β β R1 βββ R2 β β β β R3 βββ R4 β β β
β β ββββββ ββββββ β β ββββββ ββββββ β β
β β IGP β β IGP β β
β β (OSPF/RIP) β β (OSPF/RIP) β β
β ββββββββββββββββββββ ββββββββββββββββββββ β
β β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
| Aspect | IGP (Interior Gateway Protocol) | EGP (Exterior Gateway Protocol) |
|---|---|---|
| Scope | Within AS | Between ASes |
| Purpose | Optimal path in internal network | External network connection policy |
| Protocols | RIP, OSPF, EIGRP, IS-IS | BGP |
| Metrics | Hop count, bandwidth, delay, etc. | Path Attributes |
1.2 Classification by Algorithm¶
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β Routing Algorithm Classification β
βββββββββββββββββββββββββββββ¬ββββββββββββββββββββββββββββββββββββββ€
β Distance Vector β Link State β
β β β
βββββββββββββββββββββββββββββΌββββββββββββββββββββββββββββββββββββββ€
β β’ RIP β β’ OSPF β
β β’ EIGRP (Hybrid) β β’ IS-IS β
β β β
β Features: β Features: β
β - Exchange info with β - Understand entire topology β
β neighbors β - Dijkstra algorithm β
β - Bellman-Ford algorithm β - Complex, more resources β
β - Simple, fewer resources β - Fast convergence β
β - Slow convergence β β
βββββββββββββββββββββββββββββ΄ββββββββββββββββββββββββββββββββββββββ
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β Path Vector β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€
β β’ BGP β
β β
β Features: β
β - Transmits AS path information β
β - Policy-based routing β
β - Used in Internet backbone β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
1.3 Classful vs Classless¶
| Aspect | Classful | Classless |
|---|---|---|
| Subnet Mask | Not transmitted | Transmitted together |
| VLSM Support | X | O |
| CIDR Support | X | O |
| Protocols | RIPv1, IGRP | RIPv2, OSPF, EIGRP, BGP |
2. Distance Vector vs Link State¶
2.1 Distance Vector¶
"Trust the information my neighbors tell me"
Distance Vector Operation
Initial State:
R1 βββ R2 βββ R3 βββ R4
[A] [ ] [ ] [D]
Step 1: Advertise routing table to neighbors
R1: "Distance to A is 0" βββΊ R2
R4: "Distance to D is 0" βββΊ R3
Step 2: Received info + 1 (hop count)
R2: "Distance to A is 1" (via R1)
R3: "Distance to D is 1" (via R4)
Step 3: Advertise again to neighbors
R2: "Distance to A is 1" βββΊ R3
R3: "Distance to D is 1" βββΊ R2
Step 4: Final Convergence
R1: A(0), D(3)
R2: A(1), D(2)
R3: A(2), D(1)
R4: A(3), D(0)
Advantages: - Simple implementation - Low CPU/memory usage - Suitable for small networks
Disadvantages: - Slow convergence - Potential for routing loops - Hop count limitation (RIP: 15)
2.2 Link State¶
"I understand the entire network myself"
Link State Operation
Network Topology:
10 5 15
R1 ββββββ R2 ββββββ R3 ββββββ R4
β β
ββββββββββββββββββββββββββββββ
20
Step 1: Each router generates LSA (Link State Advertisement)
R1's LSA: "R1 connected: R2(cost 10), R4(cost 20)"
R2's LSA: "R2 connected: R1(cost 10), R3(cost 5)"
...
Step 2: Flood LSAs to all routers
All routers maintain identical LSDB (Link State Database)
Step 3: Calculate shortest path with Dijkstra algorithm
R1 β R4 paths:
- R1 β R4 direct: cost 20
- R1 β R2 β R3 β R4: cost 10+5+15 = 30
Selected: Direct path (cost 20)
SPF (Shortest Path First) Tree Example:
R1's SPF Tree:
R1 (root)
/ \
(10) / \ (20)
/ \
R2 R4
|
(5) |
|
R3
|
(15) |
|
R4 (duplicate - higher cost, ignored)
Advantages: - Fast convergence - Accurate topology information - No routing loops - VLSM/CIDR support
Disadvantages: - High CPU/memory requirements - Complex configuration - Initial flooding consumes bandwidth
2.3 Comparison Summary¶
| Feature | Distance Vector | Link State |
|---|---|---|
| Information Shared | Routing table | Link state (topology) |
| Algorithm | Bellman-Ford | Dijkstra |
| Updates | Periodic (complete) | On change (changes only) |
| Convergence Speed | Slow | Fast |
| Resources | Low | High |
| Scalability | Low | High |
| Representative Protocols | RIP, EIGRP | OSPF, IS-IS |
3. RIP (Routing Information Protocol)¶
3.1 RIP Overview¶
RIP is the oldest distance vector routing protocol, simple but limited.
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β RIP Features β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€
β β’ Metric: Hop Count β
β β’ Maximum Hops: 15 (16 = unreachable) β
β β’ Update Interval: 30 seconds β
β β’ Administrative Distance (AD): 120 β
β β’ Port: UDP 520 β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
3.2 RIP Version Comparison¶
| Feature | RIPv1 | RIPv2 |
|---|---|---|
| Class | Classful | Classless |
| Subnet Mask | Not transmitted | Transmitted |
| VLSM | Not supported | Supported |
| CIDR | Not supported | Supported |
| Authentication | None | Supported (MD5) |
| Transmission | Broadcast | Multicast (224.0.0.9) |
3.3 RIP Operation Process¶
RIP Routing Update Example
Network Configuration:
Network A Network B Network C
10.0.0.0/24 10.1.0.0/24 10.2.0.0/24
β β β
ββββ΄βββ ββββ΄βββ ββββ΄βββ
β R1 ββββββββββββββ R2 ββββββββββββββ R3 β
βββββββ βββββββ βββββββ
Initial Routing Table:
R1:
ββββββββββββββββββ¬βββββββ¬ββββββββββββ
β Network β Hops β Next Hop β
ββββββββββββββββββΌβββββββΌββββββββββββ€
β 10.0.0.0/24 β 0 β Direct β
ββββββββββββββββββ΄βββββββ΄ββββββββββββ
After 30 seconds (R2 receives updates from R1 and R3):
R2:
ββββββββββββββββββ¬βββββββ¬ββββββββββββ
β Network β Hops β Next Hop β
ββββββββββββββββββΌβββββββΌββββββββββββ€
β 10.0.0.0/24 β 1 β R1 β
β 10.1.0.0/24 β 0 β Direct β
β 10.2.0.0/24 β 1 β R3 β
ββββββββββββββββββ΄βββββββ΄ββββββββββββ
After 60 seconds (convergence complete):
R1:
ββββββββββββββββββ¬βββββββ¬ββββββββββββ
β Network β Hops β Next Hop β
ββββββββββββββββββΌβββββββΌββββββββββββ€
β 10.0.0.0/24 β 0 β Direct β
β 10.1.0.0/24 β 1 β R2 β
β 10.2.0.0/24 β 2 β R2 β
ββββββββββββββββββ΄βββββββ΄ββββββββββββ
3.4 RIP Timers¶
| Timer | Value | Description |
|---|---|---|
| Update | 30s | Routing update transmission interval |
| Invalid | 180s | Time until route marked invalid |
| Holddown | 180s | Time to prevent route changes |
| Flush | 240s | Time to delete from routing table |
3.5 RIP Loop Prevention Mechanisms¶
1. Split Horizon
- Don't advertise route back through interface it was learned
R1 ββββββββ R2
β
"Learned 10.0.0.0 from R1,
won't advertise 10.0.0.0 back to R1"
2. Route Poisoning
- Advertise down routes with metric 16
R1: "10.0.0.0 is down"
R1 β R2: "10.0.0.0 metric=16 (unreachable)"
3. Holddown Timer
- After route goes down, refuse new routes for period
- Prevents propagation of incorrect information
4. Triggered Update
- Send update immediately when change occurs
- Fast convergence without waiting 30 seconds
3.6 RIP Configuration Examples¶
Cisco Router:
Router(config)# router rip
Router(config-router)# version 2
Router(config-router)# network 10.0.0.0
Router(config-router)# network 192.168.1.0
Router(config-router)# no auto-summary
Linux (Quagga/FRR):
router rip
version 2
network 10.0.0.0/8
network 192.168.1.0/24
no auto-summary
4. OSPF (Open Shortest Path First)¶
4.1 OSPF Overview¶
OSPF is the most widely used link state routing protocol.
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β OSPF Features β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€
β β’ Metric: Cost = Reference Bandwidth / Interface Bandwidth β
β β’ Reference Bandwidth: Default 100 Mbps β
β β’ Administrative Distance (AD): 110 β
β β’ Protocol: IP Protocol 89 β
β β’ Multicast: 224.0.0.5 (AllSPFRouters) β
β 224.0.0.6 (AllDRouters) β
β β’ Area-based hierarchical structure β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
4.2 OSPF Cost Calculation¶
OSPF Cost = Reference Bandwidth (100 Mbps) / Interface Bandwidth
βββββββββββββββββββββββ¬ββββββββββββββββ¬βββββββββββββ
β Interface Type β Bandwidth β OSPF Cost β
βββββββββββββββββββββββΌββββββββββββββββΌβββββββββββββ€
β Serial (T1) β 1.544 Mbps β 64 β
β Ethernet β 10 Mbps β 10 β
β Fast Ethernet β 100 Mbps β 1 β
β Gigabit Ethernet β 1000 Mbps β 1 (default)β
β 10 Gigabit Ethernet β 10000 Mbps β 1 (default)β
βββββββββββββββββββββββ΄ββββββββββββββββ΄βββββββββββββ
β» For Gig+, reference bandwidth adjustment needed (e.g., 10000 Mbps)
4.3 OSPF Areas¶
OSPF Area Structure
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β β
β Area 0 β
β (Backbone) β
β β
β βββββββββββββββββββββββββββββββ β
β β β β
β βββββββββ΄βββ βββββββββββ ββββββββ΄ββββββββ β
β β ABR β β ABR β β ABR β β
β ββββββ¬ββββββ ββββββ¬βββββ ββββββββ¬ββββββββ β
β β β β β
β ββββββ΄βββββ ββββββ΄βββββ ββββββββ΄βββββββ β
β β Area 1 β β Area 2 β β Area 3 β β
β β β β β β β β
β β R1ββR2 β β R3ββR4 β β R5ββR6 β β
β β β β β β β β
β βββββββββββ βββββββββββ βββββββββββββββ β
β β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
ABR: Area Border Router
Purpose of Areas: - Reduce LSDB size - Limit SPF calculation scope - Reduce routing updates - Improve network stability
Area Types:
| Area Type | Description |
|---|---|
| Backbone (Area 0) | Central area, connects all areas |
| Standard Area | Regular area |
| Stub Area | Blocks external routes, uses default route |
| Totally Stubby | Blocks external + other area routes |
| NSSA | Allows limited external routes |
4.4 OSPF Router Types¶
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β OSPF Router Roles β
ββββββββββββββββββββ¬βββββββββββββββββββββββββββββββββββββββββββββββ€
β Internal Router β Operates within single area β
β β β
β ABR β Area Border Router β
β β Connected to multiple areas, exchanges β
β β routing info between areas β
β β β
β ASBR β AS Boundary Router β
β β Connected to external routing domain β
β β β
β Backbone Router β Router in Area 0 β
β β β
β DR β Designated Router β
β β Representative router on multi-access networkβ
β β β
β BDR β Backup Designated Router β
β β DR backup β
ββββββββββββββββββββ΄βββββββββββββββββββββββββββββββββββββββββββββββ
4.5 OSPF Packet Types¶
| Type | Name | Description |
|---|---|---|
| 1 | Hello | Neighbor discovery and relationship maintenance |
| 2 | DBD (Database Description) | Exchange LSDB summary information |
| 3 | LSR (Link State Request) | Request specific LSA |
| 4 | LSU (Link State Update) | Transmit LSA |
| 5 | LSAck | LSA receipt acknowledgment |
4.6 OSPF Neighbor States¶
OSPF Neighbor Relationship Establishment
Down β Init β 2-Way β ExStart β Exchange β Loading β Full
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β Down : Initial state, no Hello received β
β Init : Hello received, bidirectional not confirmed β
β 2-Way : Bidirectional communication confirmed (DR/BDR elect)β
β ExStart : Master/Slave decision, sequence number exchange β
β Exchange : Exchange LSDB summary via DBD packets β
β Loading : Request and receive missing LSAs via LSR/LSU β
β Full : LSDB synchronized, neighbor relationship establishedβ
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
4.7 OSPF Configuration Examples¶
Cisco Router:
Router(config)# router ospf 1
Router(config-router)# network 10.0.0.0 0.255.255.255 area 0
Router(config-router)# network 192.168.1.0 0.0.0.255 area 1
Linux (FRR):
router ospf
network 10.0.0.0/8 area 0.0.0.0
network 192.168.1.0/24 area 0.0.0.1
5. BGP (Border Gateway Protocol)¶
5.1 BGP Overview¶
BGP is the path vector protocol used for inter-AS routing on the Internet backbone.
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β BGP Features β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€
β β’ Type: Path Vector protocol β
β β’ Purpose: Inter-AS routing (EGP) β
β β’ Port: TCP 179 β
β β’ Administrative Distance: eBGP=20, iBGP=200 β
β β’ Path Selection: Policy-based (Path Attributes) β
β β’ Current Version: BGP-4 β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
5.2 eBGP vs iBGP¶
eBGP and iBGP
ββββββββββββββββββββββββ ββββββββββββββββββββββββ
β AS 100 β β AS 200 β
β β β β
β R1 βββββ iBGP βββββΊ R2 βββ eBGP βββΊ R3 βββ iBGP βββΊ R4
β β β β
β (Within same AS) β β (Within same AS) β
ββββββββββββββββββββββββ ββββββββββββββββββββββββ
β β
βββββββββββββ eBGP βββββββββββββββββββ
(Between different ASes)
eBGP (External BGP):
- Connection between different ASes
- AD: 20 (high trust)
- Typically directly connected
iBGP (Internal BGP):
- Connection between BGP routers in same AS
- AD: 200 (low trust)
- Requires full mesh or Route Reflector
5.3 BGP Path Attributes¶
| Attribute | Type | Description |
|---|---|---|
| AS_PATH | Well-known Mandatory | List of ASes path traversed |
| NEXT_HOP | Well-known Mandatory | Next hop IP address |
| ORIGIN | Well-known Mandatory | Route origin (IGP/EGP/Incomplete) |
| LOCAL_PREF | Well-known Discretionary | Local preference (iBGP use) |
| MED | Optional Non-transitive | Multi-Exit Discriminator |
| COMMUNITY | Optional Transitive | Route grouping tag |
5.4 BGP Path Selection Process¶
BGP Best Path Selection Algorithm
1. Weight (higher preferred) - Cisco proprietary
2. LOCAL_PREF (higher preferred)
3. Locally Originated (prefer self-generated routes)
4. AS_PATH (shorter preferred)
5. ORIGIN (IGP > EGP > Incomplete)
6. MED (lower preferred)
7. eBGP over iBGP
8. IGP metric (cost to next hop)
9. Oldest route
10. Router ID (lower preferred)
11. Neighbor IP (lower preferred)
Example:
ββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β Route A: AS_PATH = 100 200 300, LOCAL_PREF = 100 β
β Route B: AS_PATH = 400 500, LOCAL_PREF = 200 β
β β
β Selected: Route B (higher LOCAL_PREF) β
ββββββββββββββββββββββββββββββββββββββββββββββββββββββ
5.5 BGP Message Types¶
| Message | Description |
|---|---|
| OPEN | BGP session establishment, parameter exchange |
| UPDATE | Advertise/withdraw route information |
| KEEPALIVE | Connection maintenance check (60 second interval) |
| NOTIFICATION | Error notification, session termination |
5.6 BGP States¶
BGP State Transition
Idle β Connect β OpenSent β OpenConfirm β Established
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β Idle : Initial state, start TCP connection β
β Connect : Waiting for TCP connection β
β Active : TCP connection retry (if Connect fails) β
β OpenSent : OPEN message sent, waiting for response β
β OpenConfirm : OPEN message received, waiting for KEEPALIVE β
β Established : BGP session established, route exchange starts β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
5.7 BGP Configuration Examples¶
Cisco Router (eBGP):
Router(config)# router bgp 100
Router(config-router)# neighbor 203.0.113.1 remote-as 200
Router(config-router)# network 10.0.0.0 mask 255.0.0.0
Linux (FRR):
router bgp 100
neighbor 203.0.113.1 remote-as 200
address-family ipv4 unicast
network 10.0.0.0/8
exit-address-family
6. AS (Autonomous System)¶
6.1 What is AS?¶
An AS (Autonomous System) is a collection of networks under a single administrative domain with common routing policy.
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β AS (Autonomous System) β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ€
β β
β Definition: β
β - Collection of IP networks and routers managed by single org β
β - Follows common routing policy β
β - Identified by unique AS number (ASN) β
β β
β Examples: β
β - ISPs (KT, SKT, LGU+) β
β - Large enterprises β
β - Cloud providers (AWS, GCP, Azure) β
β - Content providers (Netflix, Google) β
β β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
6.2 AS Numbers (ASN)¶
| Range | Type | Description |
|---|---|---|
| 1 - 64,495 | Public 2-byte | Usable on Internet |
| 64,496 - 64,511 | Documentation | For RFC document examples |
| 64,512 - 65,534 | Private 2-byte | Internal use |
| 65,535 | Reserved | Not usable |
| 1 - 4,199,999,999 | Public 4-byte | Extended AS numbers |
| 4,200,000,000 - 4,294,967,294 | Private 4-byte | Internal use |
Famous AS Number Examples:
| ASN | Organization |
|---|---|
| AS7018 | AT&T |
| AS15169 | |
| AS16509 | Amazon |
| AS32934 | |
| AS4766 | KT |
| AS9318 | SKT |
6.3 Inter-AS Relationships¶
AS Peering Relationships
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
β β
β 1. Transit: β
β - Large ISP provides Internet connectivity to smaller ISP β
β - Paid relationship β
β β
β βββββββββββ β
β β Tier 1 β β Payment β
β β ISP β β
β ββββββ¬βββββ β
β β Transit β
β ββββββ΄βββββ β
β β Tier 2 β β
β β ISP β β
β βββββββββββ β
β β
β 2. Peering: β
β - Free traffic exchange between peer ISPs β
β - Connection at IXP (Internet Exchange Point) β
β β
β βββββββββββ Free Exchange βββββββββββ β
β β AS A ββββββββββββββββββββΊβ AS B β β
β βββββββββββ (Peering) βββββββββββ β
β β
βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ
6.4 Internet Hierarchy¶
Internet AS Hierarchy
βββββββββββββββ
β Tier 1 β β Can reach entire Internet
β (Global ISP)β No need to buy transit
ββββββββ¬βββββββ
β
βββββββββββββββΌββββββββββββββ
β β β
ββββββ΄βββββ ββββββ΄βββββ ββββββ΄βββββ
β Tier 2 β β Tier 2 β β Tier 2 β β Regional ISP
β β β β β β Buy transit
ββββββ¬βββββ ββββββ¬βββββ ββββββ¬βββββ from Tier 1
β β β
ββββββ΄βββββ ββββββ΄βββββ ββββββ΄βββββ
β Tier 3 β β Tier 3 β β Tier 3 β β Local ISP
β β β β β β End-user
βββββββββββ βββββββββββ βββββββββββ services
Tier 1 Examples: AT&T, NTT, Cogent, Lumen
Tier 2 Examples: Large regional ISPs
Tier 3 Examples: Small regional ISPs, cable operators
7. Practice Problems¶
Problem 1: Protocol Feature Matching¶
Match the following features with their corresponding protocol.
Features:
a) Maximum hop count 15
b) Uses Dijkstra algorithm
c) Used for inter-AS routing
d) Sends complete routing table every 30 seconds
e) Area-based hierarchical structure
f) Uses TCP port 179
Protocols: RIP, OSPF, BGP
Problem 2: OSPF Cost Calculation¶
Calculate the total OSPF cost for the following path when reference bandwidth is 100 Mbps.
R1 ββ(FastEthernet)ββ R2 ββ(Serial T1)ββ R3 ββ(GigabitEthernet)ββ R4
100 Mbps 1.544 Mbps 1000 Mbps
Problem 3: BGP Path Selection¶
Which of the following two BGP routes will be selected?
Route A:
- AS_PATH: 100 200 300
- LOCAL_PREF: 150
- MED: 100
Route B:
- AS_PATH: 400 500
- LOCAL_PREF: 150
- MED: 50
Problem 4: Routing Protocol Selection¶
Select the appropriate routing protocol for each scenario and explain why.
a) Small office with 10 routers b) Large enterprise network with 500 routers c) Connection between two ISPs d) Branch office network with single path
Answers¶
Problem 1 Answers¶
- a) Maximum hop count 15 β RIP
- b) Dijkstra algorithm β OSPF
- c) Inter-AS routing β BGP
- d) 30 second table broadcast β RIP
- e) Area-based structure β OSPF
- f) TCP 179 β BGP
Problem 2 Answer¶
FastEthernet: 100 / 100 = 1
Serial T1: 100 / 1.544 = 64 (rounded)
GigabitEthernet: 100 / 1000 = 1 (minimum 1)
Total cost = 1 + 64 + 1 = 66
Problem 3 Answer¶
Route B selected
Analysis: 1. LOCAL_PREF: Both 150 (tie) 2. AS_PATH length: Route A = 3, Route B = 2 β Route B selected for shorter AS_PATH
(MED only used for comparing routes from same neighbor AS)
Problem 4 Answers¶
a) Small office (10 routers): RIP or Static Routing - Suitable for simple networks - Easy configuration and management
b) Large enterprise (500 routers): OSPF - Fast convergence - Scalability via area division - VLSM/CIDR support
c) ISP interconnection: BGP - Standard for inter-AS routing - Policy-based path control
d) Single path branch: Static Routing (+ default route) - Dynamic routing unnecessary - Resource saving
8. Next Steps¶
Once you understand routing protocols, move on to the transport layer.
Next Lesson¶
- 10_TCP_Protocol.md - TCP 3-way handshake, flow control
Related Lessons¶
- 08_Routing_Basics.md - Basic routing concepts
- 15_Network_Security_Basics.md - Firewall, VPN
Recommended Practice¶
- Configure OSPF in GNS3/Packet Tracer
- Analyze routing tables with
show ip route - Check Internet routes with BGP Looking Glass
9. References¶
RFC Documents¶
- RFC 2453 - RIP Version 2
- RFC 2328 - OSPF Version 2
- RFC 4271 - BGP-4
- RFC 1930 - AS Operation Guidelines
Useful Tools¶
# BGP route lookup
# BGP Looking Glass: https://lg.he.net/
# AS information lookup
whois -h whois.radb.net AS15169
# Path tracing
traceroute -A google.com # Show AS numbers (Linux)
mtr google.com # Real-time trace
Learning Resources¶
- BGP Table Statistics
- Hurricane Electric BGP Toolkit
- PeeringDB
- Cisco Networking Academy
Simulators¶
- GNS3 - Uses actual router images
- Cisco Packet Tracer - Free learning simulator
- EVE-NG - Virtual network lab
Document Information - Last Updated: 2024 - Difficulty: βββ - Estimated Learning Time: 3-4 hours