Learn Computer Networking PNETLab - Dynamic Routing Fundamentals & Distance Vector (EIGRP)

#Introduction

In episode 9 you configured routes manually. Imagine a network with hundreds of routers — manual configuration becomes impossible to manage. This is where dynamic routing comes in: protocols that learn routes automatically, propagate them between routers, and adapt when the topology changes.

Episode 10 opens with the classification of routing protocols, then moves into EIGRP — Cisco's distance vector protocol that converges quickly and is bandwidth efficient. You will build neighbor adjacency, configure EIGRP in PNETLab, read the topology table, and secure the network from rogue neighbors.

#Classification of Dynamic Routing Protocols

#Static vs Dynamic and IGP vs EGP

Routing is divided into static (manual, episode 9) and dynamic (automatic). Dynamic protocols are divided by scope: IGP (Interior Gateway Protocol) runs within a single organization, while EGP (Exterior Gateway Protocol) runs between organizations. Examples of IGP are RIP, EIGRP, and OSPF; an example of EGP is BGP, which will be discussed in episode 13.

#Distance Vector vs Link-State vs Path-Vector

• Distance Vector: a router only knows the direction and distance to a destination, then forwards its neighbors' routing tables (EIGRP, RIP).
• Link-State: a router maps the entire topology, then calculates the best path itself (OSPF).
• Path-Vector: a route carries the list of ASes it has traversed (BGP).

EIGRP fixes the weaknesses of classic distance vector with the DUAL algorithm, so routing loops do not occur and convergence is faster.

#EIGRP Introduction and Configuration

#The Autonomous System Concept and K-Values

EIGRP runs within a single Autonomous System (AS) — all routers in the same AS share a database. Configuration starts with router eigrp <as>. The EIGRP metric is calculated from K-values which by default only use bandwidth and delay; enabling K2, K3, K4, K5 is only for special cases.

#EIGRP Neighbor Adjacency and Hello Packets

Before exchanging routes, EIGRP routers build neighbor adjacency by sending Hello packets periodically. Hello is sent to the multicast address 224.0.0.10. If a Hello from another router is received with matching parameters (K-values and AS number), the two routers become neighbors and begin exchanging topology.

Build R1 and R2 in PNETLab with a /30 link, then configure EIGRP on both:

configure terminal
router eigrp 100
 router-id 1.1.1.1
 network 192.168.10.0 0.0.0.255
 network 10.0.0.0 0.0.0.3
 no auto-summary
exit

network 192.168.10.0 0.0.0.255 advertises the subnet to EIGRP, and no auto-summary prevents routes from being simplified to their default class. Do not forget to configure R2's side with router eigrp 100 and the appropriate network statements.

#Understanding the EIGRP Topology Table

#Successor and Feasible Successor

Every destination has two paths in the EIGRP topology table:

• Successor: the main path with the best metric, used in the routing table.
• Feasible Successor: the loop-free backup path ready to be used when the successor fails.

Show both:

R1# show ip eigrp topology
P 192.168.20.0/24, 2 successors, FD is 2816
        via 10.0.0.2 (2816/2176), GigabitEthernet0/1
        via 10.0.0.6 (3072/2816), GigabitEthernet0/2

show ip eigrp topology displays all paths to a destination. The first path is the successor, the second is the feasible successor ready to take over without waiting for a recalculation — that is EIGRP's convergence speed.

#Preventing Rogue Neighbors

#passive-interface

Without protection, EIGRP tries to build adjacency on every interface listed in a network statement — including interfaces facing end users. A rogue neighbor router could learn the entire internal topology. Use passive-interface for interfaces that should not form adjacency:

configure terminal
router eigrp 100
 passive-interface g0/0
exit

passive-interface g0/0 keeps EIGRP advertising the network on that interface but stops sending Hello — adjacency will never form on the LAN side.

#Verifying EIGRP

Check the neighbor status and learned routes:

R1# show ip eigrp neighbors
R1# show ip route eigrp

show ip eigrp neighbors displays neighbors with established adjacency complete with hold time, while show ip route eigrp confirms that dynamic routes enter the routing table. If the adjacency column is empty, check the AS number and K-values on both sides.

#Closing

Key takeaways:

• Dynamic routing learns and propagates routes automatically.
• IGP for internal, EGP between organizations; EIGRP is an IGP.
• EIGRP is a distance vector protocol with the loop-free DUAL algorithm.
• The EIGRP metric is based on bandwidth and delay through K-values.
• The successor is the main path; the feasible successor is the backup path.
• passive-interface prevents rogue neighbors on LAN interfaces.

In the next episode, episode 11, we discuss single-area OSPF — a link-state protocol based on the Dijkstra algorithm, neighbor formation from Down to Full, DR and BDR election on multi-access networks, and full single-area OSPFv2 configuration verified with show ip ospf neighbor.