Cisco Routing Protocols OSPF EIGRP BGP for Exam Preparation

Dynamic routing is the nervous system of modern networks, and mastering OSPF, EIGRP, and BGP is non-negotiable for any Cisco certification candidate. These protocols represent the core of enterprise and service provider routing, and exams rigorously test your ability to distinguish their operations, configure them correctly, and troubleshoot their failures. A deep conceptual understanding, paired with practical verification skills, is what separates passing from excelling.

OSPF: A Link-State Protocol Built on Areas

Open Shortest Path First (OSPF) is a link-state, interior gateway protocol (IGP) that uses the Shortest Path First (SPF) algorithm (Dijkstra’s algorithm) to build a loop-free topology of the network. Unlike distance-vector protocols, OSPF routers exchange Link-State Advertisements (LSAs) to build a complete, identical link-state database (LSDB). The SPF algorithm then runs on this database to calculate the shortest path to each network, populating the routing table.

A critical scalability feature is the use of areas. All OSPF networks must contain Area 0, the backbone area. Other non-backbone areas must connect directly to Area 0. This hierarchical design confines topology changes, limiting the scope of SPF recalculations. Routers have different roles: Internal Routers have all interfaces in one area, Area Border Routers (ABRs) connect to multiple areas, and Autonomous System Boundary Routers (ASBRs) inject external routes (e.g., from EIGRP or BGP) into OSPF.

Neighbor adjacencies are foundational. OSPF neighbors progress through several states: Down, Init, 2-Way, ExStart, Exchange, Loading, and finally Full. For two routers to become adjacent, their Hello packet parameters (Hello/Dead intervals, Area ID, Authentication, and Stub area flag) must match. On multi-access networks (like Ethernet), a Designated Router (DR) and Backup Designated Router (BDR) are elected to optimize LSA flooding; all other routers form full adjacencies only with the DR and BDR.

Exam Tip: Be prepared to identify LSA types. Type 1 (Router LSA) and Type 2 (Network LSA) exist within an area. Type 3 (Summary LSA) is generated by an ABR to advertise networks from one area to another. Type 4 (ASBR Summary LSA) and Type 5 (AS External LSA) are used for external routes. Knowing which router generates which LSA is a common exam topic.

EIGRP: An Advanced Hybrid Protocol and DUAL

Enhanced Interior Gateway Routing Protocol (EIGRP) is Cisco’s proprietary advanced distance-vector protocol, often called a hybrid protocol. It uses Reliable Transport Protocol (RTP) for guaranteed delivery of certain packets and establishes neighbor relationships via Hello packets. Its core intelligence lies in the Diffusing Update Algorithm (DUAL), which provides rapid convergence and ensures a loop-free path at every instant.

The EIGRP metric is composite, calculated by default using bandwidth and delay (it can also consider load, reliability, and MTU). The formula is: $$Metric=[K1\ast Bandwidth+(K2\ast Bandwidth)/(256-Load)+K3\ast Delay]\ast [K5/(Reliability+K4)]$$ With default K-values (K1=1, K3=1, others=0), this simplifies to: $$Metric=(10^7/\text{min bandwidth})+(\text{sum of delays})\ast 256$$ You must know how to calculate this metric from given bandwidth and delay values.

DUAL operates using several key concepts. The successor is the next-hop router with the feasible distance (FD)—the best metric to reach a network. A feasible successor (FS) is a backup path. For a route to be considered a feasible successor, its advertised distance (AD)—the metric reported by the neighbor to reach the destination—must be less than the current feasible distance. This is the feasibility condition. If a successor fails and a feasible successor exists, EIGRP converges instantly by installing the feasible successor route.

Exam Strategy: You will see questions asking which route becomes the successor or if a feasible successor exists. Always calculate the metric through each path, identify the FD, then check the AD of other paths against that FD to verify the feasibility condition.

BGP: The Path-Vector Protocol of the Internet

Border Gateway Protocol (BGP) is the standardized Exterior Gateway Protocol (EGP) of the Internet. It is a path-vector protocol that makes routing decisions based on path attributes, network policies, and rule-sets. Where IGPs like OSPF and EIGRP find the best path, BGP is often used to select the best route based on policy.

A fundamental distinction is between eBGP (external BGP, between different autonomous systems) and iBGP (internal BGP, within the same autonomous system). By default, eBGP peers have a Time To Live (TTL) of 1, while iBGP peers use a TTL of 255. A critical rule is the iBGP split-horizon rule: a route learned from an iBGP peer cannot be advertised to another iBGP peer. This is why iBGP typically requires a full mesh of connections or the use of route reflectors.

BGP’s route selection process is a strict, sequential list. The router evaluates paths and eliminates candidates at each step:

1. Highest Weight (Cisco proprietary, local to router).
2. Highest Local Preference (Local Pref, propagated within the AS).
3. Prefer locally originated routes.
4. Shortest AS_Path.
5. Lowest Origin code (IGP < EGP < INCOMPLETE).
6. Lowest Multi-Exit Discriminator (MED) (compared between routes from the same neighboring AS).
7. Prefer eBGP paths over iBGP paths.
8. Path with lowest IGP metric to the Next-Hop address.
9. (And further tie-breakers...).

Exam Focus: You must know this order. A question may give you two BGP routes with different attributes and ask which one will be installed in the routing table. Work down the list step-by-step.

Configuration, Verification, and Troubleshooting Core

For the exam, you must move beyond theory to practical application.

Basic Configuration & Verification:

• For OSPF: router ospf 1 followed by network 10.1.1.0 0.0.0.255 area 0. Verify with show ip ospf neighbor, show ip ospf database, and show ip route ospf.
• For EIGRP: router eigrp 100 followed by network 10.0.0.0. Verify with show ip eigrp neighbors, show ip eigrp topology (to see successors/feasible successors), and show ip route eigrp.
• For BGP: router bgp 65001, neighbor 192.168.1.1 remote-as 65002. Verify with show ip bgp summary, show ip bgp, and show ip route bgp.

Troubleshooting Adjacency/Neighbor Issues: This is a high-frequency exam scenario.

1. Layer 1/2 Connectivity: Always check physical links and interface status first (show interfaces).
2. Mismatched Parameters:

• OSPF: Check Hello/Dead timers, Area ID, and subnet masks. An interface in a passive state will not form adjacencies (passive-interface command).
• EIGRP: Check AS numbers and that the primary interface IP addresses are on the same subnet. K-value mismatch is a common trick question.
• BGP: Check AS numbers, correct neighbor IP address, and reachability (TTL for eBGP). Authentication mismatch will prevent any of these protocols from establishing a peer relationship.

1. Access Control Lists (ACLs): An ACL blocking the protocol port (OSPF=89, EIGRP=88, BGP=179) will prevent adjacency.

Common Pitfalls

1. Confusing EIGRP Feasibility Condition: A candidate route is not a feasible successor just because it has a higher metric. Its Advertised Distance must be strictly less than the current Feasible Distance. Missing this nuance leads to wrong answers.
2. Misapplying the BGP Decision Process: Students often jump to comparing AS_Path length first. You must start at Step 1 (Weight) and proceed in order. A route with a lower Local Pref will always be preferred over a route with a shorter AS_Path.
3. Overlooking Passive Interfaces: Configuring an interface as passive for a routing protocol stops hello packets from being sent. This is a silent killer for OSPF and EIGRP neighbor formation that is easily missed in simulation questions.
4. Mixing Up OSPF LSA Types and Their Originators: Confusing a Type 3 LSA (from an ABR) with a Type 5 LSA (from an ASBR) is common. Remember: Type 1 and 2 stay in an area. Type 3 goes between areas. Type 4 and 5 are for external routes.

Summary

• OSPF is a link-state IGP that uses areas for scalability. Success depends on forming neighbor adjacencies, building a synchronized LSDB, and running the SPF algorithm. Know your LSA types and router roles (ABR, ASBR).
• EIGRP is an advanced distance-vector protocol using DUAL for fast, loop-free convergence. The feasibility condition (AD < FD) is its secret to having backup paths without active querying. Be proficient in calculating its composite metric.
• BGP is a path-vector EGP driven by policy. Memorize its step-by-step route selection process. Critically understand the operational differences between eBGP and iBGP, especially the iBGP split-horizon rule.
• For all protocols, mastery of basic configuration, key show commands for verification, and a systematic approach to troubleshooting adjacency issues (connectivity, parameter mismatches, ACLs, passive interfaces) is essential for exam success.