OSPF, EIGRP, and BGP Basics at Network+ Depth
Key Takeaways
- OSPF is a link-state interior gateway protocol that uses areas, neighbor adjacencies, and cost.
- EIGRP is an advanced distance-vector interior gateway protocol associated with fast convergence and a composite metric.
- BGP is a path-vector exterior gateway protocol used between autonomous systems and commonly at internet edges.
- For Network+, focus on what each protocol is for, how it forms relationships, and what clues indicate neighbor or advertisement problems.
- Protocol mismatches, passive interfaces, authentication mismatches, area mismatches, and filtering can prevent routes from appearing.
OSPF, EIGRP, and BGP Basics
Routing protocols automate route exchange. Network+ expects you to recognize their purpose and troubleshoot common symptoms without diving into vendor-only configuration details.
Protocol Comparison
| Protocol | Type | Scope | Main selection idea | Common use |
|---|---|---|---|---|
| OSPF | Link-state IGP | Inside one organization | Cost, often based on bandwidth | Enterprise LAN and WAN routing |
| EIGRP | Advanced distance-vector IGP | Inside one organization | Composite metric | Enterprise networks, especially Cisco environments |
| BGP | Path-vector EGP | Between autonomous systems | Path attributes and policy | Internet routing and provider edge routing |
IGP means interior gateway protocol. It runs within an administrative domain. EGP means exterior gateway protocol. BGP is the major EGP used to exchange routes between autonomous systems.
OSPF Essentials
OSPF routers form neighbor adjacencies, share link-state information, and build a topology database. Each router then calculates best paths.
| OSPF concept | Meaning |
|---|---|
| Area | Logical grouping of routers and links |
| Area 0 | Backbone area used in multi-area OSPF designs |
| Neighbor adjacency | Relationship formed between compatible OSPF routers |
| Link-state advertisement | Information describing links and networks |
| Cost | OSPF metric, commonly derived from bandwidth |
| DR/BDR | Designated router roles used on some multiaccess networks |
Common OSPF failure clues include an area mismatch, subnet mask mismatch, hello/dead timer mismatch, authentication mismatch, passive interface, or an interface not included in the OSPF process.
EIGRP Essentials
EIGRP exchanges routing information with neighbors and can converge quickly when a route fails.
| EIGRP concept | Meaning |
|---|---|
| Autonomous system number | Must match for neighbors to form |
| Neighbor table | Tracks adjacent EIGRP routers |
| Topology table | Tracks learned routes and alternatives |
| Successor | Best route to a destination |
| Feasible successor | Backup route that can be used quickly if valid |
| Composite metric | Metric based on factors such as bandwidth and delay |
At Network+ depth, know that EIGRP is an IGP and that neighbor relationships require compatible settings. If an expected EIGRP route is missing, check the neighbor relationship, advertised networks, passive interfaces, and filtering.
BGP Essentials
BGP is used for policy-based routing between autonomous systems. It is common at internet edges, between service providers, and in organizations with multiple internet connections.
| BGP concept | Meaning |
|---|---|
| Autonomous system | Network under one administrative routing policy |
| eBGP | BGP between different autonomous systems |
| iBGP | BGP within the same autonomous system |
| AS path | List of autonomous systems a route has traversed |
| Prefix advertisement | A network announced to a BGP peer |
| Route policy | Rules controlling which routes are accepted or advertised |
BGP is not chosen because it is the fastest-converging LAN protocol. It is chosen because it supports internet-scale routing and policy.
Scenario Clues
| Symptom | Likely area to check |
|---|---|
| OSPF neighbors stuck or absent | Area, timers, authentication, interface, network type |
| EIGRP route missing but interface is up | AS number, passive interface, network statement, filter |
| BGP peer down | Remote AS, neighbor IP reachability, TCP 179, authentication |
| Route learned but traffic takes unexpected path | Metric, route policy, prefix length, administrative distance |
| Route appears on one router but not another | Redistribution, filtering, summarization, or area design |
PBQ Guidance
A routing PBQ may ask you to match protocols to use cases:
| Requirement | Best match |
|---|---|
| Dynamic routing inside a multi-router enterprise LAN | OSPF |
| Dynamic routing in a Cisco-heavy enterprise with EIGRP already deployed | EIGRP |
| Exchange routes with an ISP using an autonomous system number | BGP |
| Small branch with one path to headquarters | Static or default route |
If the PBQ shows a protocol neighbor table, solve it from compatibility and reachability. Do not start by changing metrics if the neighbor is not even formed.
Common Traps
| Trap | Better reasoning |
|---|---|
| Use BGP for every dynamic routing need | BGP is mainly for interdomain and policy-based routing |
| Treat OSPF cost as hop count | OSPF uses cost, commonly bandwidth-based |
| Ignore passive interfaces | A passive interface may advertise a network but not form neighbors |
| Change VLANs to fix a BGP peer before checking reachability | BGP peers need IP reachability and TCP 179 availability |
Which routing protocol is most associated with exchanging routes between autonomous systems on the internet?
Two OSPF routers on the same link do not become neighbors. Which issue is a likely cause?
Match each routing protocol to its usual role.
Match each item on the left with the correct item on the right