OSPF, EIGRP, and BGP Basics at Network+ Depth
Key Takeaways
- OSPF is a link-state interior gateway protocol that uses areas, neighbor adjacencies, and bandwidth-based cost.
- EIGRP is an advanced distance-vector IGP known for fast convergence via feasible successors and a composite metric.
- BGP is the path-vector exterior gateway protocol used between autonomous systems and at internet edges over TCP 179.
- Network+ tests purpose, relationship formation, and the clues that explain missing routes - not full configuration.
- Area mismatches, timer mismatches, authentication mismatches, passive interfaces, and route filters all suppress expected routes.
Why Dynamic Routing Protocols Exist
Routing protocols automate route exchange so administrators do not hand-configure every path. Network+ N10-009 expects you to recognize each protocol's purpose, how it forms relationships, and how to read a symptom - not to write vendor configuration. The three you must distinguish are OSPF, EIGRP, and BGP.
| Protocol | Type | Scope | Default AD | Selection idea | Common use |
|---|---|---|---|---|---|
| OSPF | Link-state IGP | Inside one organization | 110 | Cost (bandwidth-based) | Enterprise LAN/WAN |
| EIGRP | Advanced distance-vector IGP | Inside one organization | 90 (internal) | Composite metric | Cisco-heavy enterprises |
| BGP | Path-vector EGP | Between autonomous systems | 20 eBGP / 200 iBGP | Path attributes + policy | Internet and provider edge |
An interior gateway protocol (IGP) runs within a single administrative domain. An exterior gateway protocol (EGP) runs between separate domains. BGP is the dominant EGP, exchanging routes between autonomous systems (AS) - independently administered networks each identified by an AS number.
OSPF Essentials
OSPF routers form neighbor adjacencies, flood link-state advertisements (LSAs), build an identical topology database, then independently run the Dijkstra shortest-path algorithm to pick best paths.
| OSPF concept | Meaning |
|---|---|
| Area | Logical grouping of routers and links |
| Area 0 (backbone) | Central area all other areas must touch |
| Neighbor adjacency | Relationship between compatible OSPF routers |
| Cost | Metric, commonly reference-bandwidth / interface-bandwidth |
| DR/BDR | Designated and backup designated routers on multiaccess links |
| Router ID | 32-bit ID identifying the OSPF router |
For two OSPF routers on the same link to become neighbors, several values must match. The exam loves these failure causes:
- Area number mismatch - interfaces must share the same area.
- Subnet mask / network mismatch - the link addresses must agree.
- Hello/Dead timer mismatch - default 10s hello, 40s dead on broadcast links.
- Authentication mismatch - key or type differs.
- Passive interface - advertises the subnet but refuses to form an adjacency.
- MTU mismatch - blocks adjacency at the exchange state.
EIGRP Essentials
EIGRP exchanges routing information with neighbors and uses the DUAL algorithm to converge fast when a route fails.
| EIGRP concept | Meaning |
|---|---|
| Autonomous system number | Must match between neighbors |
| Neighbor table | Tracks adjacent EIGRP routers |
| Topology table | All learned routes and alternatives |
| Successor | Current best (lowest-metric) route |
| Feasible successor | Pre-qualified backup, installed instantly on failure |
| Composite metric | Derived from bandwidth and delay by default |
At Network+ depth, know EIGRP is an IGP whose neighbors require a matching AS number and compatible settings. A feasible successor is why EIGRP can fail over almost instantly - the backup is already verified loop-free, so no recalculation is needed. If an expected EIGRP route is missing while the interface is up, check the AS number, the advertised networks, whether the interface is passive, and any route filters or distribute lists.
BGP Essentials
BGP is policy-based routing between autonomous systems, common at internet edges and in multihomed organizations.
| BGP concept | Meaning |
|---|---|
| Autonomous system | Network under one routing policy |
| eBGP | BGP between different ASes |
| iBGP | BGP within the same AS |
| AS path | List of ASes a route traversed (loop prevention) |
| Prefix advertisement | A network announced to a peer |
| TCP 179 | The port BGP peers use to establish a session |
BGP is not chosen for fast LAN convergence - it is chosen for internet-scale routing and granular policy control.
Symptom-to-Cause Map and PBQ Guidance
| Symptom | First place to look |
|---|---|
| OSPF neighbors stuck or absent | Area, timers, authentication, subnet, network type, MTU |
| EIGRP route missing but interface up | AS number, passive interface, network statement, filter |
| BGP peer down (Idle/Active) | Remote AS number, neighbor IP reachability, TCP 179, auth |
| Route learned but path is unexpected | Metric, route policy, prefix length, administrative distance |
| Route on one router but not another | Redistribution, filtering, summarization, area design |
Match protocol to requirement (PBQ-style)
| Requirement | Best match |
|---|---|
| Dynamic routing across a multi-router enterprise LAN | OSPF |
| A Cisco-heavy enterprise already running EIGRP | EIGRP |
| Exchange routes with an ISP using an AS number | BGP |
| A small branch with a single path to HQ | Static or default route |
If a PBQ shows a neighbor table, solve from compatibility and reachability first. Do not start adjusting metrics when the neighbor has not even formed.
Common traps: using BGP for every dynamic need (it is for interdomain/policy routing); treating OSPF cost as hop count (cost is bandwidth-derived, RIP uses hops); ignoring a passive interface (it advertises but will not peer); and changing VLANs to fix a BGP peer before confirming IP reachability and TCP 179.
Which routing protocol is the path-vector protocol used to exchange routes between autonomous systems across the internet?
Two OSPF routers on the same link never become neighbors. Which cause is most plausible?
An EIGRP route is missing on a router even though the connecting interface shows up/up. Which value, if mismatched, would most directly prevent the neighbor relationship?