Routing Protocols
A router picks the longest prefix match first, then the lowest administrative distance, then the lowest metric. Most routing questions are that sequence applied to a table.
Routing Protocols · firsttry.app/cheatsheets/routing-protocols · original reference written from published exam objectives. Not affiliated with any certification body.
Route selection, in order
| Step | Rule |
|---|---|
| 1 | Longest prefix match wins, whatever the source |
| 2 | Lowest administrative distance wins between sources |
| 3 | Lowest metric wins within one protocol |
| 4 | Equal-cost paths load balance |
Administrative distance
| Source | AD | Notes |
|---|---|---|
| Connected interface | 0 | Always preferred |
| Static route | 1 | Manually configured |
| EIGRP summary | 5 | |
| External BGP | 20 | |
| Internal EIGRP | 90 | Cisco proprietary |
| OSPF | 110 | Open standard |
| IS-IS | 115 | |
| RIP | 120 | Hop count, max 15 |
| External EIGRP | 170 | |
| Internal BGP | 200 | |
| Unknown / unreachable | 255 | Never installed |
Protocol comparison
| Protocol | Type | Metric | Scope |
|---|---|---|---|
| RIPv2 | Distance vector | Hop count | Small networks, 15 hop limit |
| EIGRP | Advanced distance vector | Bandwidth and delay | Cisco environments |
| OSPF | Link state | Cost, from bandwidth | Enterprise, multi-vendor |
| IS-IS | Link state | Cost | Service provider cores |
| BGP | Path vector | Path attributes | Between autonomous systems |
OSPF essentials
| Term | Detail |
|---|---|
| Area 0 | The backbone. Every other area must touch it |
| Router ID | Highest loopback, else highest active interface, else configured |
| Hello / dead timers | 10 / 40 seconds on broadcast networks |
| Cost formula | Reference bandwidth divided by interface bandwidth |
| DR and BDR | Elected on multi-access networks to limit adjacencies |
| Neighbour states | Down, Init, 2-Way, ExStart, Exchange, Loading, Full |
| Adjacency requirements | Matching area, subnet, timers, authentication, MTU |
Exams covered
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