What kind of routing protocol is EIGRP, and how is it different from RIP and OSPF?
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EIGRP is an advanced distance vector protocol. Like RIP, a router only knows what its neighbors tell it: each neighbor's distance to each prefix. Unlike RIP, it forms neighbor relationships with hellos, sends partial updates only when something changes, uses a composite metric built from bandwidth and delay, and runs DUAL so it can keep proven loop-free backup paths ready. Unlike OSPF, it never builds a full map of the network.
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The facts to have ready
- Runs directly over IP as protocol 88, not TCP or UDP.
- Hellos and most updates go to multicast 224.0.0.10 (FF02::A for IPv6).
- Cisco built it. It was published as an informational RFC, RFC 7868, in 2016, but in production you will almost only see it on Cisco gear.
- Administrative distance: 90 internal, 170 external, 5 for a summary the router creates.
- Updates are partial (only what changed) and bounded (only to the neighbors that need them). There is no periodic full-table refresh.
- Default metric uses the slowest bandwidth and the total delay along the path.
Side by side
| RIP v2 | EIGRP | OSPF | |
|---|---|---|---|
| Type | Distance vector | Advanced distance vector | Link state |
| Metric | Hop count, max 15 | Bandwidth and delay | Cost from bandwidth |
| Updates | Full table every 30 s | Partial, on change | LSAs on change, refresh every 30 min |
| Transport | UDP 520 | IP protocol 88 | IP protocol 89 |
| Loop prevention | Split horizon, holddown, max hops | DUAL feasibility condition | Full topology plus SPF |
| AD on Cisco | 120 | 90 and 170 | 110 |
What it means in practice
When the best path fails and a feasible successor exists, EIGRP switches with no messages to anyone, because the backup was proven loop-free in advance. OSPF must flood the change and rerun SPF. When no feasible successor exists, EIGRP has to ask its neighbors (queries), and that is where badly designed EIGRP networks get into trouble.
They will ask next
Why did people call EIGRP a hybrid protocol?
Cisco used to market it that way because it borrows link-state style behaviour: neighbor discovery with hellos and partial, triggered updates. The routing algorithm is still distance vector, since a router only sees its neighbors' distances. "Advanced distance vector" is the accurate answer.
Does anyone run EIGRP outside Cisco?
Rarely. RFC 7868 made the protocol documented, and FRRouting has an EIGRP daemon, but the other big vendors do not ship it. In a multivendor network you choose OSPF or IS-IS.
Traps
- Calling EIGRP a link state protocol. It never sees the whole topology.
- Saying it uses TCP or UDP. It is its own IP protocol, number 88.