VMware ESXi and vSphere Cluster Management
EIGRP Overview
Learn how EIGRP works, including its classification, capabilities, multicast delivery, administrative distance, neighbors, timers, and composite metric.
EIGRP stands for Enhanced Interior Gateway Routing Protocol. It is a dynamic routing protocol used to exchange reachability information between routers inside an autonomous system. An autonomous system is a group of networks administered under a common routing policy.
EIGRP is commonly classified as an advanced distance-vector protocol. It is also sometimes called a hybrid routing protocol because it combines distance-vector behavior with selected characteristics commonly associated with link-state protocols, such as maintaining detailed path information and sending partial updates when topology changes.
EIGRP at a Glance
| Property | Value or behavior | Why it matters |
|---|---|---|
| Protocol expansion | Enhanced Interior Gateway Routing Protocol | Identifies the protocol and its purpose as an interior routing protocol. |
| Routing-protocol classification | Advanced distance vector; sometimes described as hybrid | Explains how EIGRP combines distance-vector operation with enhanced topology and update behavior. |
| Predecessor | Interior Gateway Routing Protocol (IGRP) | EIGRP replaced the older Cisco routing protocol. |
| Original vendor status and open-standard milestone | Originally Cisco proprietary; published as an open standard in 2013 | Explains both its strong Cisco association and its later public standardization. |
| IPv4 multicast address | 224.0.0.10 | IPv4 EIGRP routers use this multicast group for protocol communication. |
| Default internal administrative distance | 90 | A lower value makes internal EIGRP preferable to default OSPF and RIP routes for the same destination. |
| Default metric inputs | Bandwidth and delay | These path characteristics determine the default EIGRP composite metric. |
| Supported network-layer protocol versions | IPv4 and IPv6 | EIGRP can support routing in both major IP environments. |
History and Platform Support
EIGRP superseded IGRP, or Interior Gateway Routing Protocol. IGRP was an earlier Cisco proprietary interior gateway protocol. EIGRP added more capable route calculation, update, and path-management features.
EIGRP was originally Cisco proprietary and was published as an open standard in 2013. In practical network administration and study environments, EIGRP is still most commonly encountered on Cisco equipment. Every router participating in the same EIGRP routing domain must support EIGRP and be configured to exchange its routing information with the other participants.
Why EIGRP Is Useful in Larger Networks
EIGRP includes capabilities that help organize and operate complex networks. These features do not remove the need for careful addressing and design, but they can reduce unnecessary routing traffic and provide multiple path choices.
Classless Routing and VLSM
Classless routing carries subnet-mask or prefix-length information with routes. This allows routers to distinguish networks by their actual prefixes rather than relying on old class A, B, or C assumptions.
Variable Length Subnet Masking (VLSM) means using different prefix lengths within one addressing plan. For example, a site might use a /24 network for users, a /27 network for infrastructure devices, and a /30 network for a point-to-point connection. EIGRP can advertise these associated prefix lengths, allowing routers to maintain classless routes.
Route Summarization
Route summarization advertises one aggregate route that represents multiple more-specific networks. For example, several contiguous branch subnets can be represented by one summary toward the rest of the network. This reduces the number of visible routes and can limit the effect of a change in one specific subnet.
EIGRP summarization can be configured at routers throughout the network where an aggregation point is useful; it is not limited to one special boundary location. The summary must be designed carefully so that it represents only the intended networks.
Incremental Updates
An incremental update communicates a routing change instead of repeatedly sending the entire routing table. If one remote network becomes unavailable after the topology has stabilized, EIGRP can communicate the relevant change rather than retransmitting every known route. This reduces unnecessary update traffic in larger networks.
Load Balancing
Load balancing uses more than one path to forward traffic. EIGRP supports the use of multiple qualifying paths, including equal-cost paths. A router with two suitable paths to a destination may forward traffic across both instead of using only one link.
Authentication and IP Version Support
EIGRP supports MD5-based authentication. MD5 authentication allows routers to validate routing-protocol exchanges using a configured authentication relationship. Authentication settings must match between participating neighbors.
EIGRP supports both IPv4 and IPv6. The details of addressing and neighbor operation differ between the two versions, but the overall purpose remains dynamic routing within an autonomous system.
EIGRP Message Delivery and Multicast
EIGRP uses Cisco Reliable Transport Protocol (RTP) to deliver EIGRP protocol messages. RTP is not the same as the multimedia transport protocol with a similar name. In EIGRP, it provides protocol-specific delivery behavior, including reliable delivery where the routing exchange requires acknowledgment and controlled handling of messages that do not require the same treatment.
For IPv4 communication, EIGRP uses the multicast address 224.0.0.10. EIGRP-capable routers on a shared IPv4 segment can listen for protocol messages sent to this group rather than requiring a separate unicast transmission to every router.
For example, three EIGRP routers connected to the same Ethernet segment can form neighbor relationships and exchange routing information using the 224.0.0.10 multicast group. Multicast communication must be able to reach the participating routers.
Administrative Distance and Route Selection
Administrative distance is a locally significant preference value used when a router learns routes to the same destination from different routing sources. It answers the question: “Which source should this router trust?” A lower administrative distance is preferred.
Administrative distance is different from an EIGRP metric. Administrative distance compares route sources, while the EIGRP metric compares candidate paths learned through EIGRP.
Default Route Preference Comparison
| Routing source | Default administrative distance | Preferred relative to competing sources |
|---|---|---|
| Internal EIGRP | 90 | Preferred over default OSPF and RIP routes to the same destination |
| OSPF | 110 | Preferred over RIP, but not over internal EIGRP at its default value |
| RIP | 120 | Less preferred than default internal EIGRP and OSPF |
Suppose a router learns the same prefix through internal EIGRP, OSPF, and RIP. With default administrative distances, the EIGRP route has a value of 90, OSPF has 110, and RIP has 120. The router selects the internal EIGRP route because 90 is the lowest value.
This comparison applies only when the routes describe the same destination prefix. A more-specific route can be selected because of longest-prefix matching before administrative distance is used to compare competing sources for that prefix.
EIGRP Metric Fundamentals
EIGRP uses a composite metric. A composite metric is a path value derived from multiple characteristics of the interfaces and links along a route. The available metric inputs are bandwidth, delay, reliability, and load.
Under the default metric calculation, EIGRP uses bandwidth and delay. Bandwidth represents the lowest configured bandwidth along the path. Delay represents the cumulative interface delay along the path.
Reliability and load are available metric components, but their default weighting values are zero. Therefore, they do not affect the default EIGRP metric. If nondefault weighting is configured, reliability or load can affect metric calculations, but changing metric weights requires careful, consistent design.
EIGRP Metric Components
| Component | Available to the metric | Used by default | Teaching note |
|---|---|---|---|
| Bandwidth | Yes | Yes | Uses the lowest configured bandwidth along the path. |
| Delay | Yes | Yes | Uses cumulative delay across the path. |
| Reliability | Yes | No | Its default weighting is zero. |
| Load | Yes | No | Its default weighting is zero. |
Consider two candidate paths. One path may have higher bandwidth but more cumulative delay. The other may have lower bandwidth but less delay. EIGRP's default composite calculation evaluates both characteristics; looking at bandwidth alone is not enough to predict the selected path.
EIGRP Capability Summary
| Capability | Conceptual benefit | Example use |
|---|---|---|
| Classless routing and VLSM | Carries prefix lengths and supports differently sized subnets. | Advertise /24, /27, and /30 networks in one addressing plan. |
| Summarization | Reduces the number of routes and limits topology detail. | Advertise several branch networks as one aggregate route. |
| Incremental updates | Sends changes instead of repeatedly sending the full routing table. | Report that one remote network is unavailable. |
| Load balancing | Uses more than one qualifying forwarding path. | Forward traffic across two equal-cost paths. |
| MD5 authentication | Validates routing-protocol exchanges. | Protect neighbor communication with matching authentication settings. |
| IPv4 and IPv6 support | Provides dynamic routing for both IP versions. | Use EIGRP in IPv4 and IPv6 network environments. |
Neighbor Relationships and Timers
EIGRP neighbor relationships are the basis for exchanging routing information. A neighbor is a directly reachable EIGRP router with which a router has established and maintained protocol communication.
EIGRP routers send hello communication to discover and maintain neighbors. The hello messages help a router confirm that another participating router is still reachable and able to exchange routing information. If expected communication stops for long enough, the router can remove the neighbor relationship and recalculate affected routes.
Hold time is the period used in neighbor maintenance before a neighbor is considered unavailable. It is part of the neighbor relationship and should not be confused with a general routing-loop prevention timer.
A holddown timer is a routing-loop mitigation concept. It can temporarily discourage acceptance of certain route information after a change, helping prevent unstable or incorrect route propagation. Neighbor hold time determines when a neighbor is considered unavailable; a holddown timer serves a different purpose. These terms should not be treated as synonyms.
How to Think About EIGRP Route Selection
- First identify the destination prefix and apply normal route-selection rules, including longest-prefix matching.
- If multiple routing sources offer the same destination prefix, compare their administrative distances.
- If EIGRP is selected as the source, compare EIGRP candidate paths using the EIGRP composite metric.
- If multiple paths qualify, EIGRP can support load balancing across them.
Conceptual Troubleshooting Examples
An Expected EIGRP Route Is Not Selected
Check whether the competing routes describe the identical prefix. Then compare the administrative-distance values of the route sources. Confirm that the EIGRP route is an internal EIGRP route before applying the default value of 90.
Routers Do Not Exchange EIGRP Information
Verify that all participating routers support and run EIGRP. Check whether they can form and maintain a neighbor relationship, whether IPv4 multicast communication to 224.0.0.10 is available on the shared segment, and whether authentication settings match when MD5 authentication is enabled.
The Selected EIGRP Path Is Unexpected
Compare the configured bandwidth and delay values on each candidate path. Keep metric evaluation separate from administrative-distance route-source selection. Also check whether nondefault metric weighting has enabled reliability or load.
A Summary Route Hides Expected Individual Routes
Identify where summarization was applied and which more-specific prefixes the aggregate represents. Confirm that the summary covers only the intended networks. An overly broad summary can make a destination appear reachable through an incorrect or less-specific path.
Key Takeaways
- EIGRP means Enhanced Interior Gateway Routing Protocol and is used within an autonomous system.
- It is an advanced distance-vector protocol and is informally described as hybrid because it combines selected distance-vector and link-state characteristics.
- EIGRP replaced Cisco's older IGRP, was originally proprietary, and became an open standard in 2013.
- Important capabilities include classless routing, VLSM, summarization, incremental updates, load balancing, MD5 authentication, and IPv4 and IPv6 support.
- EIGRP uses Cisco RTP for protocol-message delivery and uses IPv4 multicast address 224.0.0.10.
- The default administrative distance for internal EIGRP is 90.
- The default EIGRP metric uses bandwidth and delay; reliability and load have zero default weighting.
- Neighbor maintenance uses hello communication and hold time, while a holddown timer is a separate loop-prevention concept.