VMware ESXi and vSphere Cluster Management

EIGRP Neighbors: Discovery Requirements and Neighbor Table

Learn how EIGRP routers discover neighbors with Hello packets, which ASN, subnet, and K values must match, and how to verify adjacencies.

An EIGRP neighbor is a directly connected EIGRP-speaking router with which an adjacency has been established. An adjacency is the operational neighbor relationship EIGRP requires before routing information can be exchanged.

Neighbor formation and route learning are related but different steps. First, routers discover and establish a neighbor relationship. After that relationship exists, they can exchange EIGRP routing information and build their routing data.

How EIGRP Discovers Neighbors

EIGRP uses Hello packets to discover nearby EIGRP routers and maintain existing relationships. On IPv4 segments, EIGRP uses the multicast address 224.0.0.10.

Neighbor discovery occurs across a directly connected network segment. When a router receives compatible Hello packets on an enabled interface, it can form an adjacency with the sending router. Periodic Hellos continue after formation so each router can confirm that the other router is still reachable.

If Hellos stop arriving, the receiving router uses the hold timer to decide when the neighbor is no longer available. A neighbor relationship therefore depends on both successful initial discovery and continued communication.

Requirements for EIGRP Adjacency

Prospective neighbors must satisfy the required EIGRP and IP conditions. In particular, both routers must use the same EIGRP autonomous system number, their connected interfaces must belong to the same subnet, and their EIGRP K values must match.

Requirement — What must match or be true — Why it matters — Result of mismatch

EIGRP ASN — Both routers use the same EIGRP autonomous system number — The ASN identifies the EIGRP process and the group of routers intended to exchange routes — No adjacency forms.

Interface subnet — The two connected interfaces have addresses in the same subnet, using compatible masks — EIGRP Hello communication must occur across the shared IP network — Neighbor discovery fails.

K values — Metric weighting values are identical on both routers — EIGRP must agree on the metric formula used by the neighboring process — No adjacency forms until the values match.

EIGRP Autonomous System Number

An EIGRP autonomous system is a group of EIGRP-enabled routers intended to form neighbors and exchange routes with one another. The numeric identifier for that group is the EIGRP autonomous system number, or ASN.

The ASN is selected when the EIGRP routing process is configured. For example, this starts an EIGRP process using ASN 1:

Router(config)# router eigrp 1

Routers intended to become neighbors must use the same ASN on the connected link. If one router runs EIGRP ASN 1 and the other runs EIGRP ASN 2, they do not form an adjacency even if their IP addressing and physical connection are otherwise correct.

Same-Subnet Requirement

The interfaces connected to each other must have IP addresses in the same subnet. A subnet is the shared IP network to which both interfaces belong.

For example, a point-to-point link could use these compatible addresses and masks:

Router A link interface: 192.0.2.1 255.255.255.252
Router B link interface: 192.0.2.2 255.255.255.252

Both addresses belong to the 192.0.2.0/30 subnet, so the interfaces can communicate directly at Layer 3. If one interface has an incorrect address or mask that places it in a different subnet, the routers cannot properly discover each other as EIGRP neighbors.

A physical connection alone is not enough. Check both the interface IP addresses and the subnet masks when an expected neighbor does not appear.

K Values and Metric Compatibility

K values are EIGRP metric weighting parameters. They tell EIGRP which metric inputs to use and how to weight them when calculating route metrics.

Metric input — Default use — Relationship to K values

Bandwidth — Used by default — Its K value is enabled in the default metric formula.

Delay — Used by default — Its K value is enabled in the default metric formula.

Load — Not used by default — It can become an input when metric weights are changed.

Reliability — Not used by default — It can become an input when metric weights are changed.

Both neighbors must have identical K values. Changing metric weights on only one router creates a K-value mismatch and prevents adjacency formation. If a design intentionally changes the metric weights, apply the same settings consistently to every neighboring EIGRP router.

Example: Successful Two-Router Adjacency

Consider two routers connected by one Ethernet or point-to-point link:

  • Both routers run EIGRP ASN 1.
  • The connected interfaces use addresses from the same subnet.
  • Both routers retain matching default K values.
  • EIGRP Hello packets are exchanged over the directly connected segment using 224.0.0.10.

Each router should discover the other, establish an adjacency, and list the peer in its neighbor table. Once the adjacency exists, the routers can exchange routing information.

Verifying the EIGRP Neighbor Table

The neighbor table contains currently established EIGRP neighbor adjacencies. Use the following command in privileged EXEC mode to display established IPv4 neighbors:

Router# show ip eigrp neighbors

A sample entry might look like this:

H   Address         Interface       Hold Uptime   SRTT   RTO  Q Cnt  Seq Num
0   192.0.2.2       Gi0/0             12  00:18:42    10  100       0        7

The exact values change as the relationship operates. The fields provide both identification and health information.

Field — Meaning — Operational interpretation

H — Local sequential index for an established adjacency, beginning at 0 — Use it as a local entry number, not as the neighbor's address or ASN.

Address — IP address of the EIGRP neighbor — Confirms which peer is represented by the entry.

Interface — Local interface on which Hello packets from that neighbor were received — Identifies the local link used for the adjacency.

Hold (sec) — Remaining time EIGRP waits for communication before declaring the neighbor unavailable — It is refreshed when communication, including Hellos, is received.

Uptime — Elapsed time since the local router established or first heard from the neighbor — An increasing, continuous value generally indicates a stable adjacency.

SRTT (ms) — Smoothed round-trip time for reliable EIGRP communication — Helps EIGRP characterize communication timing with the neighbor.

RTO — Retransmission timeout before a reliable EIGRP packet is retransmitted — Used when EIGRP must resend a reliable packet.

Q Cnt — Number of pending EIGRP Update, Query, or Reply packets awaiting transmission — A stable value is normally 0. A value that remains above 0 may indicate impaired communication or unacknowledged reliable packets.

Seq Num — Sequence number associated with the most recently received reliable EIGRP packet — Helps track reliable packet exchange with the neighbor.

Reading a Healthy Neighbor Entry

  • Find the expected neighbor IP address in the Address column.
  • Confirm that the listed Interface is the local interface connected toward that router.
  • Observe that the Hold value is refreshed rather than reaching zero.
  • Check that Uptime continues to represent a stable relationship.
  • Review SRTT and RTO as indicators of reliable communication timing.
  • Look for a steady Q Cnt of 0 during normal operation.
  • Use Seq Num to observe reliable EIGRP packet progress.

Troubleshooting Missing Neighbors

No Neighbor Entry Appears

If show ip eigrp neighbors produces no output, or an expected router is absent, check the required formation conditions:

  1. Compare the EIGRP process ASN on both routers. The intended neighbors must use the same ASN.
  2. Compare the connected interface IP addresses and subnet masks. Both interfaces must be in the same subnet.
  3. Compare the EIGRP metric weight settings. The K values must be identical.

Correct the mismatch, then verify the neighbor table again. A router cannot exchange EIGRP routes with a peer that has not formed an adjacency.

A Neighbor Disappears After the Hold Timer Expires

This symptom means Hello packets or other communication were no longer received in time. Check whether the Hold (sec) value is being refreshed and verify the operational state and addressing of the local connected interface.

Restore the directly connected path and the exchange of EIGRP Hello packets. Once compatible communication resumes, the routers can establish the adjacency again.

Q Cnt Remains Above Zero

A persistent queue count above 0 means reliable EIGRP packets are awaiting transmission or acknowledgment. Review Q Cnt, SRTT, and RTO, then verify link stability and continued neighbor reachability.

Investigate and restore reliable packet delivery. In steady-state operation, a healthy neighbor normally has a queue count of 0.

Summary

  • EIGRP routers establish neighbor adjacencies before exchanging routing information.
  • Neighbor discovery and maintenance use Hello packets sent on directly connected segments; IPv4 EIGRP uses 224.0.0.10 for multicast communication.
  • Prospective neighbors must use the same ASN, share the same IP subnet on the connecting interfaces, and have matching K values.
  • Bandwidth and delay are default EIGRP metric inputs; load and reliability can be enabled by changing K values.
  • show ip eigrp neighbors verifies established adjacencies and displays address, interface, timers, queue, and sequencing information.
  • A missing entry usually indicates an ASN, addressing, subnet, K-value, or direct-segment communication problem.

For a focused reference, see EIGRP Neighbors.