EIGRP Neighbor Adjacency is the cornerstone of how routers trust each other and share network maps. Without this concept, routers would blindly send updates into the void, wasting bandwidth and creating routing loops. For the 300-410 exam, you need to understand exactly how two routers shake hands, stay in touch, and detect when the other goes offline.
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A simple way to picture EIGRP Neighbor Adjacency and Basics
Seven neighbours on your hallway decide to create a private, instant messaging group to coordinate deliveries for the entire 15-door building. First, they exchange phone numbers. Each neighbour sends a text: 'I am Apartment 301, confirming I am here.' The recipient must reply: 'Received, Apartment 301. I am Apartment 302.' If nobody replies, the sender tries again up to 16 times, waiting 5 seconds between each attempt. Once both confirm, they become 'adjacent' – trusted neighbours who know each other is alive. After the initial exchange, they send a heartbeat text every 5 seconds: 'Still here.' If three heartbeats in a row go unanswered, the silent neighbour is declared gone. The group then recalculates who is still present. This mirrors exactly how EIGRP routers form neighbour adjacencies. Each router sends a Hello packet (the introductory text) on its configured interface. The receiving router replies with its own Hello, and they exchange topology information. After that, they send a Hello every 5 seconds by default. If three consecutive Hellos are missed (the 'hold time' expires), the neighbour is considered down. The group – the EIGRP domain – then updates its routing table to reflect the lost path. The numbers map precisely: Hello interval (5 seconds), hold time (15 seconds, or 3 missed Hellos), and the 16 retries of the initial handshake mirror EIGRP's default maximum of 16 hops.
EIGRP (Enhanced Interior Gateway Routing Protocol) is a routing protocol created by Cisco. Its job is to help routers automatically learn about remote networks so they can forward data packets to the correct destination. Before any of that happens, two routers must first become neighbours – meaning they agree to exchange routing information. This is called forming a neighbour adjacency.
Think of it like two people meeting at a conference. They shake hands, introduce themselves, and then exchange business cards. In EIGRP, the handshake is the exchange of 'Hello' packets. A Hello packet is a small message a router sends out on every interface where EIGRP is enabled. It contains basic information such as the router's EIGRP autonomous system (AS) number – a numerical ID that must match on both sides – and the subnet mask of the interface. If two routers receive each other's Hello packets and find them compatible, they form an adjacency.
For a neighbour adjacency to form, several conditions must be met:
The routers must be in the same EIGRP autonomous system (AS number mismatch breaks the adjacency).
The primary IP addresses of the connecting interfaces must be in the same subnet.
The 'K-values' (metrics that EIGRP uses to calculate route costs) must match. If one router uses default K-values and the other uses custom values, the adjacency fails.
The authentication configuration, if any, must match (password or key chain).
The interfaces must not be in passive mode (a passive interface does not send Hellos).
Once these checks pass, the routers exchange their full routing tables using 'Update' packets. After that, they only send Hello packets every 5 seconds (by default on most Ethernet networks) to confirm they are still alive. The time a router waits for a Hello before declaring the neighbour dead is called the 'hold time', defaulting to 15 seconds (three times the Hello interval). If a router does not receive a Hello within the hold time, it removes the neighbour from its neighbour table and recalculates routes that went through that neighbour.
EIGRP uses three separate tables internally:
Neighbour Table: lists all directly connected EIGRP neighbours and their status.
Topology Table: contains all routes learned from neighbours, including backup paths.
Routing Table: contains the best (successor) routes installed for forwarding traffic.
The Hello protocol is not just for introduction. It also serves as a keepalive mechanism. If you change a router's Hello interval to, say, 10 seconds, you must also adjust the hold time to at least 30 seconds (three times the Hello interval) to avoid flapping neighbours. This is a favourite exam trap.
Why does EIGRP use a dedicated protocol instead of just sending updates? Because EIGRP is designed to be efficient. By separating the neighbour discovery and keepalive messages (Hellos) from the route updates (Updates), EIGRP saves bandwidth. Hellos are small and frequent; Updates are large and sent only when a change occurs. This design makes EIGRP converge quickly when a link fails, as the loss of Hellos triggers immediate recalculation.
Enable EIGRP on the Router
Use the global configuration command 'router eigrp [AS-number]' to start the EIGRP process. The AS number must be the same on all routers that you want to become neighbours. This step is like creating a club with a specific membership ID – only routers with the same ID can join.
Specify Which Networks to Advertise
Use the 'network [network-address] [wildcard-mask]' command under the EIGRP process to tell the router which interfaces to run EIGRP on. For example, 'network 192.168.1.0 0.0.0.255' enables EIGRP on all interfaces whose IP falls within that range. If you miss this step, the interface will not send Hellos.
Send and Receive Hello Packets
Once EIGRP is enabled on an interface, the router automatically sends Hello packets every 5 seconds (default). The neighbour router, if similarly configured, receives the Hello and sends its own Hello back. This exchange is the handshake that initiates the adjacency.
Verify Conditions for Adjacency
The routers automatically check the five conditions: AS number, subnet, K-values, authentication, and passive interface status. If all conditions match, the neighbour state transitions from Init to Two-way. This step happens in milliseconds but is critical for troubleshooting.
Exchange Routing Tables
After the handshake, the routers exchange their full routing tables using Update packets. They then compute the best routes using the Diffusing Update Algorithm (DUAL). At this point, the neighbour state reaches Full, and the routers are fully adjacent.
Maintain the Adjacency with Hellos
After the initial exchange, the routers continue to send Hello packets every 5 seconds. They also start a hold timer that resets each time a Hello is received. If three consecutive Hellos are missed, the hold timer expires, and the neighbour is declared dead.
An IT professional at a mid-sized company with two branch offices and a data centre configures EIGRP so that routers at each site can dynamically learn about networks at the other locations. The company has a router at the data centre (Router A) and one at each branch (Router B and Router C). All three are connected via leased lines.
The first step the engineer takes is enabling EIGRP on each router's WAN interface using the command 'router eigrp 100' (the '100' is the autonomous system number). They then use 'network 10.1.1.0 0.0.0.255' to tell the router which interface to run EIGRP on. After configuration, they check the neighbour status using 'show ip eigrp neighbors'. This command displays the neighbour IP address, interface, hold time, uptime, and sequence number. If a neighbour is not listed, the engineer troubleshoots:
They verify that both routers have the same AS number. Missing this is the number one cause of adjacency problems.
They ping the neighbour's IP to confirm Layer 3 connectivity.
They check that the interfaces are not administratively down or in passive mode.
They compare K-values using 'show ip protocols' to ensure they match.
In a real troubleshooting scenario, the engineer might notice that Router B and Router C both show as active but Router A's neighbour entry shows 'State = Init' (meaning the initial handshake is stuck). The engineer checks the interface settings on Router A and discovers that the interface connecting to Router B has been accidentally configured as a passive interface with 'passive-interface default'. This stops Router A from sending Hellos on that interface. The engineer removes the passive configuration, and within seconds, the neighbour state changes to 'Full', indicating a stable adjacency.
Once adjacencies are up, the engineer uses 'show ip eigrp topology' to verify that all remote networks are learned. They might also adjust the Hello and hold timers to speed up convergence on a critical link, setting the Hello interval to 1 second and the hold time to 3 seconds. This is done using 'ip hello-interval eigrp 100 1' and 'ip hold-time eigrp 100 3' on the interface. They document the change because non-default timers are a common source of confusion during audits.
The engineer also configures EIGRP authentication using MD5 or SHA-256 to prevent rogue routers from injecting false routes. They create a key chain with a password and apply it to the interface. If authentication fails, the neighbour adjacency never forms, and the engineer checks logs with 'debug eigrp packets' to see authentication errors.
The 300-410 exam tests EIGRP neighbour adjacency heavily, and it uses specific traps to try to catch you out. Here is exactly what you need to know.
First, the exam loves to ask about the conditions required for adjacency formation. You must memorise the five conditions:
Same autonomous system number
Same subnet on the connecting interface
Matching K-values
Matching authentication (if configured)
Interface not in passive mode
A typical multiple-choice question might present a scenario where two routers have mismatched K-values. They will list the K-values on both sides and ask why the adjacency is stuck in 'Init' state. The correct answer will point out the K-value mismatch.
Second, the exam tests the default timer values and their relationship. You are expected to know that default Hello interval on most Ethernet links is 5 seconds and hold time is 15 seconds. On NBMA (Non-Broadcast Multi-Access) networks like Frame Relay, the default Hello interval is 60 seconds. The exam will ask: if the Hello interval is changed to 10 seconds, what must happen to the hold time? The answer is that the hold time should be at least 30 seconds (three times the hello interval). But here is the trap: Cisco allows you to configure any hold time independently. The hold time does not auto-adjust. So if you change Hello to 10 seconds but leave hold time at 15, neighbours will flap (go up and down repeatedly) because they will miss three Hellos before the hold timer expires.
Third, the exam tests the neighbour states:
Down: No Hellos received.
Init: Hello received but not replied to yet.
Two-way: Both have received Hellos from each other.
Exstart/Exchange: They are exchanging topology information.
Loading: They are exchanging specific update packets.
Full: Adjacency fully established.
A common exam question shows 'show ip eigrp neighbors' output with a neighbour in 'Init' state and asks why. Possible correct answers include: mismatched K-values, passive interface on one side, or authentication failure.
Fourth, the exam tests the 'show' commands for troubleshooting. You must know:
'show ip eigrp neighbors': lists neighbours and their status.
'show ip eigrp interfaces': shows interfaces running EIGRP and their passive status.
'show ip protocols': shows K-values, AS number, passive interfaces, and filtering.
'debug eigrp packets': shows Hellos being sent and received (use with caution in production).
Fifth, the exam will test the concept of 'stuck in active' (SIA) but note that SIA relates to route computation, not adjacency. However, if adjacency flaps due to timer mismatch, it can cause SIA. The exam may combine these concepts.
Finally, the exam includes questions about EIGRP authentication and how it affects adjacency. You need to know that MD5 authentication requires matching key chains and that 'key-id' and 'key-string' must match on both sides. SHA-256 is also supported but less common in exam scenarios.
The traps to watch for:
They will give you K-values that are default on one side (1 0 1 0 0) and non-default on the other. If you do not memorise the default K-values (1, 0, 1, 0, 0), you will miss the trap.
They will present a scenario where the primary IPs are in different subnets but the secondary IPs match. Adjacency uses the primary IP, so it fails.
They will configure 'network' statements with wildcard masks that do not include the interface IP. This is a common exam scenario.
They will show 'passive-interface default' on one side but not globally disable it, so all interfaces stop sending Hellos.
EIGRP neighbour adjacency requires five conditions to be met: same autonomous system number, same subnet, matching K-values, matching authentication (if any), and no passive interface.\n
The default Hello interval on Ethernet links is 5 seconds, and the default hold time is 15 seconds; these are independently configurable and need not match between neighbours.\n
If a router misses three consecutive Hello packets within the hold time, it declares the neighbour dead and removes it from the neighbour table.\n
The 'show ip eigrp neighbors' command is the primary tool to verify neighbour state, which progresses from Down to Init to Two-way then Exstart/Exchange/Loading to Full.\n
Passive interfaces prevent the router from sending Hellos, thus blocking adjacency from forming while still allowing the router to advertise the connected network.\n
EIGRP uses Hello packets for neighbour discovery and keepalive, separate from Update packets for route exchange, making it bandwidth-efficient and fast-converging compared to older distance-vector protocols.
These come up on the exam all the time. Here's how to tell them apart.
EIGRP
Uses Hello interval of 5 seconds (default on Ethernet) independently configurable per router
Neighbour states: Down, Init, Two-way, Exstart/Exchange, Loading, Full
Supports unequal-cost load balancing with variance command
OSPF
EIGRP is Cisco proprietary; OSPF is an open standard.
Requires Hello and Dead intervals to match exactly between neighbours
Neighbour states: Down, Attempt, Init, Two-way, Exstart, Exchange, Loading, Full (more complex)
Hello Packet
Small packet used for neighbour discovery and keepalive (typically 48 bytes)
Sent every 5 seconds by default (on Ethernet)
Does not contain routing information
Update Packet
Large packet that contains actual routes and metrics
Sent only when a change occurs in the topology (not periodically)
Can be unicast or multicast depending on whether partial or full update
Passive Interface
Does not send Hello packets on that interface
Prevents neighbour adjacency from forming
Still advertises the connected network in route updates
Active Interface
Sends and receives Hello packets on that interface
Allows neighbour adjacency and route exchange
The default state for any interface under a 'network' command
Hold Time
Time the router waits for a Hello before declaring neighbour dead
Default is 15 seconds (three times Hello interval)
Configurable independently per interface
Hello Interval
Interval between sending Hello packets
Default is 5 seconds (on Ethernet)
Must be shorter than neighbour's hold time to avoid flapping
Mistake
EIGRP neighbour adjacency requires the routers to have the same hostname or router ID.
Correct
Router IDs are used for identifying routers in the topology table, but they do not affect adjacency formation. Two routers can form an adjacency with completely unrelated router IDs.
Beginners confuse the purpose of Router ID (used in DR/BDR election for OSPF, or for loop prevention in EIGRP) with adjacency requirements. The Router ID is not exchanged in Hello packets, so it does not prevent adjacency.
Mistake
If you change the Hello interval on one router, the neighbour automatically adjusts its own hold time to match three times the new interval.
Correct
The hold time is configured independently on each router. Changing the Hello interval does not change the hold time. You must manually adjust the hold time on the same router, and the neighbour router does not need to match either timer.
Many beginners assume that EIGRP behaves like OSPF, where Hello and Dead intervals must match. EIGRP allows each router to set its own timers. The only requirement is that the local hold time must be longer than the remote Hello interval to prevent flapping. This misunderstanding leads to incorrectly thinking timers must be symmetrical.
Mistake
EIGRP neighbours are automatically formed when you enable EIGRP globally with the 'router eigrp' command, no matter what interfaces are active.
Correct
You must explicitly specify which networks to advertise using the 'network' command under the EIGRP process. The 'network' command with a wildcard mask determines which interfaces participate in EIGRP. If an interface's IP is not covered by a 'network' command, it will not send or receive Hellos.
Global enablement suggests to beginners that EIGRP is 'on' everywhere, but Cisco's implementation requires explicit inclusion. This is because you might want to run EIGRP only on certain interfaces for security or administrative reasons.
Mistake
Two routers cannot form an EIGRP adjacency if they are in different VLANs or have different MTU sizes.
Correct
EIGRP does not require VLAN matching or MTU matching for adjacency. As long as IP connectivity exists (same subnet) and the other five conditions are met, they can form an adjacency regardless of VLAN membership. MTU mismatch can cause issues during the database exchange phase, but it does not prevent the initial hello-based adjacency.
Beginners confuse the requirements for adjacency (simple Hello exchange) with the requirements for successful route exchange. A large MTU mismatch can cause update packets to be fragmented or dropped, but the adjacency itself forms quickly. This leads engineers to misdiagnose adjacency problems as MTU issues.
Mistake
EIGRP neighbour adjacency requires that both routers have the same EIGRP version or have the same metric weights (K-values) configured, but only the default K-values work.
Correct
You can use non-default K-values as long as they match on both sides. The default values are 1, 0, 1, 0, 0, but you can change them to 0, 1, 0, 0, 0 (using only delay) as long as both neighbours use the same custom values. The adjacency will form if they match, regardless of whether they are default or custom.
Many study guides emphasise 'matching K-values' but do not clarify that custom values are allowed. Beginners assume any deviation from defaults breaks adjacency, so they panic when they see non-default values in a lab even when both sides match.
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It shows a table listing all directly connected EIGRP neighbours, their IP addresses, which interface they are connected through, the hold time remaining, uptime, and the sequence number of the last Hello received.
The neighbour is stuck in Init state because it has received a Hello from the other router but has not yet sent a Hello back. Common causes include a mismatched autonomous system number, mismatched K-values, or a passive interface on one side preventing it from replying.
Yes. EIGRP does not require Hello intervals to match. However, the hold time on each router must be long enough to accommodate the neighbour's Hello interval. If one router sends Hellos every 10 seconds, the other must have a hold time greater than 10 seconds to avoid declaring the neighbour dead.
The 'network' command tells the router which interfaces to run EIGRP on (send and receive Hellos). The 'passive-interface' command tells the router not to send Hellos out of a specific interface, preventing adjacency formation while still advertising the connected network in updates.
Yes. The two interfaces must have IP addresses in the same subnet, and there must be IP reachability between them. If you cannot ping the neighbour's interface IP, EIGRP Hellos will not be received, and the adjacency will not form.
K-values are metric weights that determine how EIGRP calculates the composite metric (cost) of a route. They must match on both neighbours, otherwise adjacency fails. The default K-values are 1 (bandwidth), 0 (load), 1 (delay), 0 (reliability), 0 (MTU).
You've finished EIGRP Neighbor Adjacency and Basics. Continue through the 300-410 study guide to build a complete picture of the exam.
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