EIGRP (Enhance Interior Gateway Routing Protocol)

What is EIGRP(Enhance Interior Gateway Routing Protocol) :- EIGRP is a hybrid or

advanced distance vector routing protocol developed by Cisco. It combines the speed
and simplicity of distance vector protocols (like RIP) with the intelligence of
link-state protocols (like OSPF).
- It's Cisco proprietary, but newer versions are open-standard and supported on
some non-Cisco devices.
- A Hybrid routing protocol has the advantage of both protocols, distance vector
and link state and merges them into a new protocol. Typically, hybrid routing
protocol are based on a Distance vector protocol but contain many of the feature
and advantage of Link state routing protocols.

- Why Use EIGRP?

- EIGRP is:
- Fast-converging (15 sec)
- Efficient with bandwidth(Incremental Update)
- Highly scalable
- Supports VLSM, CIDR, unequal-cost load balancing
- Uses the DUAL algorithm to find loop-free paths

- Key Features of EIGRP:

Feature Explanation
DUAL Algorithm Ensures loop-free and backup
paths using Feasible Successor.
Unequal Cost Load Balancing Supports multiple routes to a
destination with different costs (unique to EIGRP RIP and OSPF don’t support).
Classless Protocol Supports VLSM, CIDR, and
discontiguous networks.
Rapid Convergence Much faster than RIP, thanks to DUAL
and event-triggered updates.(15 Sec)
Uses Reliable Transport Protocol (RTP) Ensures reliable delivery of
routing info (unicast or multicast).
Partial and Bounded Updates Sends updates only when changes
occur and only to affected routers, saving bandwidth.Use IP (224.0.0.10)
Supports Multiple Network Protocols Can route IPv4, IPv6, IPX,
AppleTalk (historically).
Metric Based on Multiple Factors Bandwidth, delay (by default), and
optionally load, reliability, MTU.
Summarization It gives an auto summarization
feature.
AD(Administrative Distance) 90 Internal, 170 External.
Equal-cost load balancing Up to 4 paths by default, Up to
16 paths maximum (if configured)
EIGRP Port Number 88

- EIGRP Metrics Formula:

- EIGRP uses a composite metric, meaning it can take multiple characteristics
of the route into account:
1. Bandwidth
2. Delay
3. Reliability
4. Load
5. MTU (not used in metric calculation, but tracked)
- The metric formula is influenced by values called K-values.
- How to check on interface (#show interface fa0/0)

- EIGRP Metric Formula

 - By default, EIGRP uses:
- Metric = [(10^7 / Bandwidth) + Delay] × 256
- Bandwidth: Minimum bandwidth (in kbps) along the path
- Delay: Sum of delays (in 10s of microseconds) along the path
- This default metric considers bandwidth and delay only, unless you change
the K-values.

- K-Values in EIGRP
- K-values are tuning knobs for the metric calculation.
K-Value Represents Default Value
K1 Bandwidth 1
K2 Load 0
K3 Delay 1
K4 Reliability modifier 0
K5 MTU 0

- To View K-values:
- Router# show ip protocols
- Important:
- K-values must match on all routers in the same EIGRP AS — if they don’t,
neighbors will not form!

- EIGRP Default Metric Formula (Example)

- Metric = [(10^7 / Bandwidth) + Delay] × 256
- Bandwidth is the minimum bandwidth (in kbps) along the path.
- Delay is the sum of delays (in tens of microseconds) across all interfaces on
the path.
- Let’s say we have a path that includes two interfaces:
Interface Bandwidth Delay
Fa0/0 100,000 kbps(100 Mbps) 100 µs
S0/0 1,544 kbps (T1) 20,000 µs

- Step 1: Bandwidth Calculation

- Take the lowest bandwidth along the path:
- Bandwidth = 1,544 kbps
- Now apply:
10^7 / Bandwidth = 10,000,000 / 1544 ≈ 6476.62

- Step 2: Delay Calculation

- Sum the delays on all interfaces (in µs), then divide by 10:
Delay = (100 + 20000) / 10 = 2010

- Step 3: Plug into the Metric Formula

- Metric = (6476.62 + 2010) × 256
- Metric ≈ 8486.62 × 256
- Metric ≈ 2,174,544
- So the EIGRP metric for this route is approximately: 2,174,544

- What is the Maximum Hop Count for EIGRP?

- Default 100 hops
- Maximum 255 hops (manually configurable)

- EIGRP work with 5 Packets and 3 Table.

- How EIGRP Works:

- In EIGRP 1st established Neighborship then share update with each other.
- Neighbors discovered via hello packets (multicast on 224.0.0.10) and Hello
Packets are send every 5 second & Hold Timer 15 sec.
- Neighbor table is built
- DUAL algorithm calculates the successor (best route) and feasible successor
(backup)
- Topology table stores all learned routes
- Routing table only contains the best route(s)

- EIGRP Packets Types :- EIGRP uses 5 main types of packets to maintain and build
routing information (Hello, Update, Query, Reply, ACK).
Packet Type Purpose Reliable?
Multicast/Unicast
Hello Neighbor discovery/keepalive No
Multicast
Update Send routing information Yes
Unicast or Multicast
Query Ask about a route Yes
Multicast
Reply Respond to a query Yes
Unicast
ACK Acknowledgment of receipt No (used for reliable
ones) Unicast

1. Hello Packet
- Purpose:
- Used to discover and maintain neighbor relationships.
- Sent periodically by EIGRP-enabled interfaces.
- Details:
- No acknowledgment needed
- Multicast to 224.0.0.10 (Uses multicast address FF02::A)
- If a neighbor fails to send Hellos within the hold time, it's declared
down.

- Default Hello Timers:

Interface Type Hello Timer Hold Timer
High-speed (LAN) 5 sec 15 sec
Low-speed (WAN) 60 sec 180 sec

2. Update Packet
- Purpose:
- Used to send routing information (topology data).
- Sent only when a change occurs or new neighbors appear.
- Details:
- Contains information about routes and their metrics.
- Sent reliably — requires ACK.
- Sent multicast (224.0.0.10) to all neighbors or unicast to a specific one.
- Example: When a new neighbor comes up, the router sends its entire routing
table via Update packets.

3. Query Packet
- Purpose:
- Used to ask neighbors about a specific route.
- Triggered when a route goes into an active state (e.g., it’s lost, and no
backup is available).
- Details:
- Sent multicast
- Requires a Reply
- Sent reliably
- Example: "Hey, do you have a route to network 192.168.5.0?"
 4. Reply Packet
- Purpose:
- Response to a Query
- Contains information about the requested route (either "yes, I have it" or
"no, I don’t")
- Details:
- Sent unicast (back to the router that asked)
- Sent reliably
- Required to terminate the active state
- Example: “Yes, I still have a valid route to 192.168.5.0.”

5. ACK Packet
- Purpose:
- Acknowledge the receipt of Update, Query, or Reply packets.
- Details:
- A small, header-only packet.
- Sent unicast
- Not acknowledged itself (unreliable)

- Recap Diagram
+-----------+ Hello +-----------+
| Router A | -------------------> | Router B |
| | <------------------- | |
| | Hello | |
+-----------+ +-----------+

- Update → Sent when new routes appear

- Query → Sent when route is lost
- Reply → Sent in response to Query
- ACK → Confirms Update/Query/Reply was received

- EIGRP Uses 3 Main Tables:

Table Name Purpose
Neighbor Table Tracks directly connected EIGRP neighbors
Topology Table Stores all learned routes & their metrics
Routing Table Holds best routes installed in the router

1. Neighbor Table – "Who am I talking to?"

- Purpose: Stores information about directly connected routers that are EIGRP
neighbors.
- Established by: Receiving Hello packets.
- Stored info:
- Neighbor IP address
- Interface through which neighbor is reachable
- Hold time (how long we consider the neighbor alive)
- Uptime (how long the neighbor has been up)
- Smooth round-trip time (SRTT), used for EIGRP metrics

- Command to view it:

#show ip eigrp neighbors
- Think of it like your contacts list — it tells your router who it's peering
with in EIGRP.

2. Topology Table – "What do my neighbors know?"

- Purpose: Stores all possible routes learned via EIGRP — even non-best paths.
- Every route includes:
- Successor: Best next-hop router for a route (installed in routing table)
- Feasible Successors: Backup routers (loop-free alternative)
 - Metric info: Bandwidth, delay, etc.
- Built using updates from neighbors in the Neighbor Table.
- Command to view it:
#show ip eigrp topology
- Think of this as your "brainstorm" list of all routes you've learned —
you're not using all of them, but you keep them handy in case you need them later.

- Topology Table Route States:

Route State Meaning
Passive Route is stable and working
Active Route is being recomputed (e.g., lost path)
- Passive = good.
- Active = means DUAL is working to find a new path (uses Queries & Replies).

3. Routing Table – "What routes do I actually use?"

- Purpose: Stores only the best routes from each routing protocol.
- Routes from the topology table are installed here if:
- They’re the lowest-metric (best) route
- And the route is in passive state
- Command to view it:
#show ip route
- This is your final GPS — it's the map your router is actually using to
forward packets.

- Flow Between Tables (Visualized)

Neighbor Table --> Topology Table --> Routing Table
🛣️ 🧠
"Who are my "What routes did "Which routes
EIGRP neighbors?" I learn from them?" do I use now?"

- EIGRP Terminology:
Term Stands For Meaning
FD Feasible Distance Total metric to reach the
destination through the best path
AD Advertised Distance Metric reported by the neighbor to
reach a specific network
Successor — The best next-hop router to reach a
destination
FS Feasible Successor A backup route that meets specific
conditions to be loop-free

1. Feasible Distance (FD) :- It is the total metric (delay + bandwidth + other K-

values) from the local router to the destination via a particular path.
- Used for: Choosing the best route (Successor)
- The route with the lowest FD is installed in the routing table.
- FD = AD + Cost from local router to neighbor

2. Advertised Distance (AD) (Also called Reported Distance - RD) :- The metric
reported by the neighbor (via an EIGRP update) about how far they are from the
destination.
- It’s how far the destination is from the neighbor’s perspective.
- Think of it as “how far my neighbor says the destination is”

3. Successor :- The best path (lowest FD) to reach a destination.

- Installed in the routing table
- There can be multiple successors if multiple routes have the same FD
- These are the routes your router uses to forward packets.
 4. Feasible Successor (FS) :- A backup path to the destination that is guaranteed
loop-free
- Requirement: The AD of the FS must be less than the FD of the current
Successor.
- Feasibility Condition (FC):
# AD(FS) < FD(Successor)
- If this is true, then the backup route is stored in the topology table as a
Feasible Successor.
- If the primary path fails, the router instantly switches to the FS — no
recalculation required.