EIGRP is a proprietary routing protocol developed by Cisco that uses a composite metric and has fast convergence properties. It functions as a hybrid of distance-vector and link-state routing protocols, sending subnet mask and VLSM information in updates. EIGRP forms neighbor relationships through periodic hello messages and establishes three key tables - Neighbor, Topology, and Routing - to store neighbor, route, and best path information. It utilizes five packet types and reliable transport to efficiently share routing updates.

1. EIGRP
(Enhanced Interior Gateway Routing Protocol)
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2. ▪ EIGRP (Enhanced Interior Gateway Routing Protocol) is a hybrid
routing protocol that show both distance vector and link-state
routing protocol properties. EIGRP (Enhanced Interior Gateway
Routing Protocol) was developed by Cisco as a Cisco proprietary
protocol and it is only available on Cisco devices.
▪ EIGRP (Enhanced Interior Gateway Routing Protocol) is a classless
routing protocol. It sends the subnet masks in the update packets.
EIGRP also supports VLSM (Variable Length Subnet Mask), so EIGRP
uses the IP addresses range very efficient.

3. ▪ The most important characteristic of EIGRP is that, it is a fast
converged routing protocol. EIGRP keeps the “Backup Routes”
beside the “Best Route” to a destination. So, during a failure, EIGRP
does not need to calculate a new route. Instead, EIGRP uses the
Backup Route. This provides to EIGRP, Faster Convergence
mechanism.
▪ EIGRP (Enhanced Interior Gateway Routing Protocol) uses a very
complex metric by using bandwidth, delay, load, reliability and MTU
values.This formula is showed below:

4. ▪ As we can see, there are some K values in EGRP metric formula. By
default these values are:
– K1 (Bandwidth) = 1
– K2 (Load) = 0
– K3 (Delay) = 1
– K4(Reliability) = 0
– K5(MTU) = 0
▪ These K values need to be match in both ends to form an EIGRP
Neighbourship. we can change these values but it is not
recommended.

5. ▪ By default Bandwidth and Delay values become important only and
used as a metric ( The other parts will be 0, because of multiplying
with 0). So, we can say that, EIGRP (Enhanced Interior Gateway
Routing Protocol) uses the path with minimum bandwidth and the
total delay to a destination.
▪ Bandwidth = 107/Bandwidth in Kbps
▪ Delay = Delay / 10
▪ The default Formula becomes like below (with default K values ) :

6. ▪ EIGRP (Enhanced Interior Gateway Routing Protocol) works with IPv4, IPv6,
IPX ans AppleTalk Routed Protocols. So with this characteristic, we can say
that EIGRP is a protocol independent routing protocol.
▪ EIGRP uses DUAL (Diffusing Update Algorithm) for routing calculations.
▪ The Administrative Distance for Internal EIGRP is 90, for External EIGRP is
170 and for Summary Advertisement is 5.
▪ EIGRP uses RTP (Reliable Transport Protocol) to control sending and
acknowledging EIGRP messages.
▪ The multicast address used for EIGRP (Enhanced Interior Gateway Routing
Protocol) is 224.0.0.10.The protocol number of EIGRP is 88.

7. ▪ The default Hello Timer is 5 seconds and the Hold-Down Timer is 15
seconds in EIGRP. This is a little more for NBMA (Non Broadcast Multi
Access) networks . For NBMA networks, Hello Timer is 60 seconds
and Hold-Down Timer is 180 seconds. In OSPF the Hello and Dead
intervals must be match to form neighbourship. But in EIGRP Hello
and Hold-Down timers can be different and it is not a necessity for
neighbourship.
▪ In EIGRP, MD5 authentication mechanism is used. EIGRP does not
use simple authentication.

8. EIGRP Tables
NeighbourTable
All EIGRP Neighbours
TopologyTable
All EIGRP Routes
RoutingTable
Best EIGRP Routes
EIGRP (Enhanced Interior Gateway
Routing Protocol) has three different
tables.
These EIGRP tables are:
• NeighbourTable
•TopologyTable
• RoutingTable

9. ▪ EIGRP Neighbour Table keeps all the router that EIGRP
neighbourship is established with. EIGRP Neighbour Table is
established by Hello messages.
▪ EIGRP (Enhanced Interior Gateway Routing Protocol) has a specific
Neighbour Table for each Layer 3 protocols. For example, if we are
using both IPv4 and IPv6 protocols, we will have two different EIGRP
NeighbourTables.
▪ we can see the EIGRP Neighbour Table with “show ip eigrp
neighbors ” command.
▪ In EIGRP Neighbour Table we can see the ip address of the neighbour,
the port you access to the neighbour, the port’s uptime etc.

10. ▪ We can see an output of “show ip eigrp neighbors ” command is given
below:
▪ Router#show ip eigrp neighbors
▪ IP-EIGRP neighbors for process 100
▪ H Address Interface Hold Uptime SRTT RTO Q Seq
▪ (sec) (ms) Cnt Num
▪ 0 10.0.0.2 Fa4/0 11 00:00:08 40 1000 0 12
▪ 1 20.0.0.2 Fa5/0 10 00:00:08 40 1000 0 11

11. EIGRP Topology Table keeps all the routes to all the destinations. It keeps all
the Successor and Feasible Successor routes. We can say also, “it keeps the best
routes and the alternatives”.
You can check EIGRP Topology Table with “show ip eigrp topology ” command.
▪ Router#show ip eigrp topology
▪ IP-EIGRPTopologyTable for AS 100/ID(192.168.1.2)
▪ Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
▪ r - Reply status
▪ P 10.0.0.0/24, 1 successors, FD is 28160
▪ via Connected, FastEthernet4/0
▪ P 20.0.0.0/24, 1 successors, FD is 28160
▪ via Connected, FastEthernet5/0
▪ P 30.0.0.0/24, 1 successors, FD is 30720
▪ via 10.0.0.2 (30720/28160), FastEthernet4/0
Cont.…

12. ▪ P 40.0.0.0/8, 1 successors, FD is 30720
▪ via 20.0.0.2 (30720/28160), FastEthernet5/0
▪ P 192.168.1.0/24, 1 successors, FD is 28160
▪ via Connected, FastEthernet0/0
▪ P 192.168.2.0/24, 1 successors, FD is 30720
▪ via 10.0.0.2 (30720/28160), FastEthernet4/0
▪ P 192.168.3.0/24, 1 successors, FD is 30720
▪ via 20.0.0.2 (30720/28160), FastEthernet5/0
▪ P 192.168.4.0/24, 1 successors, FD is 33280
▪ via 10.0.0.2 (33280/30720), FastEthernet4/0

13. ▪ In EIGRP Topology Table, there are some route states. These states
are:
– Active State : The state that router has no route and trying to find
a route. Queries are sent through the all interfaces.
– Passive State : The state that the router has chosen a route.
– SIA (Stuck In Active) : The mode when no reply is get in 3 minutes.
The cause of Stuck In Active can be any high CPU and memory
usage, packet drops, unidirectional links etc.

14. ▪ Routing Table is the table which keeps the Best Routes to any destinations.This Best Routes are
Successors
▪ You can see the RoutingTable with “show ip route eigrp” command.
▪ An example output of “show ip route eigrp” command is given below:
▪ Router#show ip route Eigrp
▪ 30.0.0.0/24 is subnetted, 1 subnets
▪ D 30.0.0.0 [90/30720] via 10.0.0.2, 00:04:28, FastEthernet4/0
▪ D 40.0.0.0/8 [90/30720] via 20.0.0.2, 00:04:28, FastEthernet5/0
▪ D 192.168.2.0/24 [90/30720] via 10.0.0.2, 00:04:28, FastEthernet4/0
▪ D 192.168.3.0/24 [90/30720] via 20.0.0.2, 00:04:28, FastEthernet5/0
▪ D 192.168.4.0/24 [90/33280] via 10.0.0.2, 00:04:28, FastEthernet4/0

15. ▪ These tables are filled with updates. In EIGRP, updates are not
periodic. Whenever a change occurs in the EIGRP network, triggered
updates are sent about that change.
▪ EIGRP (Enhanced Interior Gateway Routing Protocol) is a reliable
protocol. It has window size one. If there is no acknowledgement that
the packet is received, then the next one is not sent. Even if any
packet is not acknowledged this packet is retransmitted at EIGRP.
After 16 unacknowledged retransmission.

16. Feasible Distance, Advertised Distance, Successor
and Feasible Successor
▪ In EIGRP (Enhanced Interior Gateway Routing Protocol), there are some
new terms that explains different routes to a destination according to
their metrics.These terms are :
• Feasible Distance
• Advertised Distance
• Successor
• Feasible Successor
▪ Let’s check these EIGRP specific terms one by one:
Feasible Distance (FD) :The cost of the route to the next hop, plus, next
hop to the destination.
Advertised Distance (AD) :The cost of the route from the next hop to the
destination.

18. ▪ Think about the basic topology above. From Router A and Router B
to the destination router, let’s check the Feasible Distance and
Advertised Distance values.
▪ As we can see above, Router A has 5 metric to its next hop Router B.
And from the Router B to the destination, the cost is 5+5 = 10. So, for
Router A, Advertised Distance is 10 (5+5) and Feasible Distance is 15
(5+10).
▪ This logic is same for Router B. Let’s think that our starting point is
Router B. At this time, Router B has 5 metric to its neighbour Router
C. And from its neighbour Router C to Destination, the cost is 5. So,
for Router B, the Advertised Distance is 5 and Feasible Distance is 5+5
= 10.

19. ▪ Now, let’s check the other two important terms, successor and feasible
successor.
– Successor :The Best Route to any destination.The route that has the
best Feasible Distance.
Feasible Successor :The Second Best Routes to any destination.

21. ▪ we have three ways from Router A to the destination router. These three
ways are written differently in different tables of EIGRP.
▪ In the Neighbour Table of Router A, all the neighbours connected to Router
A exist. As you can see three of them is in the NeighbourTable.
▪ After route calculations, the topology table is build and Feasible Distances
and Advertised Distances are determined. According to these values, The
Best Route to the destination, Successor, is selected. And the second best
route is also selected as Feasible Successor. These two routes are in
TopologyTable of Router A.
▪ Lastly, in the Routing table, the Successor exists. the Best Route to the
destination. In other words, the lowest metric. If Successor goes down,
then Feasible Successor become active in Routing Table. This backup best
route provides fast convergence characteristic to EIGRP.

22. EIGRP Packet Types
▪
EIGRP (Enhanced Interior Gateway Routing Protocol) uses EIGRP
messages to establish and maintain the EIGRP neighbourship. EIGRP
uses five packet types for this messages.These packet types are :
– Hello,
– Query,
– Reply,
– Update,
– ACK.

24. ▪ Let’s check these EIGRP packet’s role in EIGRP Operation.
▪ • Hello Packet : Used to establish and maintain the neighbourship. It is a
keepalive also.
• Update Packet : Used to send routing updates.
• Query Packet : Used to ask for any routing update, requests an update.
• Reply Packet: Used as a response to the Queries.
• ACK Packet : Used as a feedback to the Update, Query or Reply packets as a
feedback mechanism. It is not used for Hello Packets and Ack Packets.

25. EIGRP Neighbourship Establishment
▪ EIGRP routers establish neighborship between the EIGRP neighbors
and the full EIGRP network is build firstly with this establishments. To
establish the EIGRP Neighbourship, Hello Packets are sent between
neighbor EIGRP routers periodically. Let’s check an adjacency states
step by step.

27. ▪ As we can see above, for a neigbourship firstly Router A sends a Hello
to the EIGRP Multicast address (224.0.0.10). When Router B receives
this Hello, Router B sends its full routing table to the Router A. Beside
this, Router B also sends Hello packet to the EIGRP multicast
address(224.0.0.10). After receiving Full Routing Table of Router B,
Router A send an ACK packet. Router A also send its own Full Routing
Table to the Router B. When Router B receives this, it also send ACK
packet.
▪ To establish the EIGRP Neighborship, EIGRP neighbors must have
the same K values. But unlike OSPF, the Hello and Hold Down times
can be different in both end (Remember! In OSPF they must match
for neighbpurship establishment).

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