EIGRP –Enhanced Interior Gateway Routing Protocol. It Is the Enhanced version of IGRP. It plays an important role in identifying the best and shortest path to deliver the packet or Data at their destination network.
It is a Cisco-proprietary Hybrid routing protocol. Why it’s called Hybrid, it should be either Distance Vector or Link State, but how it works as Hybrid mode, its features, Metrics everything that you know about we will discuss here.
Why EIGRP called Hybrid
It incorporating the features of both Distance-Vector and Link-State routing protocols. Let’s understand first what is Link state and Distance vector Routing?
Routing Protocol that Calculates the distance (metric) to reach destination networks based on hop count called Distance Vector Routing Protocol
Routing Protocol that Exchange information about the state of their directly connected links with all other routers in the network is called Link State Routing Protocol.
It send the routing update for changes only not entire routing table as Link State feature and use DUAL algorithm to select alternative path to reach destination if primary link fails as Distance Vector feature. That’ why it is called Hybrid Protocol.
What is the EIGRP routing protocol:
- It is Cisco Proprietary
- Use DUAL algorithm for feasible path selection.
- It will form neighbour relationships with adjacent routers in the same Autonomous System (AS).
- Use Multicast address 224.0.0.10
- RTP- Reliable Transport protocol to ensure most packet delivery.
- Not share complete route table, only share if any changes.
- It is classless protocol.
- Internal AD is 90.
- External AD is 170
- Use MD5 as Authentication
- It has a maximum hop-count of 224, though the default, maximum hop-count is set to 100.
- Use IP Protocol 88- produces reliable updates by identifying its packets using IP protocol 88. Reliable, in a networking context, means that the receiver acknowledges that the transmission was received and understood.
- Protocol-dependent modules—supports several routed protocols independently IP, IPX, AppleTalk
- Fast Convergence
- Support VLSM
How many types of packets are there in EIGRP?
It employs five packet types:
- Hello— Multicast-Identifies neighbours. Hellos are sent as periodic multicasts and are not acknowledged directly.
- Update— Unicast or Multicast -Advertises routes. Updates are sent as multicasts only when there is a change.
- Ack— Unicast- Acknowledges receipt of an update.
- Query— Multicast -Used to ask about routes for which the previous best path has been lost. If an update indicates that a path is down, multicast queries are used to ask other neighbours if they still have a path. If the querier does not receive a reply from each of its neighbours, it repeats the query as a unicast to each unresponsive neighbor until it either gets a reply or gives up after sixteen tries.
- Reply— Unicast- Used to answer a query. Each neighbour responds to the query with a unicast reply indicating an alternative path or the fact that it does not have a path.
How many tables are in EIGRP
- Neighbour Table– The neighbour table is built from hellos and used for reliable delivery.
- Topology Table- The topology table contains routing information for best paths and loop-free alternatives.
- Routing Table- EIGRP places best routes from its topology table into the common routing table.
We can understand these table like… first create Neighbor table, based on Neighbor information create Topology table. Once Topology table created then EIGRP have all connected Neighbor and link, distance information. Now EIGRP will create Routing table.
Metrics
- It uses a sophisticated metric that considers bandwidth, load, reliability, MTU and delay
EIGRP ‘K’ Values
K Defaults Packet Types
K1 1 1 Update
K2 0 3 Query
K3 1 4 Reply
K4 0 5 Hello
K5 0 8 Ack
Metric Formula
Hello Timer
- 5 sec and 60 sec, Hold time–15 sec and 180 sec.
A neighbor table is constructed from the EIGRP Hello packets, which includes the following information:
- The IP address of the neighbouring router.
- The local interface that received the neighbour’s Hello packet.
- The Hold timer.
- A sequence number indicating the order neighbours were learned.
What is DUAL in EIGRP
DUAL” stands for “Diffusing Update Algorithm. It is algorithm which EIGRP use to calculate and find the Loop free Successor and Feasible Successor and maintain Routing Table. It helps to identify the best route to reach destination and alternative path if active link goes down.
It helps to create topology. Let’s discuss how DUAL works:
- Feasible Distance (FD) — The route with the lowest metric to each network will become the Feasible Distance (FD). I remine it like “First device to destination”. It is calculated based on various factors such as bandwidth, delay, reliability, and load.
- Advertised Distance (AD)—from neighbor to destination— I remine it like “Another Device to destination”.
- This is current best path and It should be less than of current AD.
- Feasible Successor- To allow convergence to occur quickly if a link fails, EIGRP includes backup routes in the topology table called Feasible Successors (FS). A route will only become a Successor if its Advertised Distance is less than the current Feasible Distance. This is known as a Feasible Condition (FC).
If Router-B wants to reach Router-E, then let calculate the FD and AD.
| Route Via | FD (Feasible Distance) | AD (Advertised Distance) |
| RTR-A | 90+90+120+60=360
BtoA+AtoB+BtoD+DtoE=360 |
90+120+60=270
BtoA+BtoD+DtoE=270 |
| RTR-C | 45+90+60=195
BtoC+CtoD+DtoE=195 |
90+60=150
CtoD+DtoE=150 |
| RTR-D | 120+60=180
BtoD+DtoE=180 |
60
DtoE |
Stuck-in- Active- SIA
Feasible Successors provide EIGRP with redundancy without any re-converge if a topology change occurs. But, If no Feasible Successor exists and a link fails, a route will enter an Active (converging) state until an alternate route is found that is called Stuck-In-Active.
Router status should be in Passive mode. Active mode means link is down and router is actively searching new redundant or feasible successor which is not good.
Load Balance by Variance
By default, EIGRP will automatically load-balance across equal-metric routes (four by default, six maximum).
Also supports load-balancing across routes with an unequal metric called Variance. It allows load balance between two unequal path cost.
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FAQ
Q. EIGRP Stand for?
A. EIGRP stand for Enhanced Interior Gateway Routing Protocol. It Is Enhanced version of IGRP.
Q. Why EIGRP called Hybrid?
A. EIGRP has incorporated both Link state and Distance Vector routing protocol features.
Q Is EIGRP Cisco Proprietary?
A. Yes, EIGRP Cisco Proprietary
Q. What all table EIGRP created?
A. EIGRP maintained 3 Table, Neighbor Table, Topology Table, Routing Table.
Q, What is EIGRP Matrices?
A. EIGRP matrices are bandwidth, load, reliability, MTU and delay
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