Enhanced IGRP or EIGRP is a cisco owned, hybrid routing protocol, that is designed to route traffic within the autonomous system.
It’s a hybrid protocol, which means,. it combines features from both link-state and distance vector protocols
As a distance vector, it sends out an update that contains all the routing information from the perspective of the advertising router.
And as a link state, it synchronizes routing tables between the neighbours at startup… and sends out only the partial updates when the toplogy gets changed.
On the surface, it has similar working as an OSPF protocol.
- Become the neighbours –
A neighbour Sends a hello packet, another one, checks the requirements and responding back with a hello, to become the neighbours.
A periodic hello packet is sent back and forth to ensure connection is alive.
Second step is to.. Exchange the network information.
After becoming neighbours, routers share their entire network information using a update packet – followed by an Acknowledgment.
And, when the link is changed…
only the partial updates are shared between the routers.
And to find the route.. all EIGRP routers have these tables.
It uses three tables: Neighbours Table, Topology Table & Routing Table.
Neighbour tables keep tracks of all of the adjacent routers, with their IP address and port they are connected to.
Topology table keeps track of all of the networks – that were advertised by the neighbours, along with routes to reach those networks.
This value is a metric or a cost… and
EIGRP uses a Diffusing Update Algorithm to calculate a metric for each route.
The path with the lowest metric is chosen as the successor (the best route) and is placed in the routing table.
And,, the second best route is called a feasible successor – which is a backup route in case the successor route fails.
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For example… In this network… To reach from A to C… the successor route is A-D-C
and the feasible successor is A-B-E-C
The metric to select the best route.. . is calculated using a formula:
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and there are multiple forrmulas.. in case other network factors have to be taken into consideration as well.
And finally in a scenario, where the network connection drops. The link goes from passive mode (which is working mode) to active mode, which means.. it is actively fucked.
And the router sends a query packet to the neighbours regarding the alternative routes.
If the route is available, router replies…
However,, if there is no route available – it is considered a dead network. ANd the network is removed from the routing table.
So to summarize, EIGRP quickly adapts to network changes by using backup links and sending only necessary updates, making it a fast, scalable, and reliable routing protocol.
This is all for EIGRP.. I’ll see you in the next session.