Lecture 3 - EIGRP

Reported Distance (RD)

Distance from the neighbor of source to the destination

Feasible Distance (FD)

Distance from the source to destination 

Successor

The route with the best metric to the destination and will be added to the routing table

Successor route

The route with the best metric to the destination

Feasibility condition

Next hop router must have a RD less than the FD of the current successor route

Feasible successor (FS)

backup route if the successor fails. The FS must have a RD less than the successor FD (to prevent routing loops)

Active Routes

a route that is undergoing re-computation

Passive routes

route that is in an operational/stable state

Default EIGRP metric calculation

= 𝐵𝑊𝑚𝑖𝑛 + 𝐷𝑒𝑙𝑎𝑦 ∗ 256

What does EIGRP do when it loses it’s best path

When EIGRP detects that it has lost its successor for a path, the feasible successor instantly becomes the successor route, providing a backup route. When no feasible successor exists, EIGRP DUAL performs a “Diffusing” computation

Diffusing Update Algorithm (DUAL)

when a route fails and a router does not have a feasible successor route. It causes the routers to send EIGRP Query and Reply messages to discover alternate loop-free routes.

Stuck-in-Active

When in this state, the router that lost the network sends out a query message to its first neighbors asking for a way to get back. If those neighbors do not know, it will then send a message to its neighbors (second). The process will go on until the neighbors can not ask any other router. From the bottom, the routers will send a reply saying they do not have a way to get back.

Summarization

Greatly reduces the occurrence of SIA errors

Stub Networks

Configuring the remote router as stub EIGRP routers 

distribution list

Filter routes from being advertised to neighbors