Hi Devis,
Thank you, you are welcome.
Does the EIGRP updates contains the least bandwidth and cumulative-delay
seperately by the neighbor router like AD for a given network ?
If you are asking whether the EIGRP Update contains the four (actually five) metric components - bandwidth, delay, reliability, load, MTU - separate or whether they are already "mixed" together into a single metric value, then the answer is: the metric components are always advertised as separate values. It is up to the receiving routed to compute the resulting metric from the first four components but the EIGRP Update will always convey the individual components, never the resulting metric. That is actually one of the reasons why two EIGRP routers must absolutely agree on their K-values to compute the resulting metric in the same way, otherwise, each of them would arrive at a different result.
So if a network N goes down , it is R1's responsibility to let all the
other routers to know about it since the network N is directly connected
to it .
Basically, yes. In essence, a router that detects a metric change - both increase and decrease - must inform its neighbors about the event. It's as simple as that. A network going down is the same as having the metric of the network to grow to infinity, as both indicate that the network is unreachable.
What I understand is that in EIGRP this is been happening through query
process , to let the other router to realize that the network is down .
Is that correct ? .
There is a difference in EIGRP between an update process and a query process. A router sends an update when
- the metric of a route has decreased
- the metric of a route has increased but the router still has a valid successor to the destination
In essence, update means that the metric of a route has changed but the router sending the update still knows about a valid and usable route to the destination. Therefore, there is no need to ask the neighbors for an alternate path. It is sufficient to simply let them know about our own modified distance.
A query is sent when
- the metric of a route has increased and there is neither successor nor feasible successor available anymore for the route
- the metric of a route has increased and the next shortest path is provided by a neighbor that does not qualify as a feasible successor (i.e. it does not pass the feasibility condition check but still apparently provides the next shortest path)
In essence, when a router loses its next hop to a destination and is not sure which other neighbor shall it use to reach the destination without creating a routing loop, it has to ask by sending a query to its neighbors.
Assume that the link comes up and could you please tell what happened as per my 2nd question ?
The process is fairly simple in that case: the R1 will inform R2 and R4 about the newly connected network N using an Update packet. R2 and R4 will both learn about a new network, compute the metric (adding the delay and possibly modifying the bandwidth), store the route in their routing tables, and they will subsequently send the update further to R3, having modified the delay metric component and possibly the bandwidth metric component as well. R3 will receive these two (uncoordinated) updates, increase the delay, update the bandwidth if necessary, compute the metric, choose the best path and install it in its routing table. In addition, R2 and R4 will advertise this network back to R1 as unreachable (split horizon with poisoned reverse) and so will R3 towards its next hop (possibly to both R2 and R4 if all links are of the same speed and delay).
Best regards,
Peter
