EIGRP

EdTheLad

Reading sybex it explains how EIGRPis a quite protocal and doesnt send routing updates.It goes on to say that the only way for EIGRP to learn about neighbours going down is via RTP i.e. since the router has a list of neighbours and if they dont reply to the multicast hello it will try 16 times with a unicast hello and if still no reply the neighbour will be declared dead.

Now when i read on cisco site,it explains how on a broadcast network i.e. ethernet Ack are not required for hello packets and the way EIGRP nows if a neighbour goes down is by sending a timestamp with the hello packet and if another hello packet isnt received to reset the holddown the neighbour is declared dead.

So whats the deal with the Sybex version? Is ACK for hello protocol enabled by default? Which answer should be used if a question for this came up on CCNA?

kplab

There are five types of EIGRP packets: hello, query, update, reply, and ack. Acknowledgement is required for query, update, and reply packets. If an acknowledgement is not received after 16 retransmissions of a query/update/reply packet, the neighbor is declared dead (possibly what Sybex is referring to).

Besides, EIGRP also sends hello packets to its neighbors regularly (hello time interval). If no hello packet is received from a neighbor before the hold timer expires, the neighbor is also declared dead (what cisco site is referring to).

Yankee

Rather than an ack your router looks for its ip address in the hello from its neighbor. This means the neighbor has received its hello and an adjacency can be formed. Of course this is true for both sides(they both must see their ip in the neighbor's hello for an adjacency to be formed). Then and only then can routing tables be exchanged.

Yankee

EdTheLad

Yankee wrote:

Rather than an ack your router looks for its ip address in the hello from its neighbor. This means the neighbor has received its hello and an adjacency can be formed. Of course this is true for both sides(they both must see their ip in the neighbor's hello for an adjacency to be formed). Then and only then can routing tables be exchanged.

Yankee

Yes, thanks guys this is all cleared up.I just taught what the point of sending the 16 unicast requests, as i would imagine, the next hello would or would not arrive before the unicasts indicating the neighbour was dead or alive.But since the router is waiting on the Ack probably it sends the 16 unicast one after the other in a very small time period.
Sometimes i just analyis too much.