neighbours and adjacencies

[carl_townshend]

I have just read a cisco ospf article regarding neighbors and adjacencies, I always thought these were the same thing but it appears I am wrong, Do ospf/eigrp routers etc form direct neighbours only with connected routers ? and then the 2nd step is 2 make them adjacent.... I am confused with the process, can anyone put me right on this ?

Thanks

Carl

[ankurbhasin]

Hi Carl,

Neighbors and Adjancy are 2 different things. OSPF routers form neighbors with all the routers on direct interface having same subnet and only form adjancy with DR & BDR.

So in short all the routers on same subnet in a single broadcast domain after meeting some specific conditions will form a neighbor with each other but will only be adjacent to DR & BDR.

First the routers form neighbors in 2 way state and once done they start building adjancy process with DR & BDR once they are elected.

Have a look at this link

http://www.cisco.com/warp/customer/104/1.html#t18

HTH, if yes please start rating helpful post.

Ankur

[carl_townshend]

I cannot view this link ?

[carl_townshend]

what exactly is the difference between them 2 then ? I dont know what the adjancey is ? is this the same for eigrp ?

[ankurbhasin]

Hi Carl,

Nice to see that you have started rating helpful information.

Routers that share a common segment become neighbors on that segment. Neighbors are elected via the Hello protocol. Hello packets are sent periodically out of each interface using IP multicast (Appendix B). Routers become neighbors as soon as they see themselves listed in the neighbor's Hello packet. This way, a two way communication is guaranteed

Adjacency is the next step after the neighboring process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process.

In order to minimize the amount of information exchange on a particular segment, OSPF elects one router to be a designated router (DR), and one router to be a backup designated router (BDR), on each multi-access segment.

The BDR is elected as a backup mechanism in case the DR goes down. The idea behind this is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router exchanges information with the DR and BDR. The DR and BDR relay the information to everybody else.

In mathematical terms, this cuts the information exchange from O(n*n) to O(n) where n is the number of routers on a multi-access segment.

HTH, if yes please rate the post.

Ankur

[carl_townshend]

So can they be neighbours and not adjacent, Once 2 routers are neighbours what happens then, do they look for the DR and then send updates to the DR, this then tells the other routers of the changes ?,

[ankurbhasin]

Hi Carl,

Yes they can be neighbors but not adjacent. But if they are adjacent they are 100% neighbors.

Once 2 routers are neighbors they start electing DR and BDR. And once DR and BDR is elected after going certain election procedure , rest of the routers will form adjancy with DR and BDR and they will alrady be neighbirs with each other.

Now any changes in topology will be updated by a router to their DR router and DR in turn will update other routers about the changes.

HTH, if yes please rate all post.

Ankur

[carl_townshend]

So in what way would they be 100% neighbours ? I thought this was the case anyway, what can neighbours do ?

[ankurbhasin]

Hi Carl,

Unless and until they form neighbors they do not go for adjancy process.Each neighbor generates a single Type 1 LSA to describe the status and cost (metric) of all links on the router. This LSA is flooded to each router in the OSPF area

For suppose there are 5 routers in any area. They all will become neighbors first exchan=ging hello packet and then any 2 out of 5 will become DR and BDR and rest 3 will form adjancy with DR and BDR. But will form neighborship with all 5.

So if you check in any router other then DR & BDR you will see 5 neighbors but only 2 adjancency. But if you check in DR & BDR you will see 5 adjancy and 5 neighbors. Because once router is elected as dr and bdr they form adjancy will all the routers including themselves also and because they are DR and BDR they must be neighbors first.

I hope I am clear with my explaination.

HTH, if yes please rate all post.

Ankur

[carl_townshend]

Hi Ankur, I am nearly there with this, I just want to know what the adjacency means within the networking terms ?

[ankurbhasin]

Hi Carl,

Adjacency in terms of ospf means that router will only send any topology change information to the router to whom it is adjacent and not to all the routers which means any router will send any topology change information to DR router and BDR and inturn they will send it to all other routers directly connected to them.

HTH, if yes please rate the post.

Ankur