what is the difference between adjacency and neighborship?

kobem

my questions look stupid and dull but , this thing makes me crazy
in eigrp and ospf.

dmaftei

In OSPF two routers are neighbors when they are "in touch" by means of the Hello protocol. To be able to be "in touch" the two routers must have an interface to a common network (e.g. they're both on the same ethernet segment, or they have a point-to-point link) and some things must be configured identically on those interfaces.

Two routers are adjacent when they are neigbors and they keep their link state databases synchronized. On point-to-point links, point-to-multipoint links and virtual links, two neighboring routers always form an adjacency. On broadcast and nbma, only some neighboring routers form an adjacency.

Perhaps the easiest way to figure this is to picture four routers, r1, r2, r3 and r4, connected to a common ethernet. Provided they're all configured properly, you will have six "neighborships": r1-r2, r1-r3, r1-r4, r2-r3, r2-r4, and r3-r4. Now, assuming r1 is the DR, and assuming there's no BDR, you'll have three adjacencies: r1-r2, r1-r3, and r1-r4.

HTH

DirtySouth

My understanding is the term "Adjacency" is used mainly in regards to EIGRP and the term "Neighbor" us used mainly for OSPF. Is this a good way to look at it?

EdTheLad

DirtySouth wrote:

My understanding is the term "Adjacency" is used mainly in regards to EIGRP and the term "Neighbor" us used mainly for OSPF. Is this a good way to look at it?

No, neighbor basically means physical connectivity wheras an adjacency is regarding database synchronization.

Paul Boz

Are CDP neighbors always connected, or can they be cisco equipment that's on the same internetwork?

bmauro

CDP will only show you Cisco devices that are directly connected to the device you are using CDP on.