OSPF: send hello to all?

mguy

When in non-DR router is converged with routers in its area, does it send hello packets to all packets in its area or just the DR?

elderkai

OSPF-enabled interfaces send Hello's to routers directly adjacent to them to form neighbor relationships. Their route updates, LSAs, however are only sent to the DR.

mguy

does that mean non-DR OSPF send out hellos to all of the routers in the same area they are in?

xbuzz

Only the ones they are directly (Physically) connected to.

mguy

xbuzz wrote: »

Only the ones they are directly (Physically) connected to.

I thought it multicasts Hello packets?

If it multicasts this then all of the routers will essentially receive it -- is this not true?

carmelenigma

You're thinking of Broadcast (one to all) Multicast is one to some. The Hello packets serve as a check to say " Hey are you still alive? I am alive" and its only sent to the router's neighbors.

mguy

I thought multicast is for anyone who is subscribed to the multicast address (in this case 224.0.0.5). So any neighbor (even not directly connected) will "listen-in" to the multicast and will ultimately receive it.

Am I wrong?

carmelenigma

Well think about if a router's multicast message is sent out on 224.0.0.5 or any other address it's neighboring router would receive that information. Since Hello packets are meant to help establishing adjacencies and telling the router that the link to itself is still alive, the other routers have no reason to pass that information on to their own neighbors. They will just pass on their own hello packets to their own neighbors. If every router in ospf broadcasted it's links as being up there would be no need for lsa packets being sent to the DR.

So in a sense you are right, as in all of the routers are listening on 224.0.0.5 however the neighbor routers that receive hello packets arent forwarding this information on.

NetworkVeteran

carmel, great guess and very close, but the implementation is even simpler than that.

The multicast addresses from 224.0.0.0 - 224.0.0.255 are special local multicast addresses. Hello packets sent to 224.0.0.5 normally reach all devices on the local subnet, but no further.

Given this topology--

RouterA(OSPF)---L2SWITCH---RouterB(NO-PROTOCOLS)

---

RouterC(OSPF)

If RouterA sends a Hello packet to 224.0.0.5, it will get as far as RouterB, who will ignore it.

Forsaken_GA

In a multiple access topology, the hello's are sent to the multicast address, and all OSPF routers in the area will see them. A non-DR/BDR router will form a full adjacency ONLY with the DR/BDR, it will stop at the 2WAY state with all other OSPF routers. This means that it will flood it's LSA's only to the DR and BDR, since it hasnt' reached the FULL state with any of it's other neighbors.

A DR/BDR forms a FULL adjacency with every single router on the multiaccess link.

Forsaken_GA

mguy wrote: »

does that mean non-DR OSPF send out hellos to all of the routers in the same area they are in?

Watch your terminology. An area is separate from a link type. You can have multiple access links and point to point links all in the same area.

Adjacencies are formed on a link by link basis, not on an area by area basis. The same router could be a DR on one link, a DROTHER on another link, and have absolutely no DR because it's on a point to point type on yet another link, and all of these links can be in the same area.

Forsaken_GA

NetworkVeteran wrote: »

carmel, great guess and very close, but the implementation is even simpler than that.

The multicast addresses from 224.0.0.0 - 224.0.0.255 are special local multicast addresses. Hello packets sent to 224.0.0.5 normally reach all devices on the local subnet, but no further.

Given this topology--

RouterA(OSPF)---L2SWITCH---RouterB(NO-PROTOCOLS)

---

RouterC(OSPF)

If RouterA sends a Hello packet to 224.0.0.5, it will get as far as RouterB, who will ignore it.

This is a good point. There is a distinct difference between multicast communication and multicast routing. One does not automatically indicate the other.

For all intents and purposes, OSPF multicast communication is link local.