OSPF network type

[cisc0.ameer]

hello when  asked for DMVPN configuration(phase 3):

Ensure spokes establish OSPF adjacency through a tunnel , and without attempting  to elect any DR ,

does it mean Network type should be point-2-Multipoint ?

and if Yes ? Why?

thanks and regard

[Giuseppe Larosa · Accepted solution]

Hello cisco0.ameer,

your understanding is correct the sentence "without attempting to elect a DR" clears points to using network type point to multipoint.

In point to multipoint network type a DR is not elected as the network is seen as a collection of unnumbered point to point links this is why no DR/BDR election occurs.

The good news is that multicast hellos are still used and you don't need to configure manual neighbors under router OSPF process.

The DMVPN is a virtual flat network by using NHRP resolution services, however each Spoke establish routing adjacencies only with the Hub, because only the Hub to Spoke tunnel is permanent.

Spoke to Spoke tunnels are setup on demand when traffic needs to be sent to another spoke.

DMVPN Phase 3 makes use of NHRP redirect message sent by the HUB / NHRP NHS server for a destination.

This is implemented in CEF.

This is why using network broadcast is not needed in DMVPN phase3.

Hope to help

Giuseppe

[cisc0.ameer]

thanks too much sir

regards

[omz]

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@cisc0.ameer wrote:

Ensure spokes establish OSPF adjacency through a tunnel , and without attempting  to elect any DR ,

does it mean Network type should be point-2-Multipoint ?

and if Yes ? Why?

---

Yes - with PtMP OSPF network type DR/BDR are not elected.

PtMP network type in OSPF is used when there no full connectivity between routers.

Multicasts are used to detect neighbors. Every router advertises its own IP address on the PtMP network with /32 mask.

In Phase 3, the route next hop is still the DMVPN hub but there is a CEF rewrite due to NHRP. This helps with direct spoke to spoke traffic. First packet goes through the hub but after that traffic goes direct to spoke. 

[Martin L]

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