DMVPN Tunnels - Next Hop Resolution Protocol (NHRP)
8 important questions on DMVPN Tunnels - Next Hop Resolution Protocol (NHRP)
What is Next Hop Resolution Protocol (NHRP)
- Method to provide address resolution for hosts or networks for nonbroadcast multiaccess (NBMA) networks.
- Method for devices to learn the protocol and NBMA network, allowing them to communicate directly with each other.
- Client/server protocol
- Over directly connected or disparate networks.
What is the NHRP next-hop server (NHS)?
- NHRP Next-Hop Servers (NHSs) are responsible for registering addresses or networks.
- Maintaining a NHRP repository.
- Replying on queries received by Next-Hop Clients (NHCs)
What is the NBMA address and the Protocol addres when talking about DMVPN/GRE.
- The NBMA address is the ip adress on the transport network, or underlay network.
- The protocol address is the ip address on the overlay network.
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Why does DMVPN need NHRP?
- Because DMVPN uses mGRE it needs a way to map tunnel ip addresses to to the transport (Underlay) network ip address.
How does DMVPN use NHRP?
- DMVPN spokes are Next-Hop Clients and statically configured with the IP address of the DMVPN hub (Next-Hop Servers)
- Spokes can register their tunnel and NBMA address to the Hub using NHRP.
- Spokes use the NHRP messages for the necessary information to locate each other and build a spoke-to-spoke DMVPN tunnel.
- Also allows spoke to locate a remote network.
What needs to be included in NHRP packets?
- Source NMBA address.
- Source Protocol Address.
- Destination Protocol Address.
- NHRP Message Type.
What are the 5 NHRP Message Types?
- Registration
Send from NHC (DMVPN Spoke) to NHS (DMVPN Hub) - Resolution
Request and Reply messages to exchange location of egress router for a destination. - Redirect
Essential component of DMVPN Phase 3. Intermediate router can notify the encapsulator of a more optimal path for a specific network (Spoke-To-Spoke. - Purge
Are sent to remove a cached NHRP entry. Typically sent by NHS to an NHC. - Error
Used to notify the sender of an NHRP packet that an error has occurred.
What are the 6 NHRP Message Extensions?
- Responder Address
Used to determine the address of the responding node for reply messages. - Forward transit NHS record
Contains a list of NHSs that the NHRP Request packet may have traversed. - Reverse transit NHS record
Contains a list of NHSs that the NHRP reply packets may have traversed. - Authentication
Conveys authentication information between NHRP speakers. Transmitted in plaintext. - Vendor Private
conveys vendor private information between NHRP speakers. - Network Address Translation
Used to detect the claimed NBMA address (Inside Local Address)
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