DMVPN Tunnels - DMVPN Configuration
10 important questions on DMVPN Tunnels - DMVPN Configuration
What are the required steps to configure DMVPN on a Hub router?
- Create the tunnel interface
interface tunnel tunnel-number - Identify local source of the tunnel
tunnel source ip-address | Interface-id - Configure DMVPN as an mGRE tunnel
tunnel mode gre multipoint - Allocate IP address for DMVPN network (tunnel)
ip address ip-address subnet-mask - Enable NHRP on the tunnel interface and uniquely identify the DMVPN tunnel for the virtual interface.
ip nhrp network-id 1-4294967295 - For Phase 3 DMVPN: Enable NHRP redirect functions
ip nhrp redirect
What are the optional steps to configure DMVPN on a Hub Router?
- Define the tunnel key
tunnel key 0-4294967295 - Enable multicast for NHRP
ip nhrp map multicast dynamic - Define tunnel bandwidth
bandwidth 1-10000000 - Configure mtu for tunnel interface
ip mtu mtu - Define TCP Maximum Segment Size (MSS)
ip tcp adjust-mss mss-size
What are the required steps to configure a DMVPN Spoke router?
- Create the tunnel interface
interface tunnel tunnel-number - Identify local source of the tunnel
tunnel source ip-address | Interface-id - Identify the tunnel destination (DMVPN Hub NBMA ip address)
tunnel destination ip-address - Allocate IP address for DMVPN network (tunnel)
ip address ip-address subnet-mask | DHCP - Enable NHRP on the tunnel interface and uniquely identify the DMVPN tunnel for the virtual interface.
ip nhrp network-id 1-4294967295
- Specify the address of one or more NHRP NHSs (DMVPN Hub Routers)
ip nhrp nhs nhs-address nbma nbma-address [multicast]
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What are the optional steps to configure a DMVPN Spoke Router?
- Define the tunnel key
tunnel key 0-4294967295 - Define tunnel bandwidth
bandwidth 1-10000000 - Configure mtu for tunnel interface
ip mtu mtu - Define TCP Maximum Segment Size (MSS)
ip tcp adjust-mss mss-size
What is the command to show the status of the DMVPN tunnels?
What is the command to show the NHRP cache and what does it contain?
show ip nhrp [brief]
It contains:
- Network entry for host of network and the tunnel ip address to NBMA ip address.
- Inteface number, duration of existence and when it will expire.
- The NHRP mapping entry type.
What are the required steps to confgure a DMVPN Spoke Router for Phase 3 DMVPN?
- Create the tunnel interface
interface tunnel tunnel-number - Identify local source of the tunnel
tunnel source ip-address | Interface-id - Configure the DMVPN tunnel as a GRE Multipoint tunnel
tunnel mode gre multipoint - Allocate IP address for DMVPN network (tunnel)
ip address ip-address subnet-mask | DHCP - Enable NHRP on the tunnel interface and uniquely identify the DMVPN tunnel for the virtual interface.
ip nhrp network-id 1-4294967295 - Enable NHRP Shortcut
ip nhrp shortcut - Specify the address of one or more NHRP NHSs (DMVPN Hub Routers)
ip nhrp nhs nhs-address nbma nbma-address [multicast]
What are the main differences in configuration between DMVPN Phase 1 and DMVPN Phase 3?
- For Phase 3 the ip nhrp redirect command is added on the Hub
- For Phase 3 the ip nhrp shortcut command is added on the Spoke
- For Phase 3 the tunnel is configured as mGRE with the command tunnel mode gre multipoint
- For Phase 3 the tunnel destination command is not used
How is NHRP authentication configured and what is important to note about it?
ip nhrp authentication password
It is important to note that it is a weak form of authentication because the password is stored in plaintest. Hence it is primarily used to ensure correct tunnel form.
What is a problem that can occur with the NHRP Unique message flag and how can it be prevented?
- It prevents an NHC to register with a different NBMA ip address than the NHS has stored in it's cache.
- If NHC has a DHCP address this can cause issues if it receives a different address.
- the command ip nhrp registration no-unique can be used to prevent the NHC to use the Unique flag in registration request packets to the NHS.
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