Configuring Unicast & Multicast over point-to-multipoint GRE

Feature history for unicast and multicast over point-to-multipoint GRE

This table provides release and platform support information for the features explained in this module.

These features are available in all the releases subsequent to the one they were introduced in, unless noted otherwise.

Release

Feature Name and Description

Supported Platform

Cisco IOS XE 17.18.1

Unicast and Multicast over Point-to-Multipoint GRE: The Unicast and Multicast over Point-to-Multipoint GRE feature allows to configure mGRE at the hub site and normal point-to-point GRE configuration at the spokes.

Cisco C9610 Series Smart Switches

Unicast and multicast over point-to-multipoint GRE

A point-to-multipoint GRE tunnel is a tunneling mechanism that

  • supports both unicast and multicast traffic over a single GRE interface,

  • enables dynamic routing protocol support across multiple remote endpoints, and

  • allows efficient distribution of traffic in hub-and-spoke network topologies.

Unicast transmission sends data from one sender to one receiver, whereas multicast transmission sends data from one sender to multiple receivers simultaneously. Point-to-multipoint GRE tunnels can encapsulate both types of traffic, enabling flexible and scalable communication in large, distributed networks.

Next Hop Resolution Protocol (NHRP)

A Next Hop Resolution Protocol (NHRP) is a network protocol that

  • dynamically maps non-broadcast multiaccess (NBMA) network endpoints,

  • enables systems to learn the NBMA physical addresses of other systems on the network, and

  • allows direct communication between NBMA-attached systems, avoiding manual tunnel configuration.

Next Hop Server (NHS): The hub that maintains the NHRP database of spoke public interface addresses.

Next Hop Client (NHC): The spokes that register and query addresses with the NHS.

Multipoint GRE tunnels

A multipoint GRE tunnel (mGRE tunnel) is a type of VPN tunnel interface that

  • enables communication between a central hub and multiple remote spokes across an IP network,

  • allows multiple tunnel destinations to share a single logical interface, and

  • supports both static and dynamic mapping methods for scaling large topologies.

Figure 1. Sample mGRE Configuration at Hub and Spokes

There are two different ways to configure mGRE on the hub and leave a normal GRE configuration on spokes:

  • Static NHRP mapping statements on the hub router

  • Dynamic NHRP mapping on the hub router

In static mappings, the hub router is manually configured with the spoke IP in the NHRP configuration and spokes are configured as point-to-point GRE tunnels. But if there are several branch routers, the configuration on the hub router becomes lengthy, and dynamic NHRP is used on the hub router. When using dynamic NHRP, the hub router requires that each of the spoke routers be configured to register with a Next Hop Server (NHS), which would also typically be the hub router. This NHS keeps track of the NHRP mappings so that the hub device knows where to send traffic (sent to multiple tunnel destinations). For this configuration to work correctly the IP address of the NHS server must also be statically mapped on spoke routers.

With the above hub-spoke topology, the only available way for spokes to send traffic to other spokes is to forward traffic through the hub. This requires an extra hop that may not be required when forwarding traffic. Each of the spokes has the ability to forward traffic directly to each other on the underlying IP network. When this happens, it will be more efficient for the spoke-to-spoke traffic to be routed directly between the spokes without having to jump through the hub router.

If both the hub and spokes are configured to use mGRE then the ability to set up dynamic spoke-to-spoke tunnels is permitted. With this configuration, each spoke still use the hub as an NHS which allows the hub to keep track of each of the spoke sites. It also allows mGRE and NHRP to work together to inform the spokes what the forwarding information is for the other spokes. This information can then be used for each of the spokes to dynamically set up mGRE tunnels between each of the other spokes, as required.

Configure unicast and multicast over point-to-multipoint GRE

To provide configurational information about unicast and multicast over point-to-multipoint GRE.

Configure unicast mGRE for a hub

Deploy a hub router interface using unicast multipoint GRE (mGRE) tunneling for scalable dynamic VPN connectivity.

Use this task when setting up a hub node in a DMVPN network using unicast mGRE to enable multiple dynamic spokes.

Before you begin

  • Ensure you have administrative (privileged EXEC) access to the router.

  • Plan your tunnel interface number, source interface, and address assignments.

Procedure

  Command or Action Purpose

Step 1

enable

Example:


Device>enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Device#configure terminal

Enters global configuration mode.

Step 3

interface tunnel tunnel-number

Example:


Device(config)#interface tunnel 1

Configures an interface and enters interface configuration mode.

Step 4

tunnel mode gre multipoint

Example:

Device(config-if)#tunnel mode gre multipoint

Configures multipoint GRE as the tunnel mode.

Step 5

ip ospf network point-to-multipoint

Example:

Device(config-if)#ip ospf network point-to-multipoint

If the underlying protocol is OSPF, execute this command to set the network type to point-to-multipoint.

Step 6

ip address address mask

Example:

Device(config-if)#ip address 10.1.1.1 255.255.255.255

Configures the IP address of the tunnel.

Step 7

ipv6 address address prefix

Example:

Device(config-if)#ipv6 address 2001:DB8:1::1

Configures the IPv6 address of the tunnel.

Step 8

tunnel source address

Example:

Device(config-if)#tunnel source TenGigabitEthernet1/0/3

Configures the source IP address of the tunnel.

Step 9

{ip | ipv6} nhrp network-id id

Example:

Device(config-if)#ip nhrp network-id 1

Defines the NHRP domain which differentiates if multiple NHRP domains (GRE tunnel interfaces) are available on the same NHRP router.

Step 10

{ip | ipv6} nhrp registration timeout seconds

Example:

Device(config-if)#ip nhrp registration timeout 30

Changes the interval that NHRP NHCs take to send NHRP registration requests to configured NHRP NHSs.

Step 11

{ip | ipv6} nhrp holdtime seconds

Example:

Device(config-if)#ip nhrp holdtime 400

Changes the number of seconds that NHRP NBMA addresses are advertised as valid in positive NHRP responses.

Step 12

{ip | ipv6} nhrp authentication string

Example:

Device(config-if)#ip nhrp authentication DMVPN

Specifies an authentication string.

Step 13

ip pim nbma-mode

Example:

Device(config-if)#ip pim nbma-mode

Configures a multiaccess WAN interface to be in non-broadcast multiaccess (NBMA) mode.

Step 14

end

Example:

Device(config-if)#end

Exits interface configuration mode and returns to priviledged EXEC mode.

Configure unicast mGRE at a spoke

Configure a spoke router for unicast mGRE to enable dynamic IPsec connectivity with hub routers.

Unicast mGRE (Multipoint Generic Routing Encapsulation) enables dynamic and scalable VPN connectivity between hub and spoke routers in a DMVPN (Dynamic Multipoint VPN) topology. At the spoke, you must map the NBMA (Non-Broadcast Multi-Access) addresses of the hubs and configure NHRP (Next Hop Resolution Protocol) settings. This configuration allows the spoke to register with the hub and participate in secure communications.

Before you begin

  • Ensure you have administrative access (privileged EXEC mode) to the spoke device.

  • Obtain the NBMA and tunnel IP addresses of the hub router(s).

  • Determine the tunnel interface number to use or confirm which tunnel interface will be configured.

  • Confirm that the device is running software that supports mGRE and NHRP.

  • (Optional) Back up existing router configurations.

Procedure

  Command or Action Purpose

Step 1

enable

Example:


Device>enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Device#configure terminal

Enters global configuration mode.

Step 3

interface tunnel tunnel-number

Example:


Device(config)#interface tunnel 1

Configures an interface and enters interface configuration mode.

Step 4

ip nhrp map ip-address nbma-address

Example:

Device(config-if)#ip nhrp map 10.0.0.1 192.0.0.1

Configures static IP-to-NBMA address mapping of a hub router on the spoke.

Step 5

{ip | ipv6} nhrp map multicast nbma-address

Example:

Device(config-if)#ip nhrp map multicast 10.0.0.2

Enables IP multicast and broadcast packets (example: routing protocol information) to be sent from the spoke to the hub.

Step 6

ip nhrp nhs nhs-address

Example:

Device(config-if)#ip nhrp nhs 192.0.2.1

Enables the spoke to send NHRP registration request to the hub.

  • Here nhs-address is the tunnel address of the hub.

Step 7

ipv6 nhrp nhs nhs-address

Example:

Device(config-if)#ipv6 nhrp nhs 2001:DB8:1::2

Enables the spoke to send an NHRP registration request to the hub. Here nhs-address is the IPv6 address of the hub tunnel.

Step 8

ipv6 nhrp map address/prefix nbma address

Example:

Device(config-if)#ipv6 nhrp map 2001:DB8:1::3 192.0.2.2

Configures static IPv6-to-NBMA address mapping of the hub on the spoke.

Step 9

end

Example:

Device(config-if)#end

Exits interface configuration mode and returns to privileged EXEC mode.

Configure unicast mGRE at the hub

Set up unicast mGRE on the hub router to enable spoke-to-spoke communication.

Procedure

  Command or Action Purpose

Step 1

enable

Example:


Device>enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device#configure terminal

Enters global configuration mode.

Step 3

interface tunnel tunnel-number

Example:


Device(config)#interface tunnel 1

Configures an interface and enters interface configuration mode.

Step 4

{ip | ipv6} nhrp map multicast dynamic

Example:

Device(config-if)#ip nhrp map multicast dynamic

Enables the NHRP server (hub) to create a broadcast/multicast mapping for the spoke when spoke routers register their unicast NHRP mapping with the hub.

Step 5

{ip | ipv6} next-hop-self eigrp number

Example:

Device(config-if)#ip next-hop-self eigrp 10

Enables the hub to use the next received hop while sending routing protocol updates of one spoke to another, so that hosts behind hosts can be reached directly.

Step 6

{ip | ipv6} split-horizon eigrp number

Example:

Device(config-if)#ip split-horizon eigrp 10

Enables routing protocol updates of one spoke to be sent to another spoke.

Step 7

end

Example:

Device(config-if)#end

Exits interface configuration mode and returns to privileged EXEC mode.

Configure multicast mGRE

Enable multicast on an mGRE tunnel interface by configuring PIM in NBMA and sparse mode.

Perform these steps after you have configured unicast mGRE.

Procedure

  Command or Action Purpose

Step 1

enable

Example:


Device>enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device#configure terminal

Enters global configuration mode.

Step 3

interface tunnel tunnel-number

Example:


Device(config)#interface tunnel 1

Configures an interface and enters interface configuration mode.

Step 4

ip pim nbma-mode

Example:

Device(config-if)#ip pim nbma-mode

Configures a multiaccess WAN interface to be in NBMA mode.

Step 5

ip pim sparse-mode

Example:

Device(config-if)#ip pim sparse-mode

Enables IPv4 Protocol Independent Multicast (PIM) sparse mode on an interface.

Step 6

end

Example:

Device(config-if)#end

Exits interface configuration mode and returns to priviledged EXEC mode.

Verifying the mGRE configuration

To verify the following configuration and operation of multipoint GRE (mGRE) interfaces on Cisco devices, using the CLI commands.

Use the show ip nhrp command to display IPv4 Next Hop Resolution Protocol (NHRP) mapping information. 

Spoke2#show ip nhrp 10.0.0.1

10.0.0.1/32 via 10.0.0.1
   Tunnel0 created 00:03:13, expire 00:06:47
   Type: dynamic, Flags: router used nhop 
   NBMA address: 192.0.0.1 

Spoke2#show ip nhrp 10.0.0.3

10.0.0.3/32 via 10.0.0.3
   Tunnel0 created 22:57:58, never expire 
   Type: static, Flags: used 
   NBMA address: 192.0.0.3

Use the show ipv6 nhrp command to display IPv6 Next Hop Resolution Protocol (NHRP) mapping information. 

HUB#show running-config | interface tunnel6

Building configuration...

Current configuration : 255 bytes
!
interface Tunnel6
 no ip address
 no ip redirects
 ipv6 address 2001:DB8:1::1/64
 ipv6 eigrp 10
 no ipv6 next-hop-self eigrp 10
 no ipv6 split-horizon eigrp 10
 ipv6 nhrp network-id 1
 tunnel source FortyGigabitEthernet1/0/19
 tunnel mode gre multipoint
end 

HUB#show ipv6 nhrp

2001:DB8:1::5/128 via 2001:DB8:1::5
   Tunnel6 created 02:37:30, expire 00:07:29
   Type: dynamic, Flags: registered nhop 
   NBMA address: 192.168.0.2
2001:DB8:1::2A7:42FF:FE83:CEA0/128 via 2001:DB8:1::5
   Tunnel6 created 02:37:30, expire 00:07:29
   Type: dynamic, Flags: registered 
   NBMA address: 192.168.0.2

HUB#

Spoke1#show running-config | interface tunnel6

Building configuration...

Current configuration : 292 bytes
!
interface Tunnel6
 no ip address
 no ip redirects
 ipv6 address 2001::5/64
 ipv6 eigrp 10
 ipv6 nhrp map multicast 192.168.0.3
 ipv6 nhrp map 2001:DB8:1::1/64 192.168.0.3
 ipv6 nhrp network-id 1
 ipv6 nhrp nhs 2001:DB8:1::1
 tunnel source FortyGigabitEthernet1/0/7
 tunnel mode gre multipoint
end

Spoke1#show ipv6 nhrp

2001:DB8:1::/64 via 2001:DB8:1::1
   Tunnel6 created 02:46:17, never expire 
   Type: static, Flags: 
   NBMA address: 192.168.0.3 
2001:DB8:1::2A7:42FF:FE83:CFE0/128 via 2001:DB8:1::2A7:42FF:FE83:CFE0
   Tunnel6 created 02:45:39, never expire 
   Type: static, Flags: nhs-ll 
   NBMA address: 192.168.0.3 
Spoke1#

Use the show ipv6 route command to display IPv6 content of the routing table. 

Spoke1#show ipv6 route 2001:DB8:1::/64

Routing entry for 2001:DB8:1::/64
  Known via "eigrp 10", distance 90, metric 27008000, type internal
  Route count is 1/1, share count 0
  Routing paths:
     2001:DB8:1::2A7:42FF:FE83:CFE0, Tunnel6
      From 2001:DB8:1::2A7:42FF:FE83:CFE0
      Last updated 00:03:07 ago
Spoke1#

HUB#show ipv6 route 2001:DB8:1::/64

Routing entry for 2001:DB8:1::/64
  Known via "eigrp 10", distance 90, metric 27008000, type internal
  Route count is 1/1, share count 0
  Routing paths:
    2001:DB8:1::2A7:42FF:FE83:CEA0, Tunnel6
      From 2001:DB8:1::2A7:42FF:FE83:CEA0
      Last updated 00:01:29 ago
HUB#

Use the debug nhrp detail command to display NHRP registration and packet related information.

Use the debug tunnel command to display tunnel state changes and packet related information.

Configuration examples for unicast and multicast over point-to-multipoint GRE

The following sections provide configuration examples for unicast and multicast over point-to-multipoint GRE.

Example: configuring unicast mGRE for hub

The show an example of how to do a basic configuration for a unicast multipoint GRE (mGRE) tunnel interface for a hub.
Device>enable
Device#configure terminal
Device(config)#interface tunnel 1
Device(config-if)#tunnel mode gre multipoint
Device(config-if)#ip ospf network point-to-multipoint
Device(config-if)#ip address 10.1.1.1 255.255.255.255
Device(config-if)#ipv6 address 2001:DB8:1::1
Device(config-if)#tunnel source TenGigabitEthernet1/0/3
Device(config-if)#ip nhrp network-id 1
Device(config-if)#ip nhrp registration timeout 30
Device(config-if)#ip nhrp holdtime 400
Device(config-if)#ip nhrp authentication DMVPN
Device(config-if)#ip pim nbma-mode
Device(config-if)#end

Example: configuring unicast mGRE at spoke

To show how to configure unicast mGRE at a spoke. 
Device>enable
Device#configure terminal
Device(config)#interface tunnel 1
Device(config-if)#ip nhrp map 10.0.0.1 192.0.0.1
Device(config-if)#ip nhrp map multicast 10.0.0.2
Device(config-if)#ip nhrp nhs 192.0.2.1
Device(config-if)#ipv6 nhrp nhs 2001:DB8:1::2
Device(config-if)#ipv6 nhrp map 2001:DB8:1::3 192.0.2.2
Device(config-if)#end

Example: configuring unicast mGRE for hub

The show an example of how to do a basic configuration for a unicast multipoint GRE (mGRE) tunnel interface for a hub.
Device>enable
Device#configure terminal
Device(config)#interface tunnel 1
Device(config-if)#tunnel mode gre multipoint
Device(config-if)#ip ospf network point-to-multipoint
Device(config-if)#ip address 10.1.1.1 255.255.255.255
Device(config-if)#ipv6 address 2001:DB8:1::1
Device(config-if)#tunnel source TenGigabitEthernet1/0/3
Device(config-if)#ip nhrp network-id 1
Device(config-if)#ip nhrp registration timeout 30
Device(config-if)#ip nhrp holdtime 400
Device(config-if)#ip nhrp authentication DMVPN
Device(config-if)#ip pim nbma-mode
Device(config-if)#end

Example: configuring multicast mGRE

To display an example on how to configure multicast mGRE. 
Device>enable
Device#configure terminal
Device(config)#interface tunnel 1
Device(config-if)#ip pim nbma-mode
Device(config-if)#ip pim sparse-mode
Device(config-if)#end

Example: configuring mGRE at hub and spokes

To displays examples for configuring mGRE at hub and spokes.

Configuration at hub:


Device(config)#interface Tunnel0
Device(config-if)#ip address 172.16.0.2 255.255.255.0
Device(config-if)#no ip redirects
Device(config-if)#ip nhrp authentication DMVPN
Device(config-if)#ip nhrp network-id 1
Device(config-if)#ip nhrp registration timeout 30
Device(config-if)#no ip next-hop-self eigrp 10
Device(config-if)#no ip split-horizon eigrp 10
Device(config-if)#tunnel source TenGigabitEthernet1/0/3
Device(config-if)#tunnel mode gre multipoint
Device(config-if)#tunnel key 4
Device(config-if)#end
Device(config)#interface TenGigabitEthernet1/0/3
Device(config-if)#no switchport
Device(config-if)#ip address 10.78.6.25. 255.255.255.0
Device(config-if)#end

Configuration at spoke1:


Device(config)#interface Tunnel0
Device(config-if)#ip address 172.16.0.4 255.255.255.0
Device(config-if)#no ip redirects
Device(config-if)#ip nhrp authentication DMVPN
Device(config-if)#ip nhrp map 172.16.0.2 10.78.6.25
Device(config-if)#ip nhrp map multicast 10.78.6.25
Device(config-if)#ip nhrp network-id 1
Device(config-if)#ip nhrp nhs 172.16.0.2
Device(config-if)#ip nhrp registration timeout 30
Device(config-if)#tunnel source GigabitEthernet2/0/3
Device(config-if)#tunnel mode gre multipoint
Device(config-if)#tunnel key 4
Device(config-if)#end
Device(config)#interface GigabitEthernet2/0/3
Device(config-if)#no switchport
Device(config-if)#ip address 209.165.202.129 255.255.255.0
Device(config-if)#end

Configuration at spoke2:


Device(config)#interface Tunnel0
Device(config-if)#ip address 172.16.0.3 255.255.255.0
Device(config-if)#no ip redirects
Device(config-if)#ip nhrp authentication DMVPN
Device(config-if)#ip nhrp map 172.16.0.2 10.78.6.25
Device(config-if)#ip nhrp map multicast 10.78.6.25
Device(config-if)#ip nhrp network-id 1
Device(config-if)#ip nhrp nhs 172.16.0.2
Device(config-if)#ip nhrp registration timeout 30
Device(config-if)#tunnel source GigabitEthernet1/0/3
Device(config-if)#tunnel mode gre multipoint
Device(config-if)#tunnel key 4
Device(config-if)#end
Device(config)#interface GigabitEthernet1/0/3
Device(config-if)#no switchport
Device(config-if)#ip address 192.168.0.1 255.255.255.0
Device(config-if)#end