Cisco Systems 78-11741-02 manual Nhrp on a Multipoint Tunnel Example, IPC-58

Configuring IP Addressing

IP Addressing Examples

NHRP on a Multipoint Tunnel Example

With multipoint tunnels, a single tunnel interface may be connected to multiple neighboring routers. Unlike point-to-point tunnels, a tunnel destination need not be configured. In fact, if configured, the tunnel destination must correspond to an IP multicast address. Broadcast or multicast packets to be sent over the tunnel interface can then be sent by sending the GRE packet to the multicast address configured as the tunnel destination.

Multipoint tunnels require that you configure a tunnel key. Otherwise, unexpected GRE traffic could easily be received by the tunnel interface. For simplicity, we recommend that the tunnel key correspond to the NHRP network identifier.

In the following example, routers A, B, C, and D all share a common Ethernet segment. Minimal connectivity over the multipoint tunnel network is configured, thus creating a network that can be treated as a partially meshed NBMA network. Due to the static NHRP map entries, Router A knows how to reach Router B, Router B knows how to reach Router C, Router C knows how to reach Router D, and Router D knows how to reach Router A.

When Router A initially attempts to send an IP packet to Router D, the packet is forwarded through Routers B and C. Through NHRP, the routers quickly learn the NBMA addresses of each other (in this case, IP addresses assigned to the underlying Ethernet network). The partially meshed tunnel network readily becomes fully meshed, at which point any of the routers can directly communicate over the tunnel network without their IP traffic requiring an intermediate hop.

The significant portions of the configurations for routers A, B, C, and D follow:

Router A Configuration

interface tunnel 0 no ip redirects

ip address 11.0.0.1 255.0.0.0 ip nhrp map 11.0.0.2 10.0.0.2 ip nhrp network-id 1

ip nhrp nhs 11.0.0.2 tunnel source ethernet 0 tunnel mode gre multipoint tunnel key 1

interface ethernet 0

ip address 10.0.0.1 255.0.0.0

Router B Configuration

interface tunnel 0 no ip redirects

ip address 11.0.0.2 255.0.0.0 ip nhrp map 11.0.0.3 10.0.0.3 ip nhrp network-id 1

ip nhrp nhs 11.0.0.3 tunnel source ethernet 0 tunnel mode gre multipoint tunnel key 1

interface ethernet 0

ip address 10.0.0.2 255.0.0.0

Router C Configuration

interface tunnel 0 no ip redirects

ip address 11.0.0.3 255.0.0.0 ip nhrp map 11.0.0.4 10.0.0.4

Cisco IOS IP Configuration Guide

IPC-58