NBMA: nonbroadcast multiaccess
Turning off split horizon is needed on Tunnel interface if any routing protocol is used:
When the device is connected to a spoke and not only the HUB (and some routing protocol is running) then split horizon causing problems.
When the device receives an update for example from the HUB it cannot forward the routing update towards another spoke because it should have send it on the same interface (on the tunnel interface).
Configuration of the HUB:
interface FastEthernet0/0 ip address 10.0.1.1 255.255.255.0 duplex auto speed auto ! end interface Tunnel1 ip address 192.168.88.1 255.255.255.248 no ip redirects ip nhrp map multicast dynamic #automatically add routers to the multicast NHRP mappings (needed for dynamic routing protocols) ip nhrp network-id 1 #enables NHRP on the interface; routers have to be in the same group to communicate tunnel source FastEthernet0/0 tunnel mode gre multipoint #Enables a GRE tunnel to be used in multipoint NBMA mode.
Configuration of the Spokes:
interface FastEthernet0/0 ip address 10.0.1.2 255.255.255.0 duplex auto speed auto ! end interface Tunnel1 ip address 192.168.88.2 255.255.255.248 no ip redirects ip nhrp map multicast 10.0.1.1 #destination of broadcast or multicast messages ip nhrp map 192.168.88.1 10.0.1.1 #mapping statically configured to the next hop server ( first tunnel IP, then interface IP) ip nhrp network-id 1 #enables NHRP on the interface; routers have to be in the same group to communicate ip nhrp nhs 192.168.88.1 tunnel source FastEthernet0/0 tunnel mode gre multipoint #Enables a GRE tunnel to be used in multipoint NBMA mode
R2#sho dmvpn Legend: Attrb --> S - Static, D - Dynamic, I - Incomplete N - NATed, L - Local, X - No Socket # Ent --> Number of NHRP entries with same NBMA peer NHS Status: E --> Expecting Replies, R --> Responding UpDn Time --> Up or Down Time for a Tunnel ========================================================================== Interface: Tunnel1, IPv4 NHRP Details Type:Spoke, NHRP Peers:2, # Ent Peer NBMA Addr Peer Tunnel Add State UpDn Tm Attrb ----- --------------- --------------- ----- -------- ----- 1 10.0.1.1 192.168.88.1 UP 03:00:13 S 1 10.0.1.3 192.168.88.3 UP 00:00:05 D
R2#sho ip nhrp 192.168.88.1/32 via 192.168.88.1 Tunnel1 created 03:00:53, never expire Type: static, Flags: used NBMA address: 10.0.1.1 192.168.88.2/32 via 192.168.88.2 Tunnel1 created 00:00:08, expire 01:59:53 Type: dynamic, Flags: router unique local NBMA address: 10.0.1.2 (no-socket) 192.168.88.3/32 via 192.168.88.3 Tunnel1 created 00:00:08, expire 01:59:51 Type: dynamic, Flags: router used NBMA address: 10.0.1.3
R2#sho int tun1 Tunnel1 is up, line protocol is up Hardware is Tunnel Internet address is 192.168.88.2/29 MTU 17916 bytes, BW 100 Kbit/sec, DLY 50000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation TUNNEL, loopback not set Keepalive not set Tunnel source 10.0.1.2 (FastEthernet0/0) Tunnel Subblocks: src-track: Tunnel1 source tracking subblock associated with FastEthernet0/0 Set of tunnels with source FastEthernet0/0, 1 member (includes iterators), on interface <OK> Tunnel protocol/transport multi-GRE/IP Key disabled, sequencing disabled Checksumming of packets disabled Tunnel TTL 255, Fast tunneling enabled Tunnel transport MTU 1476 bytes Tunnel transmit bandwidth 8000 (kbps) Tunnel receive bandwidth 8000 (kbps) Last input 00:04:58, output 00:04:58, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue: 0/0 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 12 packets input, 1572 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 18 packets output, 2164 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 unknown protocol drops 0 output buffer failures, 0 output buffers swapped out