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port instead of the address of other branch router’s tunnel port. Thus, the data transmission between
branches will still pass through the center router.
To solve this problem, it is required to set on the center router. When a branch subnet’s reachable
routing is announced on the port of mGRE tunnel, the "next hop" address is the address of this
branch router’s tunnel port instead of the address of the center router.
In RIP or EIGRP equidistant vector routing protocol, the function of split horizon is usually
achieved, to prevent sending the routing information back to its source port and avoid routing loop
on the adjacent routers. If RIP or EIGRP protocol runs on the DMVPN network, it is required to
turn off the split horizon function. Otherwise, the branch routers will not be able to learn the routing
to the other branch subnets.
For RIP, this is enough, because when RIP sends the routing to the routing information source port,
its "next hop" address will not be changed and remains to be the original address. When EIGRP
sends the routing to the routing information source port, its "next hop" address will change to the
address of the port. Therefore, it is necessary to turn off this feature (EIGRP is private protocol of
CISCO. The IOS command to turn off this feature is no ip next-hop-self eigrp).
OSPF is a link status type routing protocol and itself does not have the problem of split horizon.
However, in configuring OSPF network type, it is required to be configured as a broadcast rather
than the point-to-multipoint type. Otherwise, the above problems will be caused. In addition, it
should also be noted that it is required to configure the center router (Hub) of DMVPN as the
designated router (DR) of OSPF, which can be achieved by specifying a higher OSPF priority for
the center router (Hub).
12.5.2 Main Configuration Steps of DMVPN
Knowledge points involved: configuration / IPSec VPN steps of GRE tunnel / NHRP / dynamic
routing protocol:
1: Configuring IPSec VPN
2: Configuring GRE Tunnel
3: Configuring NHRP protocol
4: Configuring dynamic routing protocols: (RIP)
12.5.3 DMVPN Configuration
12.5.3.1 IPSec Phase 1
Page Wizard: VPN →IPSec →IPSec Phase 1
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