To form spoke to spoke tunnels in Phase 3 we combine summarization and exclude process switching. In Phase 3 spokes don’t need full routing table, In DMVPN Phase III We will get the specific route via NHRP. In this Phase as well as in Phase II, we want the traffic exchange between Spokes is done directly without going through the Hub, however, in Phase III will not change the behavior of the routing protocol (EIGRP), but we will use the NHRP Protocol. An ip nhrp redirect message indicates that the current route to the destination is not the best, so the receiver of the message (Spoke) should find a better route for that destination. ip nhrp shoortcut Tells the router to accept ip nhrp redirect messages and rewrites the new CEF entry. (Cisco Express Forwarding entry). If we look at the routing table in a Spoke, we will notice that the next hop for all routes will be the Hub, however, an NHRP Cache will be created that tells Hub a better route for each destination. R2 – R3 & R4 ! interface Tunnel1 ip address 192.168.1.2 255.255.255.0 ip nhrp map 192.168.1.1 190.1.1.1 ip nhrp map multicast 190.1.1.1 ip nhrp network-id 1 ip nhrp nhs 192.168.1.1 ip nhrp shortcut tunnel source 190.1.2.2 tunnel mode gre multipoint end If we look at the routing table on Spoke (R2): R2#sh ip route eigrp Gateway of last resort is 190.1.2.5 to network 0.0.0.0 10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks D 10.1.1.0/24 [90/27008000] via 192.168.1.1, 01:03:01, Tunnel1 D 10.1.3.0/24 [90/28288000] via 192.168.1.1, 01:03:01, Tunnel1 D 10.1.4.0/24 [90/28288000] via 192.168.1.1, 01:03:01, Tunnel1 Now let’s take a look at: R2#sh ip nhrp 10.1.3.0/24 via 192.168.1.3 Tunnel1 created 00:00:01, expire 01:59:58 Type: dynamic, Flags: router rib nho NBMA address: 190.1.3.3 10.1.4.0/24 via 192.168.1.4 Tunnel1 created 00:00:12, expire 01:59:47 Type: dynamic, Flags: router rib nho NBMA address: 190.1.4.4 …. If we make a traceroute from R2 to R4 we’ll see the following : R2#traceroute 10.1.4.4 Type escape sequence to abort. Tracing the route to 10.1.4.4 VRF info: (vrf in name/id, vrf out name/id) 1 192.168.1.4 0 msec 1 msec 0 msec On top of that, in Phase 3 we can summarize: R1 ! interface Tunnel1 ip address 192.168.1.1 255.255.255.0 no ip redirects no ip split-horizon eigrp 1 ip nhrp map multicast dynamic ip nhrp network-id 1 ip nhrp redirect ip summary-address eigrp 1 10.0.0.0 255.0.0.0 tunnel source Ethernet0/0 tunnel mode gre multipoint end If we see the routing table on Spokes (R2, R3, R4) R2#sh ip route eigrp … 10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks D 10.0.0.0/8 [90/27008000] via 192.168.1.1, 00:00:52, Tunnel1 If we do a traceroute from R2 to R4 we’ll see that communication between them is direct without passing by the Hub. R2#traceroute 10.1.4.4 Type escape sequence to abort. Tracing the route to 10.1.4.4 VRF info: (vrf in name/id, vrf out name/id) 1 192.168.1.4 2 msec 6 msec 5 msec R2#
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