Static/default Route to Local outgoing interface and next hop??
FrankGuthrie
Member Posts: 245
in CCIE
Ok, I'm working on the INE workbook, and in one of the explanation the ask to create a static/default route to the DMVPN hub
I've attached a screenshot of the topology.
There is 1 specific question:
R1 Tunnel 0 (DMVPN) IP address: 155.1.0.1/24
R1 Loopback 0 with IP address: 155.1.1.1/32
R2 Tunnel 0 (DMVPN) IP address: 155.1.0.2/24
R2 Loopback 0 with IP address: 155.1.2.2/32
R5 Tunnel 0 (DMVPN) IP address: 155.1.0.5/24
R5 Loopback 0 with IP address: 155.1.5.5/32
What I came up with was:
ON R1
ip route 0.0.0.0 0.0.0.0 155.1.0.5 track 1
track 1 interface Tunnel0 line-protocol
On R2
ip route 0.0.0.0 0.0.0.0 155.1.0.5 track 1
track 1 interface Tunnel0 line-protocol
On R5
ip route 150.1.1.1 255.255.255.255 155.1.0.1
ip route 150.1.2.2 255.255.255.255 155.1.0.2
The config On R5 was correct, but not on R1 and R2.
INE's solution on R1 and R2:
ON R1
ip route 0.0.0.0 0.0.0.0 Tunnel0 155.1.0.5
ON R2
ip route 0.0.0.0 0.0.0.0 Tunnel0 155.1.0.5
I've never seen this config, I always thought it was either using the local interface or the next hop IP address. Can anyone explain the configuration used by INE? I'm confused as to why the route is valid as long the tunnel interface is up. They do explain the config, but I don't understand the explanation.
I've attached a screenshot of the topology.
There is 1 specific question:
- Configure R1 and R2 with IPv4 default routes through the DMVPN cloud with a next-hop of R5.
- Ensure that the route is valid as long as the Tunnel interface is in the UP state.
R1 Tunnel 0 (DMVPN) IP address: 155.1.0.1/24
R1 Loopback 0 with IP address: 155.1.1.1/32
R2 Tunnel 0 (DMVPN) IP address: 155.1.0.2/24
R2 Loopback 0 with IP address: 155.1.2.2/32
R5 Tunnel 0 (DMVPN) IP address: 155.1.0.5/24
R5 Loopback 0 with IP address: 155.1.5.5/32
What I came up with was:
ON R1
ip route 0.0.0.0 0.0.0.0 155.1.0.5 track 1
track 1 interface Tunnel0 line-protocol
On R2
ip route 0.0.0.0 0.0.0.0 155.1.0.5 track 1
track 1 interface Tunnel0 line-protocol
On R5
ip route 150.1.1.1 255.255.255.255 155.1.0.1
ip route 150.1.2.2 255.255.255.255 155.1.0.2
The config On R5 was correct, but not on R1 and R2.
INE's solution on R1 and R2:
ON R1
ip route 0.0.0.0 0.0.0.0 Tunnel0 155.1.0.5
ON R2
ip route 0.0.0.0 0.0.0.0 Tunnel0 155.1.0.5
I've never seen this config, I always thought it was either using the local interface or the next hop IP address. Can anyone explain the configuration used by INE? I'm confused as to why the route is valid as long the tunnel interface is up. They do explain the config, but I don't understand the explanation.
Comments
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AwesomeGarrett
Member Posts: 257
If you only use the next-hop, in the event that the interface goes down, there is a possibility that the local router will attempt to route traffic to the next-hop recursively. If you use the interface as a next-hop, it will insert the route as directly connected and try to ARP the destination out the interface. If there is a router on the segment and proxy ARP is enabled, the packet will be routed. If proxy ARP is disabled on the neighboring router the packet will be dropped.
Using both will ensure that the packet is routed out the specified interface with the CEF information for the next-hop. In the event that the interface is down, the route will be removed from the route table.
I have not done this lab in a while but I was under the impression that there was a explanation. If not, review Routing TCP/IP VOL I.
EDIT: Since these are DMVPN spokes and only have a mapping to the hub, you can use the interface statement as the next-hop and only the hub router will receive the ARP request. Again, if proxy ARP is enabled, the hub router will answer the ARP request and route the packet.