Core Issue
If the tunnel interface learns that the best path to the tunnel destination is through the tunnel itself, the interface shuts down temporarily.
Resolution
To avoid recursive routing problems, keep passenger and transport network routing information disjointed with one of these methods:
- Use a different Autonomous System (AS) number or tag.
- Use a different routing protocol.
- Use static routes to override the first hop, but watch for routing loops. For more information, refer to the Special Considerations section of Configuring Logical Interfaces.
Example Scenario
Tunnel specification
Tunnel source: Loopback interface (10.1.1.1/24)
Tunnel destination : 10.1.2.1/24
Tunnel ip address : 172.16.1.1/24
Before Tunnel created
R1 has static default route sending out to R2. Therefore, destination tunnel (10.1.2.1/24) is reachable though static route.
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 192.168.1.2 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, Loopback0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
R 192.168.2.0/24 [120/1] via 192.168.1.2, 00:00:02, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 192.168.1.2
However, when we configure the tunnel interface between R1 and R2 with the tunnel source and destination as stated above. The destination tunnel is now reachable through the tunnel itself instead of through static route.
After tunnel configuration
R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 192.168.1.2 to network 0.0.0.0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Tunnel0
10.0.0.0/24 is subnetted, 2 subnets
O 10.1.2.0 [110/11112] via 172.16.1.2, 00:00:02, Tunnel0
C 10.1.1.0 is directly connected, Loopback0
C 192.168.1.0/24 is directly connected, FastEthernet0/0
R 192.168.2.0/24 [120/1] via 192.168.1.2, 00:00:05, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 192.168.1.2
From the core issue above which stated that
"If the tunnel interface learns that the best path to the tunnel destination is through the tunnel itself, the interface shuts down temporarily."
At this time the tunnel interface learns the path to the tunnel destination through the tunnel interface itself. (172.16.1.2 = tunnel interface ). Therefore, it create the tunnel recursive routing issue as shown below that the tunnel interface will be temporary disable.
01:11:39: %LINEPROTO-5-UPDOWN:
Line protocol on Interface Tunnel0, changed state to up
01:11:48: %TUN-5-RECURDOWN:
Tunnel0 temporarily disabled due to recursive routing
01:11:49: %LINEPROTO-5-UPDOWN:
Line protocol on Interface Tunnel0, changed state to down
01:12:49: %LINEPROTO-5-UPDOWN:
Line protocol on Interface Tunnel0, changed state to up
01:12:58: %TUN-5-RECURDOWN:
Tunnel0 temporarily disabled due to recursive routing
01:12:59: %LINEPROTO-5-UPDOWN:
Line protocol on Interface Tunnel0, changed state to down
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