INE R&S ATC041 - GRE Tunneling, GRE Recursive Routing Errors

GRE Tunneling

• Generic Routing Encapsulation
– IP Protocol 47
• Used to transport payload protocols over IPv4 network
– IPv4
– IPv6
– CLNS
– Etc.


GRE Tunneling

• Create tunnel interface
– interface tunnel [number]

• Define source and destination
– tunnel source [ip | interface]
– tunnel destination [ip]

• Apply payload protocols
– ip address…
– ipv6 address…

• Optional keepalive
– keepalive


GRE Recursive Failure

• Tunnel destination must not recurse to the tunnel interface itself
– 1.2.3.4 via Tunnel0
– Tunnel0 via 1.2.3.4

• IOS detects this and disables the interface
– %TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing
• Can happen due to metric or distance errors


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使用SW1與R5建立 Tunnel....當然底層有EIGRP

R5
int Tunnel 10
  ip address 10.0.0.5 255.255.255.0
  tunnel source lo 0
  tunnel destination 150.10.5.5

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要避免 GRE Recursive Routing

避免將GRE IP Address啟動動態路由

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或是下filter

R5
ip prefix-list EIGRP_FILTER deny 150.10.5.0/24
ip prefix-list EIGRP_FILTER permit 0.0.0.0/0 le 32

router eigrp 1
  distribute-list preifx EIGRP_FILTER out tunnel 10

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Google: tunnel recursive routing

The "%TUN-5-RECURDOWN" Error Message and Flapping EIGRP/OSPF/BGP Neighbors Over a GRE Tunnel
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094690.shtml