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Q&A: What happens after the hold-time expires on a GRE tunnel interface?

Question

What happens after the hold-time expires on a GRE tunnel interface?

 

Environment

  • Silverline
    • DDoS
      • Routed

 

Answer

Tunnel endpoints do not have knowledge of the state of the remote endpoint or the network condition between them.   Since GRE Tunnels are stateless, they should never go down and from a "CE"  perspective. GRE Tunnel interfaces are always UP much like a loopback interface. 

  • The GRE tunnel will stay up even though the interface cannot send or receive traffic
  • The Link status will be Up and the GRE keepalives adjacency state will be Down

This is why GRE tunnel interfaces are configured using the Operation, Administration, and Maintenance (OAM) "keepalives". 

 

Gre keepalives configured: On, Gre keepalives adjacency state: down

c1.sjc1> show interfaces descriptions | match "gr-0/2/0.99"
gr-0/2/0.99 up up defensenet-4-customer:gre-DFNSJC1000214

c1.sjc1> show interfaces gr-0/2/0.99
Logical interface gr-0/2/0.99 (Index 1976) (SNMP ifIndex 4282)
Description: defensenet-4-customer:gre-DFNSJC1000214
Flags: Up Point-To-Point SNMP-Traps 0x4000 IP-Header 107.162.128.11:107.162.9.158:47::64:0000000000002000 Encapsulation: GRE-NULL
Gre keepalives configured: On, Gre keepalives adjacency state: down
Input packets : 0
Output packets: 1462975
Protocol inet, MTU: 1476
Flags: Sendbcast-pkt-to-re, User-MTU
Addresses, Flags: Dest-route-down Is-Preferred Is-Primary
Destination: 107.162.22.32/30, Local: 107.162.22.33, Broadcast: 107.162.22.35

 

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