GRE Tunnel knows as Generic Routing Encapsulation is a tunnelling protocol developed by Cisco that provides the encapsulation of an extensive number of network layer protocols inside point-to-point links.
GRE encapsulates packets into IP packets and redirects them to an intermediate host, where they are de-encapsulated and routed to their final destination. As a result of the path to the intermediate host appear to the internal datagrams as one hop, Ethernet Switches can function as if they've a digital point-to-point reference to one another.
A GRE tunnel is utilised when packets must be despatched from one community to a different over the Web or an unsafe community. With GRE, a digital tunnel established between the 2 endpoints (Cisco routers), and packets forwarded by way of the GRE tunnel.
Create Static Routes
HQ(config)#interface tunnel 1
HQ(config-if)#tunnel source fastEthernet 0/0
HQ(config-if)#ip mtu 1400
HQ(config-if)# ip tcp adjust-mss 1360
HQ(config-if)#tunnel destination 126.96.36.199
HQ(config-if)#ip address 192.168.13.1 255.255.255.0
Branch(config)#interface tunnel 1
Branch(config-if)#tunnel source fastEthernet 0/0
Branch(config-if)#ip mtu 1400
Branch(config-if)# ip tcp adjust-mss 1360
Branch(config-if)#tunnel destination 188.8.131.52
Branch(config-if)#ip address 192.168.13.3 255.255.255.0
Since GRE is an encapsulating protocol, we adjust the maximum transfer unit (MTU) to 1400 bytes and maximum segment size (mss) to 1360 bytes. As a result of most transport MTUs are 1500 bytes and we now have an added overhead due to GRE, we should reduce the MTU to account for the additional overhead
GRE tunnels permit routing protocols equivalent to RIP and OSPF to ahead knowledge packets from one change to a different change throughout the Web. As well as, GRE tunnels can encapsulate multicast knowledge streams for transmission over the Web.
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