The SPI(Security Parameter Index) is carried in the ESP header. This enables the receiver to select the SA, under which a packet should be processed. The SPI is a 32-bit value. Both decimal and hexadecimal values are acceptable. e.g. [987654321] or [0x3ade68b1]. Each tunnel must have unique an Inbound SPI and Outbound SPI. No two tunnels share the same SPI. Notice that Inbound SPI must match the other router's Outbound SPI, and vice versa.