Ip protocol has an optional feature called source-routing. Using this feature you can ensure that an IP datagram will pass through certain interfaces en route to destination and this functionality is obtained with some IP options fields in IP header. (You can try loose or strict source routing using extended ping command). Handling of IP datagrams with source-routing option is determined on per interface basis using "ip source-route" command. This is enabled by default and generally manually disabled because source-routing creates some open doors for some kind of attacks. If you issue "no ip source-route" under an interface, IP datagrams inbound this interface with source-routing option are discarded.
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