The following sed command
sed -n '/ROUTING/,/\[ B/' filename does what you wish: the -n suppresses the default behaviour which is to print every line, the expression between single quotes selects a range of lines, then prints them (this is the meaning of ). The ranges to be selected are identified by means of matches (the initial and final one) delimited by separatrices, in my case I use the forward slash / as separatrix; the initial and final pattern match must be separated by a comma, and \[ is used to stress the literal meaning of the square parenthesis, i.e. this is not a grammatical structure but instead it is a real opening square parenthesis which must be found.