Hey Thanks Pieter for your response.
yes, that's the approach. I did something like this. As of now, the output is correct. Let me check in graph how it comes.
Really thanks for your great help!! 
InterfaceThroughput$interfaceList <- ifelse(substr(InterfaceThroughput$interfacename,1,2)== "xe","xe",
ifelse(substr(InterfaceThroughput$interfacename,1,2)== "ge","ge",
ifelse(substr(InterfaceThroughput$interfacename,1,2)== "et","et",
ifelse(substr(InterfaceThroughput$interfacename,1,3)== "irb","irb","others"))))
InterfaceThroughput1 <- InterfaceThroughput[InterfaceThroughput$interfaceList %in% c("xe","ge","et","irb"),]