It begins 400 km from Hanoi , where the massive Hoanh Son joins the sea, forming the Gate of Trung Bo, on the 18th degree of north latitude. It will end at about fifty kilometers from Saigon. Trung Bo is a land in any length , with the exception of the northern regions corresponding to the provinces from Thanh Hoa and Nghe An. Its average width is 120 km and the difficulty is reduced to 40 km up to Dong Hoi in the province Quang Binh . Guru remark is that at this level, alluvial lands included entered first foothills of the Truong Son mountain and cord coastal dunes , only a few hundred meters. "She draw a convex curve of the sea coast and do not suggest an obese belly Cape Varella be back" ( Huard ) . The Cordillera center, which serves the border with Laos, lies 600 km north- west to south - east to the province of Quang Nam Danang, before giving way to the Highlands, composed of metamorphic rocks and volcanic. The Cordillera rarely exceeds 1500 m, while its culminat point is 2 598m Ngoc Linh .