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Cisco routers use a Cisco-proprietary variation of HDLC that adds a Type field

From

Cover of CCCENT/CCNA ICND1 100-105 Official Certification Guide, First Edition

Note

What's the difference between the ISO standard of HDLC and the Cisco proprietary variation? #WAN HDLC url:https://www.safaribooksonline.com/library/view/cccentccna-icnd1-100-105/9780134440903/graphics/03fig06.jpg