Tag Archives: 2.3a

2.3.a Implement and troubleshoot HDLC

HDLC is a bit -oriented synchronous data link layer protocol. It supports various layer 3 protocols in addition to IP.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2127-2128).  . Kindle Edition.


below is from:


Cisco High-Level Data Link Controller (HDLC) is the Cisco proprietary protocol for sending data over synchronous serial links using HDLC. Cisco HDLC also provides a simple control protocol called Serial Line Address Resolution Protocol (SLARP) to maintain serial link keepalives. Cisco HDLC is the default for data encapsulation at Layer 2 (data link) of the Open System Interconnection (OSI) stack for efficient packet delineation and error control.

Note Cisco HDLC is the default encapsulation type for the serial interfaces.

When the encapsulation on a serial interface is changed from HDLC to any other encapsulation type, the configured serial subinterfaces on the main interface inherit the newly changed encapsulation and they do not get deleted.

Cisco HDLC uses keepalives to monitor the link state, as described in the “Keepalive Timer” section.

Cisco keepalives are useful for monitoring the link state. Periodic keepalives are sent to and received from the peer at a frequency determined by the value of the keepalive timer. If an acceptable keepalive response is not received from the peer, the link makes the transition to the down state. As soon as an acceptable keepalive response is obtained from the peer or if keepalives are disabled, the link makes the transition to the up state.

Note The keepalive command applies to serial interfaces using HDLC or PPP encapsulation. It does not apply to serial interfaces using Frame Relay encapsulation.

For each encapsulation type, a certain number of keepalives ignored by a peer triggers the serial interface to transition to the down state. For HDLC encapsulation, three ignored keepalives causes the interface to be brought down. For PPP encapsulation, five ignored keepalives causes the interface to be brought down. ECHOREQ packets are sent out only when LCP negotiation is complete (for example, when LCP is open).

Use the keepalive command in interface configuration mode to set the frequency at which LCP sends ECHOREQ packets to its peer. To restore the system to the default keepalive interval of 10 seconds, use the keepalive command with the no keyword. To disable keepalives, use the keepalive disable command. For both PPP and Cisco HDLC, a keepalive of 0 disables keepalives and is reported in the show running-config command output as keepalive disable .

2.3.a Implement and troubleshoot HDLC

i believe the blueprint for the new v5 is superior to v4… look at l2 WAN below…

Layer 2 WAN circuit technologies
2.3.a Implement and troubleshoot HDLC
2.3.b Implement and troubleshoot PPP
2.3.b (i) Authentication (PAP, CHAP)
2.3.b (ii) PPPoE
2.3.b (iii) MLPPP
2.3.c Describe WAN rate-based ethernet circuits
2.3.c (i) Metro and WAN Ethernet topologies
2.3.c (ii) Use of rate-limited WAN ethernet services

that’s it…

check out the vpn section:

4.0 VPN Technologies
4.1 Tunneling
4.1.a Implement and troubleshoot MPLS operations
4.1.a (i) Label stack, LSR, LSP
4.1.a (ii) LDP
4.1.a (iii) MPLS ping, MPLS traceroute
4.1.b Implement and troubleshoot basic MPLS L3VPN
4.1.b (i) L3VPN, CE, PE, P
4.1.b (ii) Extranet (route leaking)
4.1.c Implement and troubleshoot encapsulation
4.1.c (i) GRE
4.1.c (ii) Dynamic GRE
4.1.c (iii) LISP encapsulation principles supporting EIGRP OTP

4.1.d Implement and troubleshoot DMVPN (single hub)
4.1.d (i)NHRP
4.1.d (ii) DMVPN with IPsec using preshared key
4.1.d (iii) QoS profile
4.1.d (iv) Pre-classify
4.1.e Describe IPv6 tunneling techniques
4.1.e (i) 6in4, 6to4
4.1.e (ii) ISATAP
4.1.e (iii) 6RD
4.1.e (iv) 6PE/6VPE
4.1.g Describe basic layer 2 VPN —wireline

4.1.g (i) L2TPv3 general principals
4.1.g (ii) ATOM general principals
4.1.h Describe basic L2VPN — LAN services
4.1.h (i) MPLS-VPLS general principals
4.1.h (ii) OTV general principals

4.2 Encryption
4.2.a Implement and troubleshoot IPsec with preshared key
4.2.a (i) IPv4 site to IPv4 site
4.2.a (ii) IPv6 in IPv4 tunnels
4.2.a (iii) Virtual tunneling Interface (VTI)
4.2.b Describe GET VPN

2.3.a Implement and troubleshoot HDLC


it is worth mentioning… i once bumped into this quite by surprise and it shook me… serial line address resolution protocol… there is not a lot out there on it… it’s basically a dhcp mechanism for serial interfaces supporting a /30 network, whereby the side that is unknown get’s the other half of the /30… this guy has more on it here:


it is a legacy from what i have gathered…

wiki discusses it here briefly:


if in use it will show up as the method for which it came to be in show ip interface brief…

beware the SLARP monster…