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HDLC 101
Basic Configuration
The default encapsulation on serial interfaces on a Cisco Router is HDLC.
Even though HDLC exists as an open standard, Cisco routers use a modified version of the original standard that includes a new field in the header used for identification of the upper layer protocol that is encapsulated. Many other vendors can interoperate with Cisco Routers even when using this version of the protocol, but technically it is considered a non-standard implementation. If the requirements are to implement a standard encapsulation on a Serial interface, a better option would be PPP.
Since HDLC is the default encapsulation, configuration is probably the easiest ever:
! On R1:
R1(config)# interface Serial0/0
R1(config-if)# ip address 99.0.0.1 255.255.255.0
R1(config-if)# no shut
! On R2:
R1(config)# interface Serial0/0
R1(config-if)# ip address 99.0.0.2 255.255.255.0
R1(config-if)# no shut
! Test
R1# ping 99.0.0.2
!!!!!
Normally, when connecting two routers on a serial link, one of them must be the DTE, the other must be the DCE.
The DCE router must set a clock rate for the interface using:
R1(config-if)# clock rate CLOCK-RATE
In newer versions of the IOS, this is not needed anymore because by default, every serial interface has a clock rate set, but only the DCE end will use it.
How do we know which is the DCE end? The easiest method is to look at the command:
Keepalives
Cisco HDLC uses keepalives to monitor the link state. By default, keepalives are sent and expected every 10 seconds. Three missed keepalives will move the interface protocol to a “down” status.
Hitting enter will set the default value of 10 seconds. To disable keepalives completly use:
To verify keepalive value use:
Compression
HDLC can be configured to use software compression using the Stacker(LZS) algorithm. Compression must be enabled on both ends of the link:
Compression can be verified using:
Notice the compression ratio. One byte was sent after compression for each 2.366 bytes of uncompressed data. In the other direction, 1 byte of data was received for each uncompressed 2.380 bytes of date. These are of course average values. Beware that compression can affect system performance!