HDLC是CISCO路由器使用的缺省协议，一台新路由器在未指定封装协议时默认使用HDLC封装。本博文将为您详细介绍HDLC协议的有关命令及配置。
HDLC协议是一种简单、高效的点到点链路协议，主要用于点到点连接的路由器间的通信。HDLC协议有 Cisco HDLC 和 ISO DHLC 两种，两者不能兼容。
锐捷路由器的同步串行口默认封装Cisco HDLC，所以锐捷路由器可以和Cisco路由器直接相连，但如果把锐捷路由器和不支持Cisco HDLC的路由器相连，就需要采用其它协议(如PPP)。
1.有关命令
端口设置
注:1.以下给出一个显示Cisco同步串口状态的例子.
Router#show interface serial 0
Serial 0 is up, line protocol is up
Hardware is MCI Serial
Internet address is 150.136.190.203, subnet mask is 255.255.255.0
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)
Last input 0:00:07, output 0:00:00, output hang never
Output queue 0/40, 0 drops; input queue 0/75, 0 drops
Five minute input rate 0 bits/sec, 0 packets/sec
Five minute output rate 0 bits/sec, 0 packets/sec
16263 packets input, 1347238 bytes, 0 no buffer
Received 13983 broadcasts, 0 runts, 0 giants
2 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 2 abort
22146 packets output, 2383680 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets, 0 restarts
1 carrier transitions
2. 举例
设置如下：
Router1:
interface Serial0
ip address 192.200.10.1 255.255.255.0
clockrate 1000000
Router2:
interface Serial0
ip address 192.200.10.2 255.255.255.0
!
3. 举例使用E1线路实现多个64K专线连接.
相关命令:
注: 1. 当链路为T1时,channel-group编号为0-23, Timeslot范围1-24; 当链路为E1时, channel-group编号为0-30, Timeslot范围1-31.
2.使用show controllers e1观察controller状态,以下为帧类型为crc4时controllers正常的状态.
Router# show controllers e1
e1 0/0 is up.
Applique type is Channelized E1 - unbalanced
Framing is CRC4, Line Code is HDB3 No alarms detected.
Data in current interval (725 seconds elapsed):
0 Line Code Violations, 0 Path Code Violations
0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
Total Data (last 24 hours) 0 Line Code Violations, 0 Path Code Violations,
0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins,
0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs
以下例子为E1连接3条64K专线, 帧类型为NO-CRC4,非平衡链路,路由器具体设置如下:
shanxi#wri t
Building configuration...
Current configuration:
!
version 11.2
no service udp-small-servers
no service tcp-small-servers
!
hostname shanxi
!
enable secret 5 $1$XN08$Ttr8nfLoP9.2RgZhcBzkk/
enable password shanxi
!
!
ip subnet-zero
!
controller E1 0
framing NO-CRC4
channel-group 0 timeslots 1
channel-group 1 timeslots 2
channel-group 2 timeslots 3
!
interface Ethernet0
ip address 133.118.40.1 255.255.0.0
media-type 10BaseT
!
interface Ethernet1
no ip address
shutdown
!
interface Serial0:0
ip address 202.119.96.1 255.255.255.252
no ip mroute-cache
!
interface Serial0:1
ip address 202.119.96.5 255.255.255.252
no ip mroute-cache
!
interface Serial0:2
ip address 202.119.96.9 255.255.255.252
no ip mroute-cache
!
no ip classless
ip route 133.210.40.0 255.255.255.0 Serial0:0
ip route 133.210.41.0 255.255.255.0 Serial0:1
ip route 133.210.42.0 255.255.255.0 Serial0:2
!
line con 0
line aux 0
line vty 0 4
password shanxi
login
!
end
3、配置keepalive时间
HDLC每隔10秒钟就互相发送链路探测的协商报文(KeepAlive报文)，用于探查链路是否中断，每次收发的报文按序号递增，序号失序则链路中断。当接口连续3次(数据包速率超过1000packets/s时为6次)没有收到对方对自己的递增序号的确认时，HDLC协议就把链路状态由Up转变为Down，链路将不可用。
HDLC协议可配置的参数只有Keepalive的间隔时间，缺省值是10秒。可以根据链路的流量来修改这个值。
1、配置keepalive时间：
Router(config)#interface interface-id
Router(config-if)#keepalive seconds
interface 命令用于指定要配置的接口，必须是Serial口，interface-id是接口号。
keepalive 命令设置keepalive时间，seconds是以秒为单位的时间值。
2、忽略keepalive探查：
Router(config)#interface interface-id
Router(config-if)#no keepalive
使用 no keepalive 命令表示不进行HDLC链路状态探查，即不发送keepalive报文，也不处理收到的keepalive报文。
配置举例：
R1>enable R1#configure terminal R1(config)#interface s0/0 R1(config-if)#encapsulation hdlc R1(config-if)#keepalive 15
R2>enable R2#configure terminal R2(config)#interface s0/0 R2(config-if)#encapsulation hdlc R2(config-if)#keepalive 15
注意：你必须保证相连两端的keepalive时间相同。
