What is Multilink PPP?
This is a standard protocol that allows the aggregation of PPP links into a single PPP link. The bundled links can be of any type, ISDN, T1, or any other type that supports the basic PPP protocol. The multilink configuration must be done on both sides of the link, which implies that the different links must connect the same two equipment.

Topology

For the demonstration How to Configure Multilink PPP Connections for Serial interfaces, I use here two routers interconnected by two serial links. The goal is to combine them into a Multilink interface that will then be configured as any interface.

How to Configure Multilink PPP Connections for Serial interfaces

To configure this multilink PPP, you have to go through two major steps:
  1. Creating the Multilink virtual interface
  2. Configure the serial interfaces to be integrated into the bundle.
Configuration on R1

 R1 # configure terminal

 !!!!  Creating the Multilink interface !!!!

 R1 (config) #interface multilink 1

 R1 (config-if) # ppp multilink

 R1 (config-if) # ppp multilink group 1

 R1 (config-if) #ip address 192.168.0.1 255.255.255.252

 R1 (config-if) #exit

 R1 (config) #

 !!!!  Integration of the s0 / 0 interface in the bundle

 R1 (config) #interface serial 0/0

 R1 (config-if) #encapsulation ppp

 R1 (config-if) # ppp multilink

 R1 (config-if) # ppp multilink group 1

 R1 (config-if) #no shutdown

 R1 (config-if) #exit

 R1 (config) #

 !!!!  Integration of the s0 / 1 interface in the bundle

 R1 (config) #interface serial 0/1

 R1 (config-if) #encapsulation ppp

 R1 (config-if) # ppp multilink

 R1 (config-if) # ppp multilink group 1

 R1 (config-if) #no shutdown

 R1 (config-if) #exit

 R1 (config) #
Among the important details ... Of course, the multilink group number specified in the interfaces configuration must match. As this configuration shows, the interfaces S0 / 0 and s0 / 1 are not addressed. They serve only as physical liaison. Addressing is done on the multilink interface.
Note also that if you need to use a routing protocol, a NAT process, or any other element that would interfere with the routing of this interface, the multilink interface should be referenced.

Configuration on R2

 R2 # configure terminal

 !!!!  Creating the Multilink interface !!!!

 R2 (config) #interface multilink 1

 R2 (config-if) # ppp multilink

 R2 (config-if) # ppp multilink group 1

 R2 (config-if) #ip address 192.168.0.2 255.255.255.252

 R2 (config-if) #exit

 R2 (config) #

 !!!!  Integration of the s0 / 0 interface in the bundle

 R2 (config) #interface serial 0/0

 R2 (config-if) #encapsulation ppp

 R2 (config-if) # ppp multilink

 R2 (config-if) # ppp multilink group 1

 R2 (config-if) #no shutdown

 R2 (config-if) #exit

 R2 (config) #

 !!!!  Integration of the s0 / 1 interface in the bundle

 R2 (config) #interface serial 0/1

 R2 (config-if) #encapsulation ppp

 R2 (config-if) # ppp multilink

 R2 (config-if) # ppp multilink group 1

 R2 (config-if) #no shutdown

 R2 (config-if) #exit

 R2 (config) #
The configuration is almost identical, of course. Note that the number of the multilink interface must not necessarily be the same as on the other router. The same applies to the multilink group number. There is also no relationship between the number of the multilink interface and the group number.

Communication test

R1 # ping 192.168.0.2

 Type escape sequence to abort.

 Sending 5, 100-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:

 !!!!!

 Success rate is 100 percent (5/5), round-trip min / avg / max = 1/4/12 ms

 R1 #
No surprise ... everything works! 

Checking the configuration

  R1 # show interface serial 0/0

  Serial0 / 0 is up, line protocol is up

  Hardware is GT96K Serial

  MTU 1500 bytes, BW 1544 Kbit / sec, DLY 20000 usec,

  Reliability 255/255, txload 1/255, rxload 1/255

  PPP Encapsulation, LCP Open, multilink Open

  Link is a member of Multilink bundle Multilink1 , loopback not set

  Keepalive set (10 sec)

  CRC checking enabled

  Last input 00:00:09, output 00:00:03, output hang never

  Last clearing of "show interface" counters 00:13:07

  Input tail: 0/75/0/0 (size / max / drops / flushes);  Total output drops: 1

  Queuing strategy: weighted fair [suspended, using FIFO]

  FIFO output queue 0/40, 0 drops

  5 minute input rate 0 bits / sec, 0 packets / sec

  5 minute output rate 0 bits / sec, 0 packets / sec

  163 packets input, 5310 bytes, 0 no buffer

  Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

  0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

  183 packets output, 5770 bytes, 0 underruns

  0 output errors, 0 collisions, 6 interface resets

  0 unknown protocol drops

  0 output buffer failures, 0 output buffers swapped out

  0 carrier transitions

  DCD = up DSR = up DTR = up RTS = up CTS = up

  R1 #

The interface works well in PPP, and is a member of the multilink group 1.
  
  R1 # show interface multilink 1

  Multilink1 is up, line protocol is up

  Hardware is multilink group interface

  Internet address is 192.168.0.1/30

  MTU 1500 bytes, BW 3088 Kbit / sec , DLY 100000 usec,

  Reliability 255/255, txload 1/255, rxload 1/255

  PPP Encapsulation, LCP Open, multilink Open

  Open: IPCP, CDPCP, loopback not set

  Keepalive set (10 sec)

  DTR is pulsed for 2 seconds on reset

  Last input 00:00:31, output never, output hang never

  Last clearing of "show interface" counters 00:17:23

  Input tail: 0/75/0/0 (size / max / drops / flushes);  Total output drops: 0

  Queuing strategy: fifo

  Output tail: 0/40 (size / max)

  5 minute input rate 0 bits / sec, 0 packets / sec

  5 minute output rate 0 bits / sec, 0 packets / sec

  29 packets input, 5944 bytes, 0 no buffer

  Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

  0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

  29 packets output, 6266 bytes, 0 underruns

  0 output errors, 0 collisions, 1 interface resets

  0 unknown protocol drops

  0 output buffer failures, 0 output buffers swapped out

  0 carrier transitions

  R1 #
Note here the bandwidth of the interface that takes the combined value of the two physical interfaces (2x 1544kbits / s).
 R1 # show ppp multilink

 Multilink1 Bundle name: R2 Remote Endpoint Discriminator: [1] R2 Local  Endpoint   Discriminator: [1] R1 Bundle up for 00:26:33, total bandwidth 3088, load 1/255 Receive buffer limit 24000 bytes, frag Timeout 1000 ms 0/0 fragments / bytes in reassembly list 0 lost fragments / bytes, 0 lost received 0x5C received sequence, 0x5C sent sequence Member links: 2 active, 0 not active (max not set, min Not set) Se0 / 0, since 00:26:34 Se0 / 1, since 00:26:03 No inactive multilink interfaces

R1 #
Here you find the state of the multilink link. You will find the interfaces that are part of the bundle, as well as information about the name of the remote equipment and counters for the number of packets transmitted, destroyed, ...

Conclusion

The basic configuration of a multilink PPP link is quite simple and intuitive. It also makes it possible to increase the bandwidth of the link without making the topology more complex. Note however that this is a very basic configuration, it is of course possible to configure the authentication, or to set the distribution of the data on the links.

 
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