Configuring Multilink Interfaces

The standards for MLPPP and MLFR FRF.15 are defined in RFC 1990, The PPP Multilink Protocol (MP) , and in FRF.15, End-to-End Multilink Frame Relay Implementation Agreement .

The Multilink Protocol (MP) enables you to split, recombine, and sequence datagrams across multiple logical data links. The goal of multilink operation is to coordinate multiple independent links between a fixed pair of systems, providing a virtual link with greater bandwidth than any of the members . The physical connections must be E1 or T1 interfaces. The Multilink Protocol includes two encapsulation types: Multilink Point-to-Point Protocol (MLPPP) and Multilink Frame Relay (MLFR). The JUNOS software supports both MLPPP and MLFR (FRF.15 only).

Each Multilink Services PIC can support a number of multilink bundles. A multilink bundle can contain up to eight individual links, such as T1 or E1 physical interfaces. Each link is associated with a logical unit number that you configure. You must configure a link before it can join a bundle. Each bundle should consist solely of one type of link; Juniper Networks recommends not mixing T1 and E1 physical interfaces within a bundle.

Multilink Services PICs are available in three versions, as shown in Table 6.6. The PIC hardware is identical, except for different faceplates that enable you to identify which version you are installing. The software limits the unit numbers and maximum number of physical interfaces you assign to the PIC. You can install a maximum of four Multilink Services PICs per router.

Table 6.6. Multilink Services PIC Capacities

PIC Capacity	Unit Numbers	Maximum Number of T1 Interfaces	Maximum Number of E1 Interfaces
4-bundle PIC	0 through 3	32 links	32 links
32-bundle PIC	0 through 31	256 links	219 links
128-bundle PIC	0 through 127	292 links	219 links

A single PIC can support an aggregate bandwidth of 450 Mbps. You can configure a larger number of links, but the Multilink Services PIC can reliably process only 450 Mbps of traffic. A higher rate of traffic might degrade performance.

Configuring Multilink Properties

By default, the drop timeout value is disabled. You can configure a drop timeout value to provide a recovery mechanism if individual links in the multilink bundle drop one or more packets. Make sure that the value you set is larger than the expected differential delay across the links, although drop timeout is not a differential delay tolerance setting and does not limit the overall latency. To configure the drop timeout value, include the drop-timeout statement. milliseconds is the duration of the drop timer; its range is 1 through 127 milliseconds. Values less than 5 milliseconds are not recommended; a value of 0 disables the timer.

 [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]  drop-timeout  milliseconds  ; 

By default, the encapsulation on multilink interfaces is MLPPP. MLPPP and Multilink Frame Relay (MLFR) are the encapsulation types used to transmit packets within the multilink interface. To configure multilink encapsulation, include the encapsulation statement. You must also configure the T1 or E1 physical interface with the same encapsulation type.

 [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]  encapsulation (multilink-ppp  multilink-framerelay); 

By default, the fragmentation threshold parameter is disabled. For MLPPP interfaces only, you can configure a fragmentation threshold to set a maximum size for packet payloads transmitted across the individual links within the multilink circuit. The software splits any incoming packet that exceeds the fragmentation threshold into smaller units suitable for the circuit size; it reassembles the fragments at the other end, but does not affect the output traffic stream. The threshold value affects the payload only; it does not affect the MLPPP header. To configure a fragmentation threshold value, include the fragment-threshold statement, specifying the maximum fragment size, beyond which the software automatically subdivides packet payloads; its range is 128 through 16,320 bytes. Any value you set must be a multiple of 64 bytes. The default value of 0 results in no fragmentation.

 [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]  fragment-threshold  bytes  ; 

To set the minimum number of links that must be up for the multilink bundle as a whole to be labeled up, include the minimum-links statement. number can be a value from 1 through 8. The default is 1.

 minimum-links  number  ; 

The maximum received reconstructed unit (MRRU) is similar to a maximum transmission unit (MTU), but applies only to multilink bundles; it is the maximum packet size that the multilink interface can process. By default, the MRRU is set to 1,500 bytes; you can configure a different MRRU value if the peer equipment allows. The MRRU includes the original payload plus the 2-byte PPP header, but not the additional MLPPP or MLFR header applied while the individual multilink packets are traversing separate links in the bundle. To configure a different MRRU value, include the mrru statement. bytes is the MRRU size; its range is 1,500 through 4,500 bytes.

 [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]  mrru  bytes  ; 

For MLPPP, the sequence header format is set to 24 bits by default. You can configure an alternative value of 12 bits, but 24 bits is considered the more robust value for most networks. To configure a different sequence header value, include the short-sequence statement. For MLFR, the sequence header format is set to 24 bits by default. This is the only valid option.

 [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]  short-sequence; 

Configuring Physical and Logical Multilink Interfaces

To complete a multilink interface configuration, you configure both the physical interface, either a T1 or E1, and the multilink bundle, which is a logical connection (see Figure 6.2). The physical interface is usually connected to networks capable of supporting MLPPP or MLFR.

Figure 6.2. Multilink Interface Configuration

Using the topology in Figure 6.2 as an example, configure a multilink bundle over a T1 connection (for which you have already configured the T1 physical interface) with the following additional configuration statements:

1. To configure a physical T1 link for MLPPP, include the following statements:

    [edit interfaces t1-  fpc/pic/port  ]  unit 0 {   family mlppp {     bundle ml-  fpc/pic/port  ;   } } 

   You do not need to configure an IP address on this link.

   If the MLPPP bundle is interoperating with Cisco IOS, the IOS configuration must include the ppp multilink fragment-delay command within the interface multilink# configuration group . The time should be 500 milliseconds.

    interface Multilink#  ppp multilink fragment-delay milliseconds 

   To configure a physical T1 link for MLFR, include the following statements:

    [edit interfaces t1-  fpc/pic/port  ]  unit 0 {   dlci  dlci-identifier  ;   encapsulation multilink-framerelay;   family mlfr {     bundle ml-  fpc/pic/port  ;   } } 

   You do not need to configure an IP address on this link.

2. To configure the logical address for the MLPPP bundle, include the address and destination statements:

    [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]    family inet {     address  address  {       destination  address  ;     }   } } 

   When you add statements such as MRRU to the configuration and commit, the T1 interface becomes part of the multilink bundle.

   To configure the logical address for the MLFR bundle, include the address and destination statements:

    [edit interfaces ml-  fpc/pic/port  unit  logical-unit-number  ]    encapsulation multilink-framerelay;   family inet {     address  address  {       destination  address  ;     }   } } 

   For MLPPP and MLFR links, you must specify the address as /32 or /30 . Other subnet designations are treated as mismatches .