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Configuring Link and Multilink Services Logical Interfaces

 

Multilink and Link Services Logical Interface Configuration Overview

You configure multilink and link services interface properties at the logical unit level. Default settings for multilink and link services logical interface properties are described in Default Settings for Multilink and Link Services Logical Interfaces.

For general information about logical unit properties or family inet properties, see the Junos OS Network Interfaces Library for Routing Devices. For information about multilink and link services properties you configure at the family inet hierarchy level, see Configuring the Links in a Multilink or Link Services Bundle.

Note

On DS0, E1, or T1 interfaces in LSQ bundles, you can configure the bandwidth statement, but the router does not use the bandwidth value if the interfaces are included in an MLPPP or MLFR bundle. The bandwidth is calculated internally according to the time slots, framing, and byte-encoding of the interface. For more information about logical interface properties, see the Junos OS Network Interfaces Library for Routing Devices.

Default Settings for Multilink and Link Services Logical Interfaces

Table 1 lists the default settings for multilink and link services statements, together with the other permitted values or value ranges.

Table 1: Multilink and Link Services Logical Interface Statements

Option

Default Value

Possible Values

DLCI

None

16 through 1022

Drop timeout period

500 ms for bundles greater than or equal to the T1 bandwidth value and 1500 ms for other bundles.

0 through 2000 milliseconds

Encapsulation

For multilink interfaces, multilink-ppp. For link services interfaces, multilink-frame-relay-end-to-end.

multilink-frame-relay-end-to-end, multilink-ppp

Fragmentation threshold

0 bytes

128 through 16,320 bytes (Nx64)

Interleave fragments

disabled

enabled, disabled

Minimum links

1 link

1 through 8 links

Maximum received reconstructed unit (MRRU)

1504 bytes

1500 through 4500 bytes

Sequence ID format for MLPPP

24 bits

12 or 24 bits

Sequence ID format for MLFR FRF.15 and FRF.16

12 bits

12 bits

See Default Settings for Link Services Interfaces for statements that apply to link services physical interfaces only.

Multilink and link services interfaces support the following logical interface encapsulation types:

  • MLPPP

  • Multilink Frame Relay (MLFR) end-to-end

By default, the logical interface encapsulation type on multilink interfaces is MLPPP. The default logical interface encapsulation type on link services interfaces is MLFR end-to-end. For general information on encapsulation, see the Junos OS Network Interfaces Library for Routing Devices.

You can also configure physical interface encapsulation on link services interfaces. For more information, see Configuring Encapsulation for Link Services Physical Interfaces.

To configure multilink or link services encapsulation, include the encapsulation type statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

You must also configure the T1, E1, or DS0 physical interface with the same encapsulation type.

Note

ACX Series routers do not support DS0 physical interface as member links.

Caution

When you configure the first MLFR encapsulated unit or delete the last MLFR encapsulated unit on a port, it triggers an interface encapsulation change on the port, which causes an interface flap on the other units within the port that are configured with generic Frame Relay.

Configuring the Drop Timeout Period on Multilink and Link Services Logical Interfaces

By default, the drop timeout parameter is disabled. You can configure a drop timeout value to provide a recovery mechanism if individual links in the multilink or link services bundle drop one or more packets. Drop timeout is not a differential delay tolerance setting, and does not limit the overall latency. However, you need to make sure the value you set is larger than the expected differential delay across the links, so that the timeout period does not elapse under normal jitter conditions, but only when there is actual packet loss. You can configure differential delay tolerance for link services interfaces only. For more information, see Configuring Differential Delay Alarms on Link Services Physical Interfaces with MLFR FRF.16.

To configure the drop timeout value, include the drop-timeout statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

For link services interfaces, you also can configure the drop timeout value at the physical interface level by including the drop-timeout statement at the [edit interfaces ls-fpc/pic/port:channel mlfr-uni-nni-bundle-options] hierarchy level:

By default, the drop timer has a value of 500 ms for bundles greater than or equal to the T1 bandwidth value, and 1500 ms for other bundles. Any CLI-configured value overrides these defaults. Values can range from 1 through 2000 milliseconds. Values less than 5 milliseconds are not recommended, and a configured value of 0 reverts to the default value of 2000 milliseconds.

Note

For multilink or link services interfaces, if a packet or fragment encounters an error condition and is destined for a disabled bundle or link, it does not contribute to the dropped packet and frame counts in the per-bundle statistics. The packet is counted under the global error statistics and is not included in the global output bytes and output packet counts. This unusual accounting happens only if the error conditions are generated inside the multilink interface, not if the packet encounters errors on the wire or elsewhere in the network.

If you configure the drop-timeout statement with a value of 0, it disables any resequencing by the PIC for the specified class of MLPPP traffic. Packets are forwarded with the assumption that they arrived in sequence, and forwarding of fragmented packets is disabled for all classes. Fragments dropped as a result of this setting will increment the counter at the class level.

Alternatively, you can configure the drop-timeout statement at the [edit class-of-service fragmentation-maps map-name forwarding-class class] hierarchy level. The behavior and the default and range values are identical, but the setting applies only to the specified forwarding class. Configuration at the bundle level overrides configuration at the class-of-service level.

By default, compression of the inner PPP header in the MLPPP payload is enabled. To disable compression, include the disable-mlppp-inner-ppp-pfc statement at the [edit interfaces interface-name unit logical-unit-number] hierarchy level. For example:

For more information about CoS configuration, see the Class of Service User Guide (Routers and EX9200 Switches). You can view the configured drop-timeout value and the status of inner PPP header compression by issuing the show interfaces interface-name extensive command.

Limiting Packet Payload Size on Multilink and Link Services Logical Interfaces

For multilink and link services logical interfaces with MLPPP encapsulation only, you can configure a fragmentation threshold to limit the size of 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. By default, the fragmentation threshold parameter is disabled.

Note

To ensure proper load balancing:

  • For Link Services MLFR (FRF.15 and FRF.16) interfaces, do not include the fragment-threshold statement in the configuration.

  • For MLPPP interfaces, do not include both the fragment-threshold statement and the short-sequence statement in the configuration.

  • For MLFR (FRF.15 and FRF.16) and MLPPP interfaces, if the MTU of links in a bundle is less than the bundle MTU plus encapsulation overhead, then fragmentation is automatically enabled. You should avoid this situation for MLFR (FRF.15 and FRF.16) interfaces and for MLPPP interfaces on which short-sequencing is enabled.

To configure a fragmentation threshold value, include the fragment-threshold statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

For link services interfaces, you also can configure a fragmentation threshold value at the physical interface level by including the fragment-threshold statement at the [edit interfaces ls-fpc/pic/port:channel mlfr-uni-nni-bundle-options] hierarchy level:

The maximum fragment size can be from 128 through 16,320 bytes. The Junos OS automatically subdivides packet payloads that exceed this value. Any value you set must be a multiple of 64 bytes (Nx64). The default value, 0, results in no fragmentation.

Note

Only MLPPP is supported on ACX Series routers. MLFR is not supported on ACX Series routers.

You can set the minimum number of links that must be up for the multilink bundle as a whole to be labeled up. By default, only one link must be up for the bundle to be labeled up. A member link is considered up when the PPP Link Control Protocol (LCP) phase transitions to open state.

The minimum-links value should be identical on both ends of the bundle.

To set the minimum number, include the minimum-links statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

For link services interfaces, you also can configure the minimum number of links at the physical interface level by including the minimum-links statement at the [edit interfaces ls-fpc/pic/port:channel mlfr-uni-nni-bundle-options] hierarchy level:

The number can be from 1 through 8. The maximum number of links supported in a bundle is 8. When 8 is specified, all configured links of a bundle must be up.

The maximum received reconstructed unit (MRRU) is the maximum packet size that the multilink interface can process. It is similar to a maximum transmission unit (MTU), but applies only to multilink bundles. By default, the MRRU is set to 1500 bytes. You can configure a different MRRU value if the peer equipment allows this. The MRRU accounts for the original payload, for example the Layer 3 protocol payload, but does not include the 2-byte PPP header or 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:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

Note

ACX Series routers do not support logical systems.

For link services interfaces, you also can configure a different MRRU at the physical interface level by including the mrru bytes statement at the [edit interfaces ls-fpc/pic/port:channel mlfr-uni-nni-bundle-options] hierarchy level. The MRRU size can range from 1500 through 4500 bytes.

Note

If you set the MRRU on a bundle to a value larger than the MTU of the individual links within it, you must enable a fragmentation threshold for that bundle. Set the threshold to a value no larger than the smallest MTU of any link included in the bundle.

Determine the appropriate MTU size for the bundle by ensuring that the MTU size does not exceed the sum of the encapsulation overhead and the MTU sizes for the links in the bundle.

You can configure separate family mtu values on the following protocol families under bundle interfaces: inet, inet6, iso, and mpls. If not configured, the default value of 1500 is used on all except for mpls configurations, in which the value 1488 is used.

Note

ACX Series routers do not support family inet6 on MLPPP interfaces.

Note

The effective family MTU might be different from the MTU value specified for MLPPP configurations, because it is adjusted downward by the remote MRRU’s constraints. The remote MRRU configuration is not supported on M120 routers.

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:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

For MLFR FRF.15, the sequence header format is set to 24 bits by default. This is the only valid option.

Note

Only MLPPP is supported on ACX Series routers. MLFR is not supported on ACX Series routers.

Configuring CoS on Link Services Interfaces

For link services IQ (lsq-) interfaces, Junos OS class of service (CoS) is fully supported and functions as described in the Class of Service User Guide (Routers and EX9200 Switches). For more information and detailed configuration examples, see Layer 2 Service Package Capabilities and Interfaces.

On SRX Series devices, the lsq- interface is an internal interface, which is not associated with a physical interface. For information about link services on SRX Series devices, see the Junos OS Interfaces Configuration Guide for Security Devices.

For information about CoS functions and link services on M Series or T Series routers, see the following sections:

CoS for Link Services Interfaces on M Series and T Series Routers

For Link Services PIC interfaces (ls) on M Series and T Series routers, queue 0 is the only queue that you should configure to receive fragmented packets. Configure all other queues to be higher-priority queues.

Table 2 summarizes how CoS queues work on link services (ls) interfaces.

Table 2: Link Services CoS Queues

Supported Bundling Type

Queue 0

Higher-Priority Queues

Hash-based load balancing

No

Yes

MLFR FRF.15

Yes

No

MLFR FRF.16

Yes

No

MLPPP

Yes

No

For M Series and T Series routers, CoS on link services (ls) interfaces works as follows:

  • On all platforms, the Link Services PIC currently supports up to four queues: 0, 1, 2, and 3.

  • Queue 0 uses MLFR FRF.15, MLFR FRF.16, or MLPPP to bundle packets.

  • Higher-priority queues (1, 2, and 3) use hash-based load balancing to bundle packets. IP and MPLS header information is included in the hash.

  • MLPPP packets traversing link services interfaces using queue 0 are fragmented and distributed across the constituent links. Queue 0 packets are sent on the least utilized link, proportional to its bandwidth. The queue 0 load balancer attempts to maintain even distribution of all traffic across all constituent links. In situations with a small number of high-priority traffic flows (queues 1, 2, and 3), queue 0 traffic might be unevenly distributed.

  • For the MLFR FRF.16 protocol, only queue 0 works. If you configure a bundled interface to use MLFR FRF.16 with queue 0, then you must ensure the classifier does not send any traffic to queues 1, 2, and 3 on that interface.

  • To carry high-priority traffic correctly on MLFR FRF.16 interfaces, you must configure an output firewall filter that forces all traffic into queue 0 on the ls-fpc/pic/port.channel interface.

  • MLFR FRF.15 and MLPPP interfaces support CoS through packet interleaving. The MLFR FRF.16 standard does not support packet interleaving, so all packets destined for an FRF.16 PVC interface must egress from the same queue.

  • For constituent link interfaces of Link Services PICs, you can configure standard scheduler maps.

  • For input packets and fragments received from constituent links, you can use regular input firewall filters and standard CoS classifiers on the link services interface.

  • For packets that pass through a link services interface and are destined for a constituent link interface, all traffic using queue 0 is fragmented. Traffic using higher-priority queues (1, 2, and 3) is not fragmented.

  • For MLFR FRF.15 and MLPPP, routing protocol packets smaller than 128 bytes are sent to queue 3; routing protocol packets that exceed 128 bytes are sent to queue 0 and fragmented accordingly. For MLFR FRF.16, queue 0 is used for all packet sizes.

  • You must configure output firewall classification for egress traffic on the link services interface, not directly on the constituent link interface directly.

  • Inverse multiplexing for ATM (IMA) is not supported on link services interfaces.

For more information, see Configuring Delay-Sensitive Packet Interleaving on Link Services Logical Interfaces and the Routing Policies, Firewall Filters, and Traffic Policers User Guide.

Example: Configuring CoS on Link Services Interfaces

Configure CoS on a link services interface and its constituent link interfaces.

Note

This example applies to M Series and T Series routers. For examples that apply to SRX Series devices, see the Junos OS Interfaces Configuration Guide for Security Devices.

Packets that do not match the firewall filters are sent to a queue that performs load balancing by sending fragments to all constituent links.

Packets that match the firewall filters are sent to a queue that does not support packet fragmentation and reassembly; instead, this traffic is load-balanced by sending each packet flow to a different constituent link. Each packet that matches a firewall filter is subjected to a hash on the IP source address and the IP destination address to determine the packet flow to which each packet belongs.

When you configure the MLPPP encapsulation type or the multilink FRF.15 Frame Relay end-to-end encapsulation type, routing protocol packets smaller than 128 bytes are sent to the network-control queue on the constituent link interface. This keeps routing protocols operating normally, even when low-speed links are congested by regular packets.