Configuring LSQ Interfaces for Single Fractional T1 or E1 Interfaces Using MLPPP and LFI
When you configure a single fractional T1 interface, it is called a logical interface, because it can represent, for example, a routing adjacency.
The logical link services IQ interface represents the MLPPP bundle. Four queues are associated with the logical interface. A scheduler removes packets from the queues according to a scheduling policy. Typically, you designate one queue to have strict priority, and the remaining queues are serviced in proportion to weights you configure.
To configure a single fractional T1 interface using MLPPP and LFI, you associate one DS0 (fractional T1) interface with a link services IQ interface. To associate a fractional T1 interface with a link services IQ interface, include the bundle statement at the [edit interfaces ds-fpc/pic/port:channel unit logical-unit-number family mlppp] hierarchy level:
 | Note: Link services IQ interfaces support both T1 and E1 physical interfaces. These instructions apply to T1 interfaces, but the configuration for E1 interfaces is similar. |
To configure the link services IQ interface properties, include the following statements at the [edit interfaces lsq-fpc/pic/port unit logical-unit-number] hierarchy level:
For MLPPP, assign a single scheduler map to the link services IQ (lsq) interface and to each constituent link. The default schedulers for M Series and T Series routers, which assign 95, 0, 0, and 5 percent bandwidth for the transmission rate and buffer size of queues 0, 1, 2, and 3, are not adequate when you configure LFI or multiclass traffic. Therefore, for MLPPP, you should configure a single scheduler with nonzero percent transmission rates and buffer sizes for queues 0 through 3, and assign this scheduler to the link services IQ (lsq) interface and to each constituent link and to each constituent link, as shown in Example: Configuring an LSQ Interface for a Fractional T1 Interface Using MLPPP and LFI.
| Note: For M320 and T Series routers, the default scheduler transmission rate and buffer size percentages for queues 0 through 7 are 95, 0, 0, 5, 0, 0, 0, and 0 percent. |
To configure and apply the scheduling policy, include the following statements at the [edit class-of-service] hierarchy level:
For link services IQ interfaces, a strict-high-priority queue might starve all the other queues because traffic in a strict-high priority queue is transmitted before any other queue is serviced. This implementation is unlike the standard Junos CoS implementation in which a strict-high-priority queue receives infinite credits and does round-robin with high-priority queues, as described in the Junos OS Class of Service Configuration Guide.
After the scheduler removes a packet from a queue, a certain action is taken. The action depends on whether the packet came from a multilink encapsulated queue (fragmented and sequenced) or a nonencapsulated queue (hashed with no fragmentation). Each queue can be designated as either multilink encapsulated or nonencapsulated, independently of the other. By default, traffic in all forwarding classes is multilink encapsulated. To configure packet fragmentation handling on a queue, include the fragmentation-maps statement at the [edit class-of-service] hierarchy level:
If you require the queue to transmit small packets with low latency, configure the queue to be nonencapsulated by including the no-fragmentation statement. If you require the queue to transmit large packets with normal latency, configure the queue to be multilink encapsulated by including the fragment-threshold statement. If you require the queue to transmit large packets with low latency, we recommend using a faster link and configuring the queue to be nonencapsulated. For more information about fragmentation maps, see Configuring CoS Fragmentation by Forwarding Class on LSQ Interfaces.
When a packet is removed from a multilink-encapsulated queue, it is fragmented. If the packet exceeds the minimum link MTU, or if a queue has a fragment threshold configured at the [edit class-of-service fragmentation-maps map-name forwarding-class class-name] hierarchy level, the software splits the packet into two or more fragments, which are assigned consecutive multilink sequence numbers.
If you do not include the fragment-threshold statement in the fragmentation map, the fragmentation threshold you set at the [edit interfaces interface-name unit logical-unit-number] hierarchy level is the default for all forwarding classes. If you do not set a maximum fragment size anywhere in the configuration, packets are fragmented if they exceed the smallest MTU of all the links in the bundle.
Even if you do not set a maximum fragment size anywhere in the configuration, you can configure the maximum received reconstructed unit (MRRU) by including the mrru statement at the [edit interfaces lsq-fpc/pic/port unit logical-unit-number] hierarchy level. The MRRU is similar to the MTU, but is specific to link services interfaces. By default the MRRU size is 1500 bytes, and you can configure it to be from 1500 through 4500 bytes. For more information, see Configuring MRRU on Multilink and Link Services Logical Interfaces.
When a packet is removed from a multilink-encapsulated queue, the software gives the packet an MLPPP header. The MLPPP header contains a sequence number field, which is filled with the next available sequence number from a counter. The software then places the packet on the fractional T1 link. Traffic from another queue might be interleaved between two fragments of the packet.
When a packet is removed from a nonencapsulated queue, it is transmitted with a plain PPP header. The packet is then placed on the fractional T1 link as soon as possible. If necessary, the packet is placed between the fragments of a packet from another queue.
The fractional T1 interface links to another router, which can be from Juniper Networks or another vendor. The router at the far end gathers packets from the fractional T1 link. If a packet has an MLPPP header, the software assumes the packet is a fragment of a larger packet, and the fragment number field is used to reassemble the larger packet. If the packet has a plain PPP header, the software accepts the packet in the order in which it arrives, and the software makes no attempt to reassemble or reorder the packet.
Example: Configuring an LSQ Interface for a Fractional T1 Interface Using MLPPP and LFI
Configure a single fractional T1 logical interface:
