7705 SAR Interfaces
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Interface Configuration Guide
3HE 11011 AAAC TQZZA Edition: 01
3.2.1.9 T1/E1 Link Hold Timers
T1/E1 link hold timers (or MLPPP link flap dampening) guard against the node
reporting excessive interface transitions. Timers can be set to determine when link
up and link down events are advertised; that is, up-to-down and down-to-up
transitions of the interface are not advertised to upper layer protocols (are
dampened) until the configured timer has expired.
3.2.2 Multi-Class MLPPP
The 7705 SAR supports multi-class MLPPP (MC-MLPPP) to address end-to-end
delay caused by low-speed links transporting a mix of small and large packets. With
MC-MLPPP, large, low-priority packets are fragmented to allow opportunities to send
high-priority packets. QoS for MC-MLPPP is described in QoS in MC-MLPPP.
MC-MLPPP allows for the prioritization of multiple types of traffic flowing over
MLPPP links, such as traffic between the cell site routers and the mobile operator’s
aggregation routers. MC-MLPPP, as defined in RFC 2686, The Multi-Class
Extension to Multi-Link PPP, is an extension of the MLPPP standard. MC-MLPPP is
supported on access ports wherever PPP/MLPPP is supported, except on the 2-port
OC3/STM1 Channelized Adapter card. It allows multiple classes of fragments to be
transmitted over an MLPPP bundle, with each class representing a different priority
level mapped to a forwarding class. The highest-priority traffic is transmitted over the
MLPPP bundle with minimal delay regardless of the order in which packets are
received.
Figure 4 shows the original MLPPP header format that allowed only two implied
classes. The two classes were created by transmitting two interleaving flows of
packets; one with MLPPP headers and one without. This resulted in two levels of
priority sent over the physical link, even without the implementation of multi-class
support.
Figure 5 shows the short and long sequence number fragment format MC-MLPPP
headers. The short sequence number fragment format header includes two class bits
to allow for up to four classes of service. Four class bits are available in the long
sequence number fragment format header, but a maximum of four classes are still
supported. This extension to the MLPPP header format is detailed in RFC 2686.
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