Posts Tagged ‘Frame-Relay’

Jun
14

In this short blog post, we are going to give condensed overview of the four main flavors of Frame-Relay Traffic Shaping (FRTS). Historically, as IOS evolved with time, different methods have been introduced, having various level of feature support. Two main features, specific to Frame-Relay Traffic-Shaping are per-VC shaping and queueing and adaptive shaping in response to Frame-Rleay congestion notifications (e.g. BECNs). You’ll see that not every flavor supports these two features. We begin with the «fossil» known as Generic Traffic Shaping.

Generic Traffic Shaping

This feature was initially designed to shape packet traffic sent over any media, be it Ethernet, Frame-Relay, PPP etc. The command syntax is traffic-shape {rate|group} and allows specifying traffic scope using an access-list (notice that different ACL types are supported). You may tune the Bc/Be values as well as the shaping queue depth (amount of buffers). If the shaper delays traffic, the queue service strategy would be fixed to WFQ with the queue size equal to the buffer space allocated. Additional WFQ parameters such as number of flows and congestive discard threshold could not be tuned and set based on the shaper rate automatically.

An unique feature of GTS is the ability to apply multiple shapers to a single interface. However, shapers are not cascaded, but rather a packet is assigned to the first matching shaper rule. In the example below, there are three rules, with the last one being “fallback”, matching all packets that didn’t match access-lists 100 and 101. Unlike using the legacy CAR feature (rate-limit command) you cannot «cascade» multiple traffic-shape statements on the same interface, i.e. there is no “continue” action.

traffic-shape group 100 128000
traffic-shape group 101 64000
traffic-shape group 199 256000
!
access-list 199 permit ip any any

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