Should we wait for the BECN?

Adaptive Shaping could only be configured to shape traffic rates when frame-relay congestion notifications where received “Yes – (BECN – or fecn-adapt, q922 test frame)”, this could result that delay sensitive traffic experience more delay or simple get dropped, until the router waits for the signal in order to slow down to the adaptive rate.

We all recall from our CCNA studys, that in-band congestion signaling [like FECNs, BECNs, (and DE) in the Q.922 header] where set by the Frame-relay Switch to notify end devices that they should start their configured congestion avoidance procedures, recall that if a router receives any BECN during a TC interval, the router decreases the tx rate 25%, and the router will continue to decrement with each BECN (limited to TC intervals) until the traffic rate hits the configured adaptive (minCIR) rate. After the rate had decreased the router takes 16 times intervals of receiving no BECN to start increasing by (Be + Bc)/16, per TC interval.

Now, Cisco, created the Frame-relay Voice-Adaptive Traffic Shaping (FR-VATS) feature, FR-VATS just monitors the LLQ Queue for traffic, and the concept is really simple. When there is traffic on this Priority queue, FR-VATS shapes to the minCIR, for the other non priority classes, leaving the traffic in Priority class to be inserted directly into the Dual Fifo High Priority Queue, just before the TXRing. Also other features like fragmentation and interleaving could be configured to work in conjunction with VATS,  similarly, if there is no traffic in the LLQ Queue for a configurable deactivation time (30 sec by default) the FR-VATS shaper resumes shaping to CIR (not the minCIR), in other words, no more shapers working at night for Bulk Traffic Class 🙂

a sample configuration could be

R2(config)#class-map PREC5
R2(config-cmap)#ma ip prec 5
R2(config-cmap)#exit
R2(config)# class-map PREC0
R2(config-cmap)#ma ip prec 0
R2(config-cmap)#ma access-gr 106
R2(config-cmap)#do show ip access-list
Extended IP access list 106
    10 permit icmp any any (6141842 matches)
R2(config-cmap)#
R2(config-cmap)#policy-map VATS
R2(config-pmap)#class PREC5
R2(config-pmap-c)#priority 10
R2(config-pmap-c)#exit
R2(config-pmap)#class PREC0
R2(config-pmap-c)#band 10
R2(config-pmap-c)#exit
R2(config-pmap)#exit
R2(config)#policy-map SHAPPER
R2(config-pmap)#class class-default
R2(config-pmap-c)#shape average 128000 1005
R2(config-pmap-c)#shape adaptive 10050
R2(config-pmap-c)#shape fr-voice-adapt deactivation 30
R2(config-pmap-c)#service-policy VATS
R2(config-pmap-c)#exit
R2(config-pmap)#map-class frame-relay BINDINGS
R2(config-map-class)#service-policy out SHAPPER
R2(config-map-class)#exit
R2(config)#int s0/1
R2(config-if)#frame-relay framentation voice-adaptive deactivation 30
R2(config-if)#frame-relay fragment 80 end-to-end
R2(config-if)#frame-relay interface-dlci 666
R2(config-fr-dlci)#class BINDINGS

Note: end to end fragmentation is on the interface and not in the class map. (no difference at all)

In Summary, VATS feature is automatically trigged throttling back CIR to minCIR for the non priority traffic, if packets enters the LLQ Queue.

A Networker Blog

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