Junos Enterprise Routing, 2nd Edition

Adaptive Shapers and Virtual Channels

This section focuses on SRX and J-series specific CoS capabilities that are designed to work with Frame Relay. The virtual channel and adaptive shaping features help to optimize Frame Relay–based transport. Note that currently you cannot combine the functionality of an adaptive shaper with that of a virtual channel.

Configure Adaptive Shaping

Recall that Bock and Porter are connected via a 0 CIR Frame Relay service terminating in a 500 Kbps port. With the newly added DiffServ-based CoS infrastructure now in place, the idea of a 0 CIR service has been revisited. The result is a decision to pay extra for a guaranteed CIR of 256 Kbps, with the ability to burst to port speed via an EIR of 244 Kbps (CIR + EIR = port speed).

Simply configuring a scheduler or shaper that allows the router to send at maximum speed, all the time, is problematic because during network congestion only the CIR traffic is guaranteed for delivery. Ideally, you want to send at the EIR rate only when the network is not congested, and then fall back to the CIR when congestion is detected, in an effort to ensure that congestion-induced discards do not negate your CoS SLAs. This capability is exactly what adaptive shaping provides.

The configurations at Bock and Porter are updated to support adaptive shaping. The modified configuration at Bock is shown:

[edit class-of-service]
lab@Bock# show adaptive-shapers
becn_shaper {
    trigger becn shaping-rate 256k;
} [edit class-of-service] lab@Bock# ...