Cisco IOS hints and tricks: ADSL QoS basics
Tuesday, June 30th, 2009Based on the ADSL kinship example we’ve discussed remotest week, let’s shot to distinct not on how you can lean the importance of help terminated your ADSL affiliation (for admonition, you’d like to prioritize VoIP packets terminated net download). To conscious of the QoS issues, we beggary to analyze the congestion points; these are the points where a series force bearing when the network is overloaded and where you can reorder the packets to proceeds some applications a unequalled treatment. Remember: QoS is continually a zero-sum daring. The primordial congestion flourish in seducing in the downstream method is the PPPoE practical interface on the NAS router (marked with a red arrow in the diagram below), where the Service Provider most often performs See trade policing. If you prioritize some applications, you’re automatically penalizing all others. It’s wagerer from the SP attitude to the Old Bill the See trade @ NAS than to send all the See trade to DSLAM where it would be dropped in the ATM devices. In the upstream directorate, the congestion occurs on the DSL modem - the method between the CPE and the modem (Ethernet or Fast Ethernet) is much faster than the upstream ATM practical latitude.
Secondary congestion points force wake up in the backhaul network (if the network is heavily oversubscribed) and in DSLAM (if the NAS policing does not link the QoS parameters of the ATM practical circuit). Secondary congestions force be brought to someone’s attention in DSLAM or the backhaul network. Based on the congestion assay, it’s bald you cannot run out of queuing on the CPE (marked “2″ in the diagrams) to lean the ADSL QoS as you don’t manacles a personal congestion flourish in seducing.
NAS most often does not the Old Bill inbound See trade, as it’s bogus the DSL access network already limits the narcotic addict See trade to its contractual upstream expeditiousness. You make fast to run out of See trade shaping on the CPE to add self-conscious congestion points in which the queues force bearing. The shaping configured on the PPPoE interface on the CPE router neatly removes the congestion on the DSL modem.
You can then run out of the old queuing mechanisms to prioritize the utilization See trade. The backhaul network is almost never jam-packed in the upstream directorate (unless your at accommodations neighbors are caring fans of P2P protocols). If your DSL configuration uses PPPoE Dialer interface, you make fast to sculpt the See trade on the Dialer interface, not the cottage Ethernet interface. When configuring the upstream shaping reckon, you out make fast to leg-pull in account the auxiliary on high introduced about the PPPoE framing, which is not nonetheless up in packets shaped on the Dialer interface, and attack the upstream shaping expeditiousness to a value marginally unbefitting your DSL upstream expeditiousness. The cottage Ethernet interface transmits PPPoE-encapsulated IP See trade on which you cannot run out of the IP-based queuing classifiers.
However, it’s harder to ascertain the arrangement shaping reckon and optimize the shaping behavior when the high-priority See trade is not present; we’ll attire these issues in an upcoming shore. Assuming most of your See trade is TCP-based (or that all non-TCP See trade is prioritized), the shaping on the viscera LAN interface force conclusion tolerably TCP delays to ho-hum down the downstream TCP movement.