beef410's version from 2018-05-19 22:41


Question Answer
sh int <>Will list queueing strategy.
Tail DropQueue is full, everything else is dropped. Doesn't care about traffic type, can trigger TCP global synch
TCP Global SynchAs packets are dropped TCP will slow speed, as congestion then clears speed will ramp back up.
TCP GS + Tail DropCauses a roller coaster of inconsistent bw as TCP GS bounces between too much and too little throughput as tail drop occurs and clears
DSCPDifferentiated Services Code Point, packet type label
Bandwidth StarvationToo many high priority traffic types can cause the router to starve bandwidth from lower priority traffic as it tries to service all the high priority packets first.
ShapingTypically delaying packets. Overflow gets delayed until BW is available
PolicingTypically dropping packets.
IntServIntegrated Services model, better than best-effort(UDP) using RSVP. Lacks scalability, enough voip using RSVP will eventually cause starvation.
RSVPResource Reservation Protocol, creates a high-priority end-to-end path of reserved BW before the stream starts. Basically for voip. Potential for starvation.
DiffServDifferentiated Service model, BW is not reserved in advance like RSVP each router along the path is allowed to makes its own next-hop decision while reserving BW.
Markingtagging data with a value. Should be marked as close to source as possible
ClassificationUsing marking to make queueing and transmission decisions.
CoSCode of Service value, switch uses this tag to apply QoS.
ECNExplicit Congestion Notification
ToSType of service field
L3 DiffServWill use ToS to assign a DSCP and ECN value using 8bits. 6DSCP/2ECN
BoundaryThe ingress port of the first switch that trusts QoS markings is the trust boundary.

Queueing strategies

Question Answer
WFQWeighted Fair Queue, splits streams into high and low bandwidth. Low BW goes in front and the rest is split fairly/interleaved between the high BW streams. Is the default for serials running E1 or slower
WFQ exceptionsDoesn't work with tunneling or encryption
FIFODefault on all non-serial interfaces running E1(2.048Mbps) or below
Random Early Detection (RED)An alternative to tail drop to prevent TCP GS. Drops packets indiscriminately before the queue fills to smooth out TCP GS behavior.
Weighted REDRED plug - uses IP Precedence or DSCP to make an intelligent decision on what packets to drop.
LLQLow Latency Queueing, add on to CBWFQ that creates a strict priority queue for delay-sensitive traffic, primarily voip. Prevent voice stream jittering. Recommended to use highest priority queue only for voip.
CBWFQClass Based Weighted Fair Queueing. Allows creation of admin-defined classes by which traffic can be sorted into separate queues. Match criteria can include protocols, access control lists (ACLs), and input interfaces.
Round RobinEqually distributes bandwidth between all queues one at a time, in order.
Weighted Round RobinAdmin can add weights to each queue in the cycle.
Deficit Weighted Round RobinIf a transmission in a queue needs to go over its limit (mid-packet) the router will deduct that BW from its allotment for the next cycle.

DiffServ DSCP class selector values

Question Answer
6bits 2 partsClass Selector and Drop Precedence
Class 7 111Network control traffic STP, routing protocols, etc
Class 6 110Internetwork control, same as Network Control
Class 5 101Expedited Forwarding, voip and time sensitive. Almost guaranteed to not be dropped
Class 1-4Assured Forwarding, allows QoS to manage traffic for lower priority traffic.
Class 0Best Effort Forwarding, default.


Question Answer
BinaryQoS is either trusted or not
IntranetIntranet devices are typically trusted
InternetDevices outside our administrative control are typically untrusted
Trust BoundaryIs between the egress port marking traffic and the ingress port trusting the marked traffic.
PC TrustApplications have been known to use high priority DSCP values. Best practice is not to trust the PC.
Phone TrustIP Phones are considered trustworthy and part of the trust boundary

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