VLANs and QoS

Domains

Areas

802.1Q

Layer 2 header, Ethernet

Layer 2 header, 802.1Q

Layer 2 header, 802.1Q

802.1p priority

  1. priority
  2. immediate
  3. flash
  4. flash override
  5. critical
  6. internetwork control
  7. network control

More About 802.1p

For more on 802.1p, I like Bringing Prioritization Services to Ethernet, then maybe Directing Your Network Traffic and Implementing Prioritization for IP Networks if you want to read more. It's really a pretty painless introduction to 802.1p.

VLAN Facts of Life

  1. 802.1Q adds 4 bytes to Ethernet packets between the Layer 2 (Ethernet) header and the Layer 3 leader, increasing the maximum packet size from 1518 to 1522 bytes; 3 bits are for the 802.1p Ethernet class of service and 12 bits are for the 802.1Q VLAN ID.
  2. A VLAN, by definition, provides broadcast containment. Broadcasts (Windows browser elections, Bonjour) are contained within the VLAN.

VLAN Facts of Life

  1. Your IP address determines your IP subnet (of course). Your IP subnet uniquely determines your 802.1Q VLAN. Each VLAN can have several subnets, but each subnet has one and only one VLAN. If the VLAN on your switch port doesn't match your IP address subnet, you can't communicate on the network (except at Layer 2, but the user reports no network connectivity).
  2. For traffic to travel between VLANs, it must go to the router port defined for that VLAN (your gateway). In the simplest case, it then goes through the routing engine and out the router port for the destination VLAN. (It could also go from the routing engine on one router to the routing engine on another router, and then out the router port for the destination VLAN.) Each router knows what subnets match the VLANs it manages, and it knows what router to send packets for the VLANs it doesn't manage.

QoS

QoS Categories

Simple QoS

QoS: IP ToS

QoS: 802.1p

QoS: 802.1p priorities

  1. priority
  2. immediate
  3. flash
  4. flash override
  5. critical
  6. internetwork control
  7. network control

QoS: DiffServ

QoS: DiffServ

Best Practices for DSCP

Best Practices for DSCP

DSCPBinary valueDecimal value
Default0000000
CS10010008
AF1100101010
AF1200110012
AF1300111014
CS201000016
AF2101001018
AF2201010020
AF2301011022
CS301100024
AF3101101026
AF3201110028
AF3301111030
CS410000032
AF4110001034
AF4210010036
AF4310011038
CS510100040
EF10111046
CS611000048
CS711100056

QoS: IntServ

QoS: SBS

WAN

MPLS

MPLS separates packet forwarding from packet routing

QoS Comparison

NameNumber of LevelsOSI LayerHard or SoftStatic or DynamicBandwidth Broker or Policy ServerFunctional TypesEngineeringActive or Proactive
more bandwidth11Soft, RelativeStatic, Managementnone ($)Bandwidth ControlNetwork EngineeringProactive, Overprovisioning
traffic shaping (like Packeteer)11Soft, RelativeStatic, ManagementPolicy ServerTraffic ControlTraffic EngineeringActive, Oversubscription
SBS21Soft, RelativeStatic, Managementnone ($)PrioritizationNetwork EngineeringProactive, Overprovisioning
802.1p82Soft, RelativeStatic, ManagementPolicy ServerBandwidth Control, PrioritizationPer-Device (Network) EngineeringActive, Oversubscription
switch queues82Soft, RelativeStatic, ManagementPolicy ServerPrioritizationPer-Device EngineeringActive, Oversubscription
ToS83Soft, RelativeStatic, Managementnone (per-device)PrioritizationPer-Device EngineeringActive, Oversubscription
DiffServ643Soft, RelativeStatic, ManagementPolicy ServerBandwidth Control, PrioritizationPer-Device EngineeringActive, Oversubscription
IntServ2563Hard, AbsoluteDynamic, SignalingBandwidth BrokerBandwidth Control, PrioritizationPer-Device EngineeringActive, Oversubscription

When to use QoS

Where to use QoS

edge switches run at about 0.2% CPU utilization
drop before congestion
expedite where congestion is more likely thanks to aggregation

Which QoS is Best?

QoS Selector

QoS flowchart

QoS and Network Management

VLAN Reminders

  1. Your IP address determines your IP subnet (of course). Your IP subnet uniquely determines your 802.1Q VLAN. Each VLAN can have several subnets, but each subnet has one and only one VLAN. If the VLAN on your switch port doesn't match your IP address subnet, you can't communicate on the network (except at Layer 2, but the user reports no network connectivity).
  2. 802.1Q adds 4 bytes to Ethernet packets between the Layer 2 (Ethernet) header and the Layer 3 leader, increasing the maximum packet size from 1518 to 1522 bytes; 3 bits are for the 802.1p Ethernet class of service and 12 bits are for the 802.1Q VLAN ID.

VLAN Reminders

  1. A VLAN, by definition, provides broadcast containment. Broadcasts (Windows browser elections, Bonjour) are contained within the VLAN.
  2. For traffic to travel between VLANs, it must go to the router port defined for that VLAN (your gateway). In the simplest case, it then goes through the routing engine and out the router port for the destination VLAN. (It could also go from the routing engine on one router to the routing engine on another router, and then out the router port for the destination VLAN.) Each router knows what subnets match the VLANs it manages, and it knows what router to send packets for the VLANs it doesn't manage.

Example: DHCP

Light Reading

Reading

Homework