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Virtual LAN as A Network Control Mechanism. Tzi-cker Chiueh Computer Science Department Stony Brook University. Ethernet Routing. Spanning tree topology Source Learning to populate the forwarding table Broadcast if don’t know what to do

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Virtual lan as a network control mechanism

Virtual LAN as A Network Control Mechanism

Tzi-cker Chiueh

Computer Science Department

Stony Brook University

EdgeNet2006 Summit

Ethernet routing
Ethernet Routing

  • Spanning tree topology

  • Source Learning to populate the forwarding table

  • Broadcast if don’t know what to do

  • Question: How to control the routes on large L2 networks of commodity Ethernet switches? VLAN

EdgeNet2006 Summit

Virtual lan ieee 802 1q
Virtual LAN (IEEE 802.1Q)

  • Originally proposed to support multiple IP subnets on a L2 network without L3 routers

    • VLAN limits the scope of a broadcast packet

  • 4-byte 802.1Q header inserted between SRC MAC and Type/Length

    • 2-byte 802.1Q tag type = 0x8100

    • 3 bits for priority (IEEE 802.1P)

    • 1 bit for Canonical Format Indicator

    • 12 bits for VLAN ID

EdgeNet2006 Summit

Vlan in practice
VLAN in Practice

  • 802.1Q tag is added at the hosts or edge switches

  • Packets are exchanged between two VLANs through a router

  • Conceptually, each VLAN is like a physical LAN that has its own

    • Spanning tree

    • L2 routing table

  • 802.1S allows per-VLAN spanning tree

  • Number of VLANs supported in real switches is hundreds

  • VLAN specification is port-based or host-based

    • Configuration can be based on SNMP or web requests or CLI

EdgeNet2006 Summit

Viking project
Viking Project

  • Goal: A network resource management system for campus-wide L2 network backbone or Metro Ethernet Services

  • A large number of low-port-density switches vs. a small number of high-port-density switches

    • Larger geographic coverage

    • More cost-effective (economy of scales)

    • More redundancy at the physical connectivity level

    • Higher aggregate back-plane throughput

EdgeNet2006 Summit

Problem with existing ethernet
Problem with Existing Ethernet

  • Main problem: single spanning tree

    • Inefficient

    • Inflexible routing

    • Longer failure recovery

EdgeNet2006 Summit

Traffic engineering
Traffic Engineering

  • Constantly measure traffic load matrix

  • Compute an active-backup path for each node pair to balance loads among links and use shorter links whenever possible mesh rather than tree

  • Force a path’s route by setting up a dedicated logical VLAN for it  ATM-like behavior on Ethernet

  • Need to combine multiple logical VLANs into one physical VLAN, which corresponds to a spanning tree; active and path paths belong to different VLANs

EdgeNet2006 Summit

Big picture
Big Picture

  • Each host in a single IP subnet participates in multiple VLANs, and uses different VLANs to reach different destination

  • Fast failure recovery: Switch to a different 802.1S VLAN to reach a destination when the current VLAN fails

    • The failure recovery time of the Viking prototype is less than 500 msec, most of which is SNMP trap

  • Next step: Edge-based traffic shaping and 802.1P for QoS guarantee

EdgeNet2006 Summit

Igmp snooping
IGMP Snooping

  • Why: Avoid using L2 broadcast when supporting L3 multicast

  • How: Snoop on IGMP packets to infer a L2 distribution tree for an IP multicast group on top of a L2 network’s spanning tree

  • Supported by most commodity Ethernet switches

  • Real switches can only track a small number of IP multicast groups

  • Configuration: Sending IGMP packets to the root, which acts as the default router

EdgeNet2006 Summit

Cassini project
Cassini Project

  • Goal: Leverage commodity Ethernet switches as building block for storage area network

  • Multicast is an important primitive

  • Idea: Use VLAN/IGMP snooping to support tree-based L2 multicast

  • Transparent Reliable Multicast:

    • Multiple L3 connections (e.g. TCP) layered on on top of a L2 multicast connection

    • ACK/Retransmission on individual L3 unicast connection

EdgeNet2006 Summit


  • Many innovative features in commodity Ethernet switches that are largely exploited

  • CLI or SNMP or HTTP provides the possibility of on-the-fly reconfiguration according to workloads and/or hardware health status

  • Interesting application scenarios:

    • Large-scale L2 network

    • Storage area network

    • Compute cluster interconnect: program-specific topology

EdgeNet2006 Summit

Virtual lan as a network control mechanism

Thank You!


EdgeNet2006 Summit

Mariner project
Mariner Project

  • Goal: Leverage advanced features of commodity Gigabit Ethernet switches to build scalable compute cluster interconnects (~1000 nodes)

  • Programmable application-specific interconnect topology

  • Fault management: asynchronous state check-pointing and pessimistic message logging

  • Scalable multicast state management

EdgeNet2006 Summit