Comparing Traditional Bridge Mode to VLAN-aware Bridge Mode

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Summary

The Cumulus Linux bridge driver operates in two modes: VLAN-aware and a traditional Linux mode. There are many minor syntax differences between the two modes, which are outlined below. However, the following behaviors apply no matter which mode you use for the driver:

  • Network interfaces are configured under /etc/network/interfaces.
  • Both modes support Spanning Tree Protocol (see the example below).
  • You manage interfaces configured with both modes with ifupdown commands (ifup bridge, ifdown bridge).

The reasons why you would use VLAN-aware mode for bridges are:

  • Scale: The new VLAN-aware mode can support 2000 concurrent VLANs while the traditional mode supports only 200 concurrent VLANs.
  • Simplicity: VLAN-aware mode has a simpler configuration.

The only reasons to use the traditional mode are:

  • Familiarity with traditional Linux syntax.
  • VXLAN support: As of Cumulus Linux 3.1, VXLAN is supported by VLAN-aware mode bridges. For VXLAN support on earlier releases, use traditional mode.
  • PVSTP+ interoperability: The traditional mode currently runs an instance of spanning tree per bridge. The VLAN-aware STP mode is compatible with other types of spanning tree but only runs single instance MST. To achieve Per-VLAN STP/RSTP the traditional bridge mode must be used.

Two Trunks Containing 200 VLANs for swp1 and swp2

Traditional VLAN-Aware
auto br-vlan1
iface br-vlan1
  bridge-ports swp1 swp2
auto br-vlan2
iface br-vlan2
  bridge-ports swp1.2 swp2.2
auto br-vlan3
iface br-vlan3
  bridge-ports swp1.3 swp2.3
 .
 .
auto br-vlan200
iface br-vlan200
  bridge-ports swp1.200 swp2.200
auto bridge
iface bridge
  bridge-vlan-aware yes
  bridge-ports swp1 swp2
  bridge-vids 1-200
  bridge-pvid 1

Note: The ... is an abbreviated output. If you're creating a trunk in traditional mode, you would need 200 stanzas for the 200 interfaces. Remember, a bridge configured in traditional mode is limited to 200 VLANs.

Creating an SVI

An SVI is a switch VLAN/virtual interface, also known as a layer 3 VLAN interface.

Traditional VLAN-aware
auto bridge
iface bridge
  bridge-ports swp1.10 swp2.10
  address 192.168.10.1/24
  address 2001:db8::1/32
auto bridge
iface bridge
  bridge-vlan-aware yes
  bridge-ports swp1 swp2
  bridge-vids 1-200
  bridge-pvid 1

auto bridge.10
iface bridge.10
  address 192.168.10.1/24
  address 2001:db8::1/32

Note: The comparison above is not exactly apples to apples, if the traditional mode configuration was the only configuration applied to the switch, there would be only VLAN 10 traffic for swp1 and swp2 as indicated by the tags. However the VLAN-aware bridge driver has two trunks for 200 VLANs. This behavior is very different.

Creating Access Ports (also known as Untagged Ports)

Traditional VLAN-aware
auto bridge
iface bridge
  bridge-ports swp1 swp2
auto bridge
iface bridge
  bridge-vlan-aware yes
  bridge-ports swp1 swp2
  bridge-vids 1-200
  bridge-pvid 1

auto swp1
iface swp1
  bridge-access 10

auto swp2 iface swp2 bridge-access 10

Note: The above illustrates how the traditional mode has no concept of VLANs, just untagged or tagged traffic. It's basically identical to the previous example except there is no .10 after the switch ports here.

Two Trunks Containing Two VLANs with Spanning Tree Enabled

Traditional VLAN-Aware
auto br-vlan100
iface br-vlan100
    bridge-ports swp1.100 swp2.100
    bridge-stp on

auto br-vlan200
iface br-vlan200
    bridge-ports swp1.200 swp2.200
    bridge-stp on
auto bridge
iface bridge
    bridge-vlan-aware yes
    bridge-ports swp1 swp2
    bridge-vids 100 200
    bridge-stp on

Note: The bridge-stp on option is identical for the two modes; however, the VLAN-aware bridge mode only needs to have it specified once.

See Also

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