# Bridging
This is the most central part of the system. A bridge is a switch, and
a switch is a bridge. In Linux, setting up a bridge with ports
connected to physical switch fabric, means you manage the actual switch
fabric!
## MAC Bridge
In Infix ports are by default not switch ports, unless the customer
specific factory config sets it up this way. To enable switching, with
offloading if you have a switch chipset, between ports you create a
bridge and then add ports to that bridge. Like this:
admin@example:/> configure
admin@example:/config/> edit interface br0
admin@example:/config/interface/br0/> up
admin@example:/config/> set interface eth0 bridge-port bridge br0
admin@example:/config/> set interface eth1 bridge-port bridge br0
admin@example:/config/> leave
Here we add two ports to bridge `br0`: `eth0` and `eth1`.
> [!TIP]
> The CLI has several built-in helpers governed by convention. E.g.,
> naming bridges `brN`, where `N` is a number, the type is *inferred*
> automatically and unlocks all bridge features. Other conventions are
> `vethNA`, where `N` is a number and `A` is a letter ('a' for access
> port and 'b' for bridge side is common), and `ethN.M` for VLAN M on
> top of `ethN`, or `dockerN` for a IP masquerading container bridge.
>
> Note, this inference only works with the CLI, configuring networking
> over NETCONF or RESTCONF requires setting the type explicitly.
It is possible to create multiple MAC bridges, however, it is
currently[^1] _not recommended_ to use more than one MAC bridge on
products with Marvell LinkStreet switching ASICs. A VLAN filtering
bridge should be used instead.
## VLAN Filtering Bridge
By default bridges in Linux do not filter based on VLAN tags. This can
be enabled when creating a bridge by adding a port to a VLAN as a tagged
or untagged member. Use the port default VID (PVID) setting to control
VLAN association for traffic ingressing a port untagged (default PVID:
1).
admin@example:/config/> edit interface br0
admin@example:/config/interface/br0/> up
admin@example:/config/> set interface eth0 bridge-port bridge br0
admin@example:/config/> set interface eth0 bridge-port pvid 10
admin@example:/config/> set interface eth1 bridge-port bridge br0
admin@example:/config/> set interface eth1 bridge-port pvid 20
admin@example:/config/> edit interface br0
admin@example:/config/interface/br0/> set bridge vlans vlan 10 untagged eth0
admin@example:/config/interface/br0/> set bridge vlans vlan 20 untagged eth1
This sets `eth0` as an untagged member of VLAN 10 and `eth1` as an
untagged member of VLAN 20. Switching between these ports is thus
prohibited.
To terminate a VLAN in the switch itself, either for switch management
or for routing, the bridge must become a (tagged) member of the VLAN.
admin@example:/config/interface/br0/> set bridge vlans vlan 10 tagged br0
admin@example:/config/interface/br0/> set bridge vlans vlan 20 tagged br0
To route or to manage via a VLAN, a VLAN interface needs to be created
on top of the bridge, see section [VLAN Interfaces](ethernet.md#vlan-interfaces)
for more on this topic.
> [!NOTE]
> In some use-cases only a single management VLAN on the bridge is used.
> For the example above, if the bridge itself is an untagged member only
> in VLAN 10, IP addresses can be set directly on the bridge without the
> need for dedicated VLAN interfaces on top of the bridge.
## Multicast Filtering and Snooping
Multicast filtering in the bridge is handled by the bridge itself. It
can filter both IP multicast and MAC multicast. For IP multicast it
also supports "snooping", i.e., IGMP and MLD, to automatically reduce
the broadcast effects of multicast. See the next section for a summary
of the [terminology used](#terminology-abbreviations).
> [!IMPORTANT]
> Currently there is no way to just enable multicast filtering without
> also enabling snooping. This may change in the future, in which case
> a `filtering` enabled setting will be made available along with the
> existing `snooping` setting.
When creating your bridge you must decide if you need a VLAN filtering
bridge or a plain bridge (see previous section). Multicast filtering is
supported for either, but take note that it must be enabled and set up
per VLAN when VLAN filtering is enabled -- there are no global multicast
settings in this operating mode.
In the following example we have a regular 8-port bridge without VLAN
filtering. We focus on the multicast specific settings:
admin@example:/> configure
admin@example:/config/> edit interface br0
admin@example:/config/interface/br0/> set bridge multicast snooping
admin@example:/config/interface/br0/> set ipv4 address 192.168.2.1 prefix-length 24
admin@example:/config/interface/br0/> leave
admin@example:/> copy running-config startup-config
Here we enable snooping and set a static IPv4 address so that the switch
can take part in IGMP querier elections. (MLD querier election
currently not supported.) We can inspect the current state:
admin@example:/> show ip multicast
Multicast Overview
Query Interval (default): 125 sec
Router Timeout : 255
Fast Leave Ports :
Router Ports :
Flood Ports : e0, e1, e2, e3, e4, e5, e6, e7
br0 192.168.2.1 Up 125 None 3
br0 224.1.1.1 e3, e2
br0 ff02::6a br0
This is a rather small LAN, so our bridge has already become the elected
IGMP querier. We see it is ours because the timeout is `None`, and we
recognize the IP address the system has detected, as ours. We can also
see two ports that have joined the same IPv4 multicast group, 224.1.1.1,
and one join from the system itself for the IPv6 group ff02::6a.
Now, let us see what happens when we add another bridge, this time with
VLAN filtering enabled. We skip the boring parts about how to move
ports e4-e7 to `br1` and assign them to VLANs, and again, focus on the
multicast bits only:
admin@example:/> configure
admin@example:/config/> edit interface br1
admin@example:/config/interface/br1/> set bridge vlans vlan 1 multicast snooping
admin@example:/config/interface/br1/> set bridge vlans vlan 2 multicast snooping
admin@example:/config/interface/br1/> leave
admin@example:/> copy running-config startup-config
Let us see what we get:
admin@example:/> show ip multicast
Multicast Overview
Query Interval (default): 125 sec
Router Timeout : 255
Fast Leave Ports : e5
Router Ports : e1, e2, e5, e6, e7
Flood Ports : e1, e2, e3, e4, e5, e6, e7, e8
br0 192.168.2.1 Up 125 None 3
br1 1 0.0.0.0 Up 125 None 3
br1 2 0.0.0.0 Up 125 None 3
br0 224.1.1.1 e2
br0 ff02::fb br0
br0 ff02::6a br0
br0 ff02::1:ff00:0 br0
br1 1 224.1.1.1 e5
br1 2 224.1.1.1 e7
br1 1 ff02::fb br1
br1 1 ff02::1:ff00:0 br1
In this setup we have a lot more going on. Multiple multicast router
ports have been detected, and behind the scenes someone has also added
an IGMP/MLD fast-leave port.
### Terminology & Abbreviations
- **IGMP**: Internet Group Membership Protocol, multicast subscription
for IPv4, for details see [RFC3376][]
- **MLD**: Multicast Listener Discovery (Protocol), multicast
subscription for IPv6, for details see [RFC3810][]
- **Unknown/Unregistered multicast**: multicast groups that are *not*
in the multicast forwarding database (MDB)
- **Known/Registered multicast**: multicast groups that *are* in the
multicast forwarding database (MDB)
- **MDB**: the multicast forwarding database, consists of filters for
multicast groups, directing where multicast is allowed to egress. A
filter entry consists of a group and a port list. The bridge filters
with a unique database per VLAN, in the same was as the unicast FDB
- **Join/Leave**: the terminology used in earlier versions of the two
protocols to subscribe and unsubscribe to a multicast group. For
more information, see *Membership Report*
- **Membership Report** A membership report is sent by end-devices and
forwarded by switches to the elected querier on the LAN. They
consist of multiple "join" and "leave" operations on groups. They
can also, per group, list which senders to allow or block. Switches
usually only support the group subscription, and even more common
also only support filtering on the MAC level[^2]
- **Querier election**: the process of determining who is the elected
IGMP/MLD querier on a LAN. Lowest numerical IP address wins, the
special address 0.0.0.0 (proxy querier) never wins
- **Proxy querier**: when no better querier exists on a LAN, one or
more devices can send proxy queries with source address 0.0.0.0 (or
:: for IPv6). See **Query Interval**, below, why this is a good
thing
- **Query interval**: the time in seconds between two queries from an
IGMP/MLD querier. It is not uncommon that end-devices do not send
their membership reports unless they first hear a query
- **Fast Leave**: set on a bridge port to ensure multicast is pruned as
quickly as possible when a "leave" membership report is received. In
effect, this option marks the port as directly connected to an
end-device. When not set (default), a query with timeout is first
sent to ensure no unintentional loss of multicast is incurred
- **Router port**: can be both configured statically and detected at
runtime based on connected devices, usually multicast routers. On
a router port *all* multicast is forwarded, both known and unknown
- **Flood port**: set on a bridge port (default: enabled) to ensure
all *unknown* multicast is forwarded
- **Router timeout**: the time in seconds until a querier is deemed to
have been lost and another device (switch/router) takes over. In the
tables shown above, a *None* timeout is declared when the current
device is the active querier
> [!TIP]
> The reason why multicast flooding is enabled by default is to ensure
> safe co-existence with MAC multicast, which is common in industrial
> networks. It also allows end devices that do not know of IGMP/MLD to
> communicate over multicast as long as the group they have chosen is
> not used by other IGMP/MLD aware devices on the LAN.
>
> As soon as an IGMP/MLD membership report to "join" a group is received
> the group is added to the kernel MDB and forwarding to other ports
> stop. The only exception to this rule is multicast router ports.
>
> If your MAC multicast forwarding is not working properly, it may be
> because an IP multicast group maps to the same MAC address. Please
> see [RFC 1112][RFC1112] for details. Use static multicast router
> ports, or static multicast MAC filters, to mitigate.
[RFC1112]: https://www.rfc-editor.org/rfc/rfc1112.html
[RFC3376]: https://www.rfc-editor.org/rfc/rfc3376.html
[RFC3810]: https://www.rfc-editor.org/rfc/rfc3810.html
## Forwarding of IEEE Reserved Group Addresses
Addresses in the range `01:80:C2:00:00:0X` are used by various bridge
signaling protocols, and are not forwarded by default. Still, it is
sometimes useful to let the bridge forward such packets, this can be
done by specifying protocol names or the last address *nibble* as
decimal value `0..15`:
admin@example:/config/> edit interface br0 bridge
admin@example:/config/interface/br0/bridge/> set ieee-group-forward # Tap the ? key for alternatives
[0..15] List of IEEE link-local protocols to forward, e.g., STP, LLDP
dot1x 802.1X Port-Based Network Access Control.
lacp 802.3 Slow Protocols, e.g., LACP.
lldp 802.1AB Link Layer Discovery Protocol (LLDP).
stp Spanning Tree (STP/RSPT/MSTP).
admin@example:/config/interface/br0/bridge/> set ieee-group-forward
The following example configures bridge *br0* to forward LLDP packets.
admin@example:/config/interface/br0/bridge/> set ieee-group-forward lldp
admin@example:/config/interface/br0/bridge/>
[^1]: MAC bridges on Marvell Linkstreet devices are currently limited to
a single MAC database, this may be a problem if the same MAC address
appears in different MAC bridges.
[^2]: For example, IPv4 groups are mapped to MAC multicast addresses by
mapping the low-order 23-bits of the IP address in the low-order 23
bits of the Ethernet address 01:00:5E:00:00:00. Meaning, more than
one IP multicast group maps to the same MAC multicast group.