ng_bridge - Ethernet bridging netgraph node type


#include <sys/types.h>
#include <netgraph/ng_bridge.h>


The bridge node type performs Ethernet bridging over one or
more links.
Each link (represented by a connected hook) is used to
transmit and
receive raw Ethernet frames. As packets are received, the
node learns
which link each host resides on. Packets unicast to a known
host are
directed out the appropriate link only, and other links are
spared the
traffic. This behavior is in contrast to a hub, which al
ways forwards
every received packet to every other link.


The bridge node incorporates a simple loop detection algo
rithm. A loop
is when two ports are connected to the same physical medium.
Loops are
important to avoid because of packet storms, which severely
degrade performance. A packet storm results when the same packet is
sent and
received over and over again. If a host is detected on link
A, and is
then detected on link B within a certain time period after
first being
detected on link A, then link B is considered to be a looped
back link.
The time period is called the minimum stable time.
A looped back link will be temporarily muted, i.e., all
traffic received
on that link is ignored.


Processing of IP packets via the ipfirewall(4) mechanism on
a per-link
basis is not yet implemented.


This node type supports up to NG_BRIDGE_MAX_LINKS hooks.
Each connected
hook represents a bridged link. The hooks are named link0,
link1, etc.
Typically these hooks are connected to the lower hooks of
one or more
ng_ether(4) nodes. To connect the host machine to a bridged
simply connect the upper hook of an ng_ether(4) node to the
bridge node.


This node type supports the generic control messages, plus
the following:
Set the node configuration. This command takes a
ng_bridge_config as an argument:
/* Node configuration structure */
struct ng_bridge_config {
u_char ipfw[NG_BRIDGE_MAX_LINKS]; /* enable ipfw
u_char debugLevel; /* debug level */
u_int32_t loopTimeout; /* link loopback
mute time */
u_int32_t maxStaleness; /* max host age be
fore nuking */
u_int32_t minStableAge; /* min time for a
stable host */
The ipfw array enables ipfirewall(4) processing of IP
received on the corresponding links. The debugLevel
field sets the
debug level on the node. At level of 2 or greater, de
tected loops
are logged. The default level is 1.
The loopTimeout determines how long (in seconds) a
looped link is
muted. The default is 60 seconds. The maxStaleness
determines how long a period of inactivity before a
host's entry is
forgotten. The default is 15 minutes. The minSta
bleAge determines
how quickly a host must jump from one link to another
before we
declare a loopback condition. The default is one sec
Returns the current configuration as a struct
Causes the node to forget all hosts and unmute all
links. The node
configuration is not changed.
This command takes a four byte link number as an argu
ment and
returns a struct ng_bridge_link_stats containing
statistics for the
corresponding link, which must be currently connected:
/* Statistics structure (one for each link) */
struct ng_bridge_link_stats {
u_int64_t recvOctets; /* total octets rec'd on
link */
u_int64_t recvPackets; /* total pkts rec'd on
link */
u_int64_t recvMulticasts; /* multicast pkts rec'd
on link */
u_int64_t recvBroadcasts; /* broadcast pkts rec'd
on link */
u_int64_t recvUnknown; /* pkts rec'd with un
known dest addr */
u_int64_t recvRunts; /* pkts rec'd less than
14 bytes */
u_int64_t recvInvalid; /* pkts rec'd with bogus
source addr */
u_int64_t xmitOctets; /* total octets xmit'd on
link */
u_int64_t xmitPackets; /* total pkts xmit'd on
link */
u_int64_t xmitMulticasts; /* multicast pkts xmit'd
on link */
u_int64_t xmitBroadcasts; /* broadcast pkts xmit'd
on link */
u_int64_t loopDrops; /* pkts dropped due to
loopback */
u_int64_t loopDetects; /* number of loop detec
tions */
u_int64_t memoryFailures; /* times couldn't get mem
or mbuf */
This command takes a four byte link number as an argu
ment and clears
the statistics for that link.
Same as NGM_BRIDGE_GET_STATS, but also atomically
clears the statistics as well.
Returns the current host mapping table used to direct
packets, in a
struct ng_bridge_host_ary.


This node shuts down upon receipt of a NGM_SHUTDOWN control
message, or
when all hooks have been disconnected.


Example script showing how to set up a bridging


bridge(4), netgraph(4), ng_ether(4), ng_hub(4),
ng_one2many(4), ngctl(8)


The ng_bridge node type was implemented in FreeBSD 4.2.


Archie Cobbs <>
BSD August 31, 2000
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