Re: Flow Control in Trunk


Albert Manfredi wrote:
"Sailing" <sailing511@xxxxxxxxx> wrote:

well, what's the difference between "a trunk segment of a VLAN" and
link aggregation?
i don't think i have a clear idea referring to VLAN segment.
i looked up some documents and found the following definition:
a trunk link is a lan segment containing vlan-aware bridges and
vlan-aware end stations.
but can you explain it more explicately?

Link aggegation is a scheme whereby you tie together multiple Ethernet
segments to behave as if it were only one, faster link. Typically,
between two switches. This technique is sometimes referred to as
"inverse multiplexing." This is described in 802.3-2005 Clause 43.

VLANs are described in 802.1Q. The idea is to have one physical Ethernet
(or other link layer) behave as if you had multiple Ethernets. By adding
overhead fields to the Ethernet frame, a VLAN trunk segment can carry
frames which are meant to belong to different LANs. Typically, these
trunk segments are only used between switches or routers, although in
principle they could also be used between two hosts, between hosts and
routers, between hosts and switches.

You're really doing two opposite things here. Either bundling multiple
physical Ethernet segments to behave as if they were one, or adding
overhead to Ethernet frames to allow a single segment to behave as if it
were multiple physcal Ethernet segments.

So how does this relate to flow cntrol. Flow control operates on
individual physical Ethernet segments, between switches, or between
switches and hosts. So flow control would not work if you have multiple
physical segments aggregated into a single trunk. There's no scheme (as
of now) to guarantee that the flow control operation can be synchronized
precisely among multiple Ethernet segments, which choreography would be
needed if you expect to be able to aggregate multiple segments. But
since flow control operates independently of the Ethernet data frames,
it should work just fine in a VLAN trunk example. Flow control doesn't
know or care whether that segment is pretending to be many different
segments. It just applies backpressure as buffers reach a certain level.

Bert

thank you very much for your detailed explanation...
i still got sth. confusing:
the trunk link under the VLAN context is a special access link which
connects vlan-awared equipments on both ends, isn't it?
as it is a single physical segment, so can be applied to flow control,
right?

.

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