Flow Control in Trunk

as we all know, the function of flow control can be set on a single port. but i'm wondering can it be applied to a trunk? i feel the answer should be no, however, if we regard a trunk is a logic link, confused...

Reply to
Sailing
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IEEE 802.3x (full-duplex flow control) resides (architecturally) below the link aggregation sublayer. Thus, it applies only to an individual port, and not to an aggregation (so-called "trunk group").

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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Reply to
Rich Seifert

You're probably right that the original post was referring to link aggregation, but possibly he was referring to a trunk segment of a VLAN setup?

I was thinking about VLANs when I first read the post. In which case, flow control would work on trunk segments.

Bert

Reply to
Albert Manfredi

Thank you for your help... 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?

Reply to
Sailing

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

Reply to
Albert Manfredi

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?

Reply to
Sailing

Correct.

Correct.

Bert

Reply to
Albert Manfredi

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