[CentOS] how to enable Flow Control on CentOS?
Giles Coochey
giles at coochey.net
Sun Jul 10 08:46:11 UTC 2011
On 10/07/2011 10:22, Charles Polisher wrote:
> Gigabit is different.
> My reading of the spec is that when a port is configured for 1GbE
> over 1000BASE-T (copper), "disabling" auto-negotiation disables the
> advertising of the auto-negotiation for 10BASE-T and 100BASE-T, but
> auto-negotiation is still advertised and operational for 1GbE.
> Auto-negotiation cannot be disabled for 1000BASE-X (optical fiber).
>
> Deviation from the spec would mean such kit is Ethernet-like.
> An ability to "set" auto-negotiation one way in the user interface
> while leaving the hardware in a different - standards conforming -
> state is possible.
>
Fiber is not a CSMA/CD medium, it's a Point to Point medium - Duplex is
meaningless.
I've been referring to the Spec of 10/100 ports. For Gigabit ports
1000Base-T, auto-negotiation is mandatory.
Quoting from Wikipedia [http://en.wikipedia.org/wiki/Autonegotiation]
Interoperability problems
The first version of the autonegotiation specification, IEEE 802.3u, was
open to different interpretations. Although most manufacturers
implemented this standard in one way, some others, including network
giant Cisco, implemented it in a different way. Autonegotiation between
devices that implemented it differently failed. This led many network
administrators to not depend on autonegotiation and instead manually set
the speed and duplex mode of each network interface card. Even Cisco
recommended its customers not to use autonegotiation. However, the use
of manually set configuration often led to duplex mismatches, in
particular when two connected devices are:
* One manually set to half duplex and one manually set to full duplex
* One set to autonegotiation and one manually set to full duplex
* Both sides manually set to full duplex where one side still expects
an autonegotiating link partner and the other side has
autonegotiation completely disabled (the side that expects an
autonegotiating link partner will fall back to half duplex because
it does not detect a partner capable of full duplex)^[/citation
needed/]
Duplex mismatch problems are difficult to diagnose because the network
is apparently working, and simple programs used for network tests such
as ping report a valid connection; however, the network is much slower
than expected.
The debatable portions of the autonegotiation specifications were
eliminated by the 1998 release of 802.3. This was later followed by the
release of IEEE 802.3ab in 1999. The new standard specified that gigabit
Ethernet over copper wiring requires autonegotiation. Currently, all
network equipment manufacturers—including Cisco^[3] —recommend to use
autonegotiation on all access ports. Cisco also recommends that you
check back with them yearly for any potential changes in their
recommendation as this has caused much confusion over the years. ^[4]
In some large installations that have had to deal with negotiation
issues, network staff may believe that "autonegotiation doesn't work",
and consider turning it off a best-practice. This should be avoided -
once autonegotiation is turned off, it will not work by definition,
creating a self-enforcing problem.
--
Best Regards,
Giles Coochey
NetSecSpec Ltd
NL T-Systems Mobile: +31 681 265 086
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Business Email: giles.coochey at netsecspec.co.uk
Email/MSN/Live Messenger: giles at coochey.net
Skype: gilescoochey
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