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Posted by vicky on July 2, 2008, 8:59 am
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> > I 've query but first i write a statement of IEEE 802.1q (1998) of
> > about CFI (canonical format indicator) ...
>
> > In an Ethernet-encoded tag header, transmitted using 802.3/Ethernet
> > MAC methods, CFI has the following
> > meanings:
> > 1) =A0When set, indicates that the E-RIF field is present in the tag
> > header, and that the NCFI bit in the
> > =A0 =A0 =A0RIF determines whether MAC Address information that may be
> > present in the MAC data carried
> > =A0 =A0 =A0by the frame is in Canonical (C) or Non-canonical (N) format;=
> > 2) =A0When reset, indicates that the E-RIF field is not present in the
> > tag header, and that all MAC
> > =A0 =A0 =A0Address information that may be present in the MAC data carri=
ed
> > by the frame is in Canonical
> > =A0 =A0 =A0format (C).
>
> > --------------
> > Now my query is =A0when the CFI bit is set in case of this ethernet
> > frame.
> > please tell me
>
> The CFI bit is not set in the case of Ethernet bridging Ethernet LANs.
> Ethernet uses canonical address format, therefore:
>
> 1. The CFI bit must not be set (i.e. set to 0), and
>
> 2. You cannot use so-called "source routing."
>
> On the other hand, if the bridge is "tunneling," or bridging, an FDDI
> frame from one FDDI LAN to another FDDI LAN, for example, and there
> are some Ethernet segments between the two FDDI LANs, then the VLAN
> tag can carry that FDDI "source routing" information. It's a way of
> allowing frames from different MACs to be bridged, without losing
> extra header information available in the non-Ethernet LANs.
>
> Here's the note in 802.1Q Clause 9 that explains this:
>
> "NOTE 2=97A decision to use native source-routing on FDDI or to use an
> embedded routing information field in the VLAN tag depends on local
> knowledge in a Bridge or end station of the capabilities of the other
> stations attached to the FDDI LAN. The VLAN tag E-RIF allows source-
> routing information to be transparently 'tunneled' across LANs that do
> not support source routing and through MAC Bridges and VLAN-aware
> Bridges that discard native source-routed frames."
>
> Bert- Hide quoted text -
>
> - Show quoted text -
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Hello
As u mentioned here above in u'r point that
if the bridge is "tunneling," or bridging, an
FDDI
> frame from one FDDI LAN to another FDDI LAN,
for example, and there
> are some Ethernet segments between the two FDDI
LANs, then the VLAN
> tag can carry that FDDI "source routing"
information. It's a way of
> allowing frames from different MACs to be
bridged, without losing
> extra header information available in the non-
Ethernet LANs.
so for doing that function is a brigde is capable of doing routing (is
bridge is source routed bridge) or it done at normally also.
One more thing is about that type of tunneling is it done
automatically or a programmer has also play a role for this , and if a
programmer has a role than also tell me some about of this role.
Thanks
Vikrant
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