Re: route field

On Jul 1, 8:23 am, vicky <vikrant.pan...@xxxxxxxxx> wrote:

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)  When set, indicates that the E-RIF field is present in the tag
header, and that the NCFI bit in the
     RIF determines whether MAC Address information that may be
present in the MAC data carried
     by the frame is in Canonical (C) or Non-canonical (N) format;
2)  When reset, indicates that the E-RIF field is not present in the
tag header, and that all MAC
     Address information that may be present in the MAC data carried
by the frame is in Canonical
     format (C).

--------------
Now my query is  when 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—A 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
.

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