well then  thanks for clearing that up..

However if i run 802.3 and no SNAP what does novel do for its 1byte layer 2
DSAP / SSAP?
is it the same as the one given below or what?

Rgds
Alok

> -----Original Message-----
> From: William Chops Westfield [SMTP:billw@CISCO.COM]
> Sent: Monday, December 04, 2000 10:51 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]:Novell Ether ?
>
>     as far as i remember if i set encap of the ethernet to SNAP instead of
>     ARPA..i cant use IPX on a cisco router..or can i?
>
> Encapsulations have been per-protocol, and sometimes per-destination, for
> quite some time.  For IP, for example, we (can) track which type of ARP a
> system responded to, so that we know how to encapsulate the IP datagrams
> for that host...
>
>
>     the SAP values that is Service Access Points are given below
>     Decimal Hex SAP Description
>
>     0 0000 Unknown
>     1 0001 User
>     2 0002 User Group
>     3 0003 Print Queue or Print Group
>     4 0004 File Server (SLIST source)
>
> These are Novell service numbers, not IEEE SAP numbers (the latter are
> one byte and have some internal structure.  For instance SAP AA is SNAP.)
>
>
>     well then again id like to dispute.. the XNS checksum header.. is a
>     layer 3 spec, if i make my own protocol tomorrow ill put what i want
>     in Layer 3.. i really cant see any checksum in the layer 2 spces.. so
>     which is this checksum ur talking abt?
>
> I am talking about the layer 3 XNS checksum (IPX uses the basic XNS
> packet format, althought NOT any (?) of the rest of the XNS stack.)
>
> A Novell Ethernet packet looks like [dst][src][len][xns...]
> In a multiprotocol network, there's no way to tell that this is a
> Novell packet - there's no "type" identifier.  Fortunately, the first
> two bytes of an XNS packet is the (layer 3) packet checksum, which is
> always 0xFFFF for a Novell ethernet.  Presumably, if you want to turn
> on layer-three checksums for IPX, you need to use SNAP on ethernet.
>
> Novell Ether packet: [dst][src][len][0xffff...xns]
>
> Novell Snap packet: [dst][src][len][0xAAAA][0x3][0x000000][0x8137][xns...]
>                                   dsap/ssap/ctl/ oui      /Novell/...
>
>
> >>        how come SAP on token ring?
>
> DamifIknow.
>
> >> Token Bus was more popular on Novel..my school used it i still
> remember!
>
> IIRC, Token Bus was preferred on broadband (cable, oldstyle) networks.  It
> never got much use outside of a few schools that got talked into it
> because
> of existing CATV wiring, I think.  Token <anything> is pretty dead these
> days.  Ethernet won.  (yeah!)
>
> > [IP uses type-1 ethernet format instead of SAP/SNAP]
> > why IP.. anything now has to follow SNAP .. the whole idea of SNAP
> > was to let 802.3 and 802.2 coexist..
>
> It's not just IP.  I don't think ANYTHING popular uses SAP (and/or snap)
> format on ethernets.  XNS, Novell, Appletalk, DecNet, LAT, MOP, IP, all
> seem to use the ethernet format.  (oops - looks like NETBEUI and Banyan
> use
> IEEE.  I guess those are popular.)  I think the main problem is that it
> was
> never abundantly clear whether the IEEE encapsulations extended the
> payload
> size to include the extra encapsulation bytes (which would have meant
> obsoleting existing HW!)  And 1492 or 1497 bytes are just awfully awkward
> packet sizes, even compared to 1500...
>
> BillW
>
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