ericbrouwers wrote:
>
> Brian,
>
> This is a copy from IOS Interface Configuration Guide,
> Specifying an
> Ethernet Encapsulation Method:
>
> Currently, there are three common Ethernet encapsulation
There are four encapsulations if you count novell-ether.
> methods:
> - The standard Advanced Research Projects Agency (ARPA)
Isn't that awful that they make it sound like Ethernet is a standard that
came from ARPA, which it certainly did not. Ethernet Version 2.0 (and 1.0,
for that matter) came from DEC, Intel, and Xerox. In this case, I would not
have even spelled out ARPA since it's not relevant, but I'm sure the editor
would have insisted on spelling it out.
> Ethernet Version 2.0
> encapsulation, which uses a 16-bit protocol type code (the
> default
> encapsulation method).
Ethernet Version 2.0 is the default for IP (and maybe older versions of
DECnet?) Nothing else.
>
> - Service access point (SAP) IEEE 802.3 encapsulation, in which
> the type
> code becomes the frame length for the IEEE 802.2 LLC
> encapsulation
> (destination and source Service Access Points, and a control
> byte).
The length field is the length of the entire packet. That sentence certainly
needs editing since they make it sound like it's the length of the LLC
header that follows.
>
> - The SNAP method, as specified in RFC 1042, Standard for the
> Transmission
> of IP Datagrams Over IEEE 802 Networks, which allows Ethernet
> protocols to
> run on IEEE 802.2 media.
There's no such thing as 802.2 media. 802.2 is a protocol.
>
> The encapsulation method that you use depends upon the routing
> protocol that
> you are using, the type of Ethernet media connected to the
> router or access
> server, and the routing or bridging application that you
> configure.
The encapsulation method depends on the protocol being carried by the
Ethernet frame. It certainly doesn't depend on the media.
A router has three fundamental jobs: forwarding, routing, and management.
For forwarding, the encapsulation depends on the type of protocol being
forwarded.
With regards to management, CDP, STP (if it's bridging), etc. use their
defined encapsulations (defined by industry standards and by IOS code, and
not changeable).
For routing protocols, it depends on the routing protocol, I guess, though
that's missing the main point that it depends on the protocol being carried
mostly. RTMP uses snap. That's because it's an AppleTalk routing protocol.
OSPF, RIP, IGRP, EIGRP use Ethernet II. That's because they are IP.
IS-IS, is a weird one. It uses 802.3 and 802.2 (so awfully named by Cisco as
sap.)
>
> To establish Ethernet encapsulation of IP packets, use one of
> the following
> commands in interface configuration mode:
The document says that you can change the encapsulation for IP packets? You
can't do that on my routers! There's no need to do it anyway. There is
essentially no modern operating system that doesn't use Ethernet II for IP.
I'd be interested to see if anyone else is able to change the Ethernet
encapsulation for IP packets on their routers.
Thanks,
Priscilla
>
> Command
> Purpose
> Router(config-if)# encapsulation arpa Selects
> ARPA Ethernet
> encapsulation.
> Router(config-if)# encapsulation sap Selects
> SAP Ethernet
> encapsulation
> Router(config-if)# encapsulation snap Selects
> SNAP Ethernet
> encapsulation.
>
> For an example of selecting Ethernet encapsulation for IP, see
> the "Ethernet
> Encapsulation Enablement Example" section.
> ...
> ...
> Ethernet Encapsulation Enablement Example:
> These commands enable standard Ethernet Version 2.0
> encapsulation on the
> Ethernet interface processor in slot 4 on port 2 of a Cisco
> 7500 series
> router:
>
> interface ethernet 4/2
> encapsulation arpa
>
> Eric
>
> ----- Original Message -----
> From: "Priscilla Oppenheimer"
> To:
> Sent: Wednesday, June 18, 2003 2:08 AM
> Subject: RE: encap for ethernet interface ? [7:70802]
>
>
> > brian dell wrote:
> > >
> > > say if cdp is enabled for an interface, then since cdp uses
> sap
> > > encapsulation (as Priscilla mentioned), then i don't think
> one
> > > configures encapsulation for that interface as sap ?
> > >
> > > the question is that why is this statement ("encap sap")in
> the
> > > configuration not needed if an interface has cdp enabled ?
> > > (i guess "encap arpa" is by default understood for an
> ethernet
> > > interface ? is that correct ? )
> >
> > No. There's no default for an interface. There's only
> defaults for
> > particular protocols. CDP uses snap (not sap). IP uses ARPA.
> Spanning Tree
> > uses sap. Novell users novell-ether.
> >
> > With the exception of Novell, VLAN tagging, and ARP, you
> can't change the
> > encapsulation that will be used for Ethernet frames for the
> various
> protocols.
> >
> > Try it on a real router. It's nothing like changing
> encapsulation on a
> WAN,
> > which causes all traffic across the WAN link to use that
> encapsulation.
> Here
> > are some hints:
> >
> >
> > Albany#config t
> > Enter configuration commands, one per line. End with CNTL/Z.
> > Albany(config)#ipx routing
> > Albany(config)#int e0
> > Albany(config-if)#ipx network 400 encapsulation ?
> > arpa Novell Ethernet_II
> > hdlc HDLC on serial links
> > novell-ether Novell Ethernet_802.3
> > sap IEEE 802.2 on Ethernet, FDDI, Token Ring
> > snap IEEE 802.2 SNAP on Ethernet, Token Ring, and
> FDDI
> > Albany(config-if)#ipx network 400 encapsulation snap
> > Albany(config-if)#ipx network 100 encapsulation arpa secondary
> > Albany(config-if)#ipx network 200 encapsulation sap secondary
> > Albany(config-if)#ipx network 300 encapsulation novell-ether
> secondary
> >
> >
> >
> >
> > Albany#config t
> > Enter configuration commands, one per line. End with CNTL/Z.
> > Albany(config)#int e0
> > Albany(config-if)#arp ?
> > arpa Standard arp protocol
> > frame-relay Enable ARP for a frame relay interface
> > probe HP style arp protocol
> > snap IEEE 802.3 style arp
> > timeout Set ARP cache timeout
> > Albany(config-if)#arp snap
> >
> >
> >
> > If you try just the encapsulation command in interface
> configuration mode,
> > you get a choice of VLAN tagging methods. I can't show you
> that because my
> > routers don't support it. VLAN tagging is a topic for another
> disertation,
> > not really related to the question you are asking.
> >
> > Priscilla
>
>
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