At 03:34 PM 5/29/01, Marty Adkins wrote:
>Priscilla Oppenheimer wrote:
> >
> > If a Cisco IOS router receives IP frames encapsulated in an 802.3 header
> > from a device, the router sends to that device in 802.3 also. You can't
> > configure the Ethernet frame format for IP with Cisco IOS. But you can
> > configure the frame format for ARP. Can you think why?
> >
>Wow, I didn't know that! And I always thought that the way one changes
>the encap for IP is via the interface command "encapsulation xxx".
My routers don't recognize such a command in Ethernet interface
configuration mode.
Albany(config)#int e0
Albany(config-if)#encapsulation ?
% Unrecognized command
The only way I can configure encapsulation is if I first add the ipx
routing command. Even then it doesn't seem to do anything and claims to be
unrecognized although let's me enter it.
Albany(config)#ipx routing
Albany(config)#int e0
Albany(config-if)#encapsulation ?
% Unrecognized command
Albany(config-if)#encapsulation
% Incomplete command.
Albany(config-if)#encapsulation sap
Albany(config-if)#
I have to also add it as a parameter to the ipx network command. Sniffing
with EtherPeek revealed that otherwise the command does nothing. Once I
added the following command I saw that RIPs were going out with sap encap.
Albany(config-if)#ipx network 100 encapsulation sap
Albany(config-if)#
We had this conversation before and I think it was generally agreed that
the encapsulation command for Ethernet is useless otherwise with one other
exception. The encapsulation command also comes into play with routing
between VLANs. You can configure isl or dot1q or sde. But as far as simply
configuring the IP encap, there doesn't seem to be a way to do this.
This is a case where the documentation doesn't help because it doesn't
explain the usage well enough. But from tinkering, those are the
conclusions I have drawn.
Regarding ARP, here is what we surmised last time:
A router sends IP packets using the encapsulation that it receives them in
(for a particular device). But what's the first packet it will probably
receive from an IP workstation? Probably an ARP. So ARP has to be configured.
By the way, configuring an 802.3 ARP causes the router to use both 802.3
and Ethernet II. It uses Ethernet II if it gets no response with 802.3.
Priscilla
>AFAIK, it doesn't have "ip" in front of it because it predates when
>Cisco routers became multi-protocol.
>
>Change the frame format for ARP? Hmmm, other than HP-probe, I can't
>think of a reason.
>
>- Marty
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
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