[vortex] Re: 802.1q VLAN problem

Ruben van der Leij ruben@blacklisted.nl
Thu Apr 18 13:28:02 2002

+++ SCHIMMEL Guillermo TECHTEL [17/04/02 15:03 -0300]:

> I do not intend to start  playing with 4k packets, I just want to do inter
> vlan routing with a PC and a Cisco 2900 switch.

Note that ISL and 802.1q/802.10 differs with regards to packetsize.

Cisco says this about the subject:

"The biggest implication for systems using ISL encapsulation is that the
encapsulation is a total of 30 bytes and fragmentation is not required.
Therefore, if the encapsulated packet is 1518 bytes long, the ISL packet
will be 1548 bytes long for Ethernet. Additionally, if packets other than
Ethernet packets are encapsulated, the maximum length can be greatly
increased. This length change must be considered when evaluating whether a
MAC can support ISL packets.

Another system implication is that ISL packets contain two FCSs. One on the
internal encapsulated packet, and another covering the entire ISL packet. If
the original data does not contain a valid CRC, two will have to be
calculated as the packet is transmitted and the invalid CRC will not be
detected until the ISL header is stripped off and the end device checks the
encapsulated FCS. This typically is not a problem for switching hardware,
but may be difficult for routers and Network Interface Cards (NICs)."


HTH, HAND. :) 

Ruben van der Leij

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