Subject: [Isis-wg] Help needed for ISIS error message in Cisco router

From: "Amudhavalli Narayanan" <[email-masked]>
Date: Fri, 20 Jul 2001 18:23:00 +0530

Hi,
We are trying an interoperability testing with Cisco. I get the following
error message in Cisco router when it receives packets from the router under
test
"%CLNS-3-BADPACKET: ISIS: LAN L1 hello, Packet(1500) or wire(1497) Length
invalid from 2101:0102:0304 (Ethernet 1/0).

It would be of great help, if someone can throw some light on this error
message.

Thanks,
Amudha

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From: [email-masked] (Jeremy Hall)
Date: Fri, 20 Jul 2001 08:59:10 -0400 (EDT)

GOOD! it seems MTU checking works.

ISIS wants the MTU on all interfaces to be the same so it can ensure that
packets on a LAN are not dropped. IT means some device is reporting the
MTU of 1497 rather than 1500.

_J

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From: Henk Smit <[email-masked]>
Date: Fri, 20 Jul 2001 06:29:13 -0700 (PDT)

> GOOD! it seems MTU checking works.

   I am not sure what you mean by this.
  But ISIS implementations should *not* explicitely check the size of
 incoming hellos to see if they are exactly MTU size. If there is
 a problem with the reception of packet because of the size, that
 should be detected via indirect ways. Example, if one router sends
 hellos that are so big that the interface driver of the other router
 can not receive them, then of course the ISIS process will never
 receive the packet either.

> packets on a LAN are not dropped.

  Huh ?
 All interfaces of an ISIS router can be configured with a different MTU
 size. No problem. As long as:
 1) all the other routers on those interfaces have matching MTUs configured
 2) the MTUs are not smaller than MaxLSPBuffersize (which is typically 1492)

> IT means some device is reporting the

> > invalid from 2101:0102:0304 (Ethernet 1/0).

  Inside the ethernetheader there is a length field.
 The device your are testing has put value 1497 in there.
 This seems to be a correct value. 14 bytes ethernet header, 4 bytes CRC,
 and 3 bytes 802.2 header leave 1497 bytes available from the 1518 bytes
 in an ethernet frame.
 Inside the ISIS packets there is a length field too. The device you are
 testing has put value 1500 in there. This seems wrong. No way you can
 fit 1500 bytes inside 1497 bytes.

  Questions like these should typically be directed to the appropriate
 vendor's support channel ([email-masked]).
 Hope this helps,

   henk.