>The mystery is solved. These CRC errors are coming every 10 seconds.
>Surprise surprise, if I turn off keepalives on the 7500/PA-FE-TX, the
>CRC errors on the 3548XL stop.
Interesting. I set up a lab test and found that a 7513/PA-FE-TX
configured with an ISL trunk connected to a 2924XL sends the keepalive
packets out onto the trunk un-encapsulated. (I don't have a 3548 to
play with, so I used a 2924. Also, I used a Shomiti inline tap to
eavesdrop on the trunk link. The router is running 12.0(9)S code)
Normally, everything on an ISL trunk is encapsulated in a frame sent
to the Cisco ISL mcast addr: 01-00-0C-00-00-00. So the 2924 at the
other end of the trunk expects to receive every payload frame wrapped
inside an ISL trunk mcast frame. The ISL encapsulating frame is CRC
checked and unpacked, and then the payload frame is CRC checked and
unpacked.
Since the keepalive packet is not encapsulated the second CRC check
has no CRC field to work on, and returns an error on the 2924 trunk
port. Sure enough, disabling keepalive packets clears up the CRC
errors.
In my tests there were no further errors. CDP packets appear to be
correctly encapsulated in ISL mcast frames.
I also noticed that when keepalive is disabled, the router interface
doesn't respond to the link state correctly. Disconnecting the cable
so that the link light is off causes the router interface to toggle
down and then back up. After which the interface stays up/up, even
though the link is down. After re-enabling keepalives the router
correctly flags the "line protocol down" state when the link is
disconnected.
-Charles
Charles E. Spurgeon / UT Austin Networking Services
c.spurgeon@mail.utexas.edu / 512.475.9265
This archive was generated by hypermail 2b29 : Sun Aug 04 2002 - 04:12:13 EDT