Is there an any option in cisco Router we can restrict MTU size to something smaller? default is 1514 but for experiment i want to allow 512 only and if packet if bigger than 512 discard them.
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Do you want to restrict packets sent or received?– Ron TrunkApr 3, 2016 at 16:43
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A router will normally fragment unless the DF bit is set in the packets.– Ron Maupin ♦Apr 3, 2016 at 16:44
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Received, but i thought MTU is sent on Interface then why you asking send or receive.. doesn't it work both way?– SatishApr 3, 2016 at 16:44
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The sender is the one which controls the DF bit.– Ron Maupin ♦Apr 3, 2016 at 16:45
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In short there is no way we can say only and only allow 512 bytes and drop rest????????– SatishApr 3, 2016 at 16:46
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1 Answer
You can do this with policy based routing, although this is an example of filtering any packet bigger than 512 bytes, it does not make your router fragment packets to 512 bytes.
ip access-list extended Filter
permit ip any any
router-map Filter
match ip address Filter
match length 1 512
set interface Null0
On the inbound interface:
ip policy route-map filter
This method will only work on inbound traffic coming in through the interface you have set the policy on
If you want to filter outbound traffic from the router, instead of an interface level command, use the following global command:
ip local policy route-map filter
You can also match only certain traffic by modifying what the access-list matches, or if using the global level command, and you want to filter outbound traffic only on a certain interface, you can do an extra match <out-bound interface> command in the route map
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You should also explain that this will cause process routing, which will slow things, and drive up the router CPU utilization.– Ron Maupin ♦Apr 3, 2016 at 17:27
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I have tried your solution but but somehow its sending all packet to
Null0even smaller packet too.– SatishApr 25, 2016 at 19:52 -
1Hey Satish, Yup, the command
match length 1 512will match anything between 1 and 512 bytes, adjust the two values to get your min and max desired packet size match. I think I misunderstood your initial request when I wrote that :) Apr 27, 2016 at 4:50 -
I am sending
20000bytes packet but that policy not matching.. something weird going on.. did you test this solution before?– SatishMay 12, 2016 at 17:17