Eigrp metric - MTU value - Redistribution

heelo gents,

i have read from a reliable source and tested myself that when redistributing Routes into eigrp and changing the MTU size, this does not effect the bandwidth calculation.

the reson stated was that the value you use in the redistribution command is the "actual" value of the MTU size.

i have tried this config which works fine

R1 --RIP--R2---EIGRP---R3

router eigrp 1
redistribute rip metric 1 1 255 1 1

my question is , when packets are sent from R3 to the redistributed networks advertised from R1.

do these packets need to have an MTU size of 1 ??,

would the eigrp peer R2 not accept packets with a larger MTU size ?

if the MTU is not taken into accout in the bandwidth calc , and its not utilised when sending packets between peers .

what on earth is it needed for ???

many thanks


  • i have found this explanation below on where the MTU metric is used  , which helps ,

    but i would still like to know if i set the eigrp MTU to 900 and i send 800 byte packets to the eigrp destiantions,

    will they be dropped by the incoming router?, could this be a troubleshooting type senario i may encouter in the lab ?

    am i worrying way, way to much about this ??.


    (the number of equal-cost paths exceeds the value of the maximum-paths router configuration parameter), it ignores those with the lowest MTU metric.

    The MTU metric behaves similarly to the load and reliability metrics:

    A change in the MTU does not trigger a routing update.
    Whenever a “real” change in topology triggers an EIGRP update, the MTU metric is updated to reflect the then-current interface ip mtu value.

    When processing inbound EIGRP updates,
    the minimum of the MTU value in the inbound UPDATE packet and the interface MTU is stored in the EIGRP topology database.
    No MTU processing is done on the outbound updates.
    The MTU metric thus represents the minimum unidirectional MTU between the current router and the originating router.

  • Hi secondsight,

    That's not true when you set the MTU is just to affect the metric calculation. EIGRP uses K values to drive metric calculations (K1, K2, K3, K4 and K5).

    By default, it only used K1 and K3, which is bandwidth and delay, respectively. Therefore, you can put any value you want on the other K values (K2, K4 and K5) and the metric will not change, because the other K values are not factored by default.

    If you want to change the way EIGRP calculates its metric, you can, by using the metric weights command under the EIGRP process.


    Good luck! 

  • MTU is advertised with the routing updates and when you have equal paths to same destination MTU comes in play. But don't assume that as soon as you change EIGRP MTU metic, it changes interface MTU.


    Good Luck

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