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DMVPN With EIGRP - MTU adaptation

Hi all,

I am managing a Hub and Spoke network with more than 1000 Spokes. The initial MTU of the tunnel interface we've configured is 1400 (on both hubs and Spokes).

I am facing now some issue with customers that have a WAN line with a lower MTU: ping [tunnel_IP] size 1400 doesn't give any reply.

The problem is that:Two routers with MTU of 1400, but in the path between them there is apparently a hop with a smaller MTU.  Hello and neighbor-ship is working (I assume that  Hello are smaller than 1400)

Route from Spoke are received on Hub (displayed in sh ip route ei) but the route from Hubs are not received by the Spoke. I assume that's due to the fact that there are not a lot of route advertised by the spoke (so packet size lower) and more advertised by the hub reaching MTU size....

I did some tests with these customer Spokes connecting them to another hub router used for R&D, adapting the MTU on both size to a lower value: working !


I have now 2 solutions

- Ask all my concerned customers to contact their ISP to check if they can increase MTU or find another gateway for my router... Most of my customer doesn't have any IT people and won't understand at all what I want... smiley

- Modify MTU on my router: modify in on the Spoke... but do I need to reconfigure it on the hub to avoid neighbor flapping... So I need to reconfigure all my 1000+ Spokes angry


 So my question is: is there a way to adapt MTU size of the tunnel interface depending on the destination. 


I tried to find a solution like a route map that will modify MTU when matching an ACL that contains my impacted Spokes but it's not possible to adapt MTU with a set in a route-map...



Any idea on your side ? workaround ?



Best regards !

  • WAN Routing and Switching
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