This is the problem I recently faced in my work environment.
Background
The switches are connected as shown in the diagram via L2
port channel. The switch C6509 is forming EIGRP neighborship with 7018 1 nexus
switch on VLAN 230. The switch C6509 is also forming EIGRP neighborship with
7018 2 nexus switch on VLAN 240. All layer 3 interfaces are MPLS enables and
LDP neighborship was formed among each other.
Issue
The clients at downstream were not able to access the
application hosted somewhere in upstream.
Since MPLS accommodate extra bytes in the packet header so
it cannot take 1500 byte of default packet size therefore the MTU size was
needed to raise on the L3 interfaces.
When MTU size on SVI interfaces of VLAN 230 and VLAN 240 was
raised to 1536 (as per the standard of my customer). I faced the strange
behaviour that after changing the MTU size the EIGRP neighborship went down
after appx. 1 hour on both the SVIs.
Solution
The default MTU size on port-channel was 1500 so when MTU on
L3 interface was changed to 1536 in that case the underlying MTU must also
support the larger MTU size therefore I had to change the MTU size of both the
port-channel to mtu 9216.
After changing the MTU of the port-channel to a larger size
the EIGRP neighborship became stable and never went down.
