EIGRP issue - flapping between two routers

Static, RIP, EIGRP, OSPF, BGP, IBGP, IS-IS and any other routing.
swordfish
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Joined: Thu Sep 27, 2018 11:24 am
Location: Slovenia

EIGRP issue - flapping between two routers

Post by swordfish » Thu Sep 27, 2018 11:56 am

Dear all,

I have a issue on network which is really starting to get on my nerves as I cannot determine the cause of failure in working.
The problem is the EIGRP neighbour adjacency between two routers. They are connected over vlan, let say VLAN ID 123, on two different locations. These two locations are connected with two switches, one on each side (border switches). Each router is connected to each of these switches.

Router 1 keep losing/flapping the connection between them with log:
EIGRP: Holdtime expired
%DUAL-5-NBRCHANGE: EIGRP-IPv4 "asnumber": Neighbor 10.1.1.1 (GigabitEthernet0/3.123) is down: holding time expired
Going down: Peeer 10.1.1.1 total=1 stub 0, iidb-stub=0 iid-all=0
Peer 10.1.1.1 going down
EIGRP: Handle deallocation failure [1]
EIGRP: Neighbor 10.1.1.1 went down on GigabitEthernet0/3.123
Last peer deleted from GigabitEthernet0/3.123

Few seconds after that another log:
EIGRP: Neighbor (10.1.1.1) not yet found.

On router 2 log is slightly different:
%DUAL-5-NBRCHANGE: EIGRP-IPv4 "asnumber": Neighbor 10.1.1.2 (GigabitEthernet0/3.123) is down: Interface PEER-TERMINATION received
Going down: Peeer 10.1.1.2 total=2 stub 0, iidb-stub=0 iid-all=0
Peer 10.1.1.2 going down
EIGRP: Handle deallocation failure [2]
EIGRP: Neighbor 10.1.1.2 went down on GigabitEthernet0/3.123
Last peer deleted from GigabitEthernet0/3.123

Few seconds after that:
%DUAL-5-NBRCHANGE: EIGRP-IPv4 "asnumber": Neighbor 10.1.1.2 (GigabitEthernet0/3.123) is up: new adjacency

I did change hello-interval and hold-time (tried 5/15, 10/30, 10/120, 40/120, 60/1809 but no luck. I set ip mtu to 1370 and the connection was alive a bit longer but eventually disconnected.

I also noticed that it is always the R1 that runs out od hold time under sh ip eigrp neighbor. I did debug eigrp packets retry, terse and hello but I cannot find the problem.

The thing is that R2 always first establishes the neighbor connection, few seconds after that, sometimes even minute, R1 also establishes neighbor connection, but the one who first "disconnects" is R1. Also, the connections are up few minutes and than slowly hold time on R1 keeps getting smaller and eventually disconnects.
Looks like Hello packets are no longer received on R2.

Any help/comment will be very helpful as this thing is causing problems for the last two days.

Kind regrads,
A