Reason for Route Flap

[gauravpundir231]

Hi All,

Can anyone advise on below point.

We have a P2P link between site A and site B running eigrp. i have observed route flap for a destination IP however eigrp neighbor ship is stable. Bandwidth utilization is also normal between sites. Please advise what i can check further to find the route cause

[Mark Malone]

Hi

i would debug it see whats going on at packet level , these are the reasons it could be flapping

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/21324-trouble-eigrp.html

Reasons for Neighbor Flapping

The stability of the neighbor relationship is of primary concern. A failure in the neighbor relationship is accompanied by increased CPU and bandwidth utilization. EIGRP neighbors can flap for these reasons:

• Underlying link flaps. When an interface goes down, EIGRP takes down the neighbors that are reachable through that interface and flushes all routes learned through that neighbor.

• Misconfigured hello and hold intervals. The EIGRP hold interval can be set independently of the hello interval if you issue the ip hold-time eigrp command. If you set a hold interval smaller than the hello interval, it results in the neighbors flapping continuously. Cisco recommends that the hold time be at least three times the hello interval. If the value is set less than 3 times the hello interval, there is the chance for link flapping or neighborship flapping.

  R1(config-if)#ip hello-interval eigrp 1 30 
  R1(config-if)#ip hold-time eigrp 1 90
  

• Loss of hello packets: Hello packets can be lost on overly congested links or error-prone links (CRC errors, Frame errors, or excessive collisions).

• Existence of unidirectional links. A router on a unidirectional link can be able to receive hello packets, but the hello packets sent out are not received at the other end. The existence of this state is usually indicated by the retry limit exceeded messages on one end. If the routers generating retry limit exceeded messages has to form neighborship, then make the link bidirectional for both unicast and multicast. In case tunnel interfaces are used in the topology make sure that the interfaces are advertised properly.

• Route goes stuck-in-active. When a router enters the stuck-in-active state, the neighbors from which the reply was expected are reinitialized, and the router goes active on all routes learned from those neighbors.

• Provision of insufficient bandwidth for the EIGRP process. When sufficient bandwidth is not available, packets can be lost, which causes neighbors to go down.

• Bad serial lines.

• Improperly set bandwidth statements.

• One-way multicast traffic.

• Stuck in active routes.

• Query storms.

[Julio E. Moisa]

Hi

if it was working fine since the beginning it could be a carrier problem or physical problem like SFP, Patch cord or module or port, have you verified the errors or lost packets through: show interface <interface>?