I am facing a weird problem for which I am trying to find an answer but in vain.
What's happening is that we have about 70 sites(spoke) connected over MPLS to HO (hub) using IPSec/GRE Tunnels, HO WAN router 2800 is connected using Vlan leg to Internal Switch 6500. All site including HO Router and Switch are using EIGRP same AS nos.
Out of all these locations 3 sites are suddendly becomes unavailable from the switch LAN where as if I ping these site from the HO router I can do so without any problem so for sure their is no WAN connectivitiy loss or any issues on the WAN side.
Then the big question "what is causing only these 3 sites to have such a behaviour", I have tried to verify all configuration and up till now have found any un-usual , no changes were made in the last few weeks also and everything else is just working as smooth as it should it.
HO Router is an EIGRP neighbour of Switch
HO Router is an EIGRP neighbour for all sites including these 3
On Switch SH IP ROUTE EIGRP TOPOLOGY shows that these site become unavailable for access and then without doing anything they comes back in routing table, as if the link was flapping but when I check from the HO Router I can access them continously without any problem.
In there something happening on the VLAN leg between 6500 and 2800 , if so then why only these 3 sites. Also these 3 sites have identical configuration on the routers and HO Router has same configuration for these and all other sites connections, absoutely no difference expect for their respective IP ADDRESS LAN and WAN.
Any suggestion would be awesome and really apperciated.
Honestly speaking, it would be very difficult to find a root cause for this issue when its no longer there...but here are few things which could cause this:
- EIGRP neighborship flapping between the HO router and these 3 spoke sites. You can check this using the command "sh ip eigrp neigh"
- In case this issue happens again, you need to check if the routes from the spoke routers are being learnt on the 6500 swtich or not and vice versa i.e HO routes are being learnt on the Spoke router (or atleast the default route)
- you also need to trace the packet from 6500 switch towards one of those spoke sites and vice versa
- there could be high CPU on any of these devices involved i.e 6500, HO WAN router & spoke routers which can cause the routes to flap or processing of these routes to be stopped
Hope the above options/steps help you in tracking the issue in case of reoccurrance
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