I would like to seek your help guys on deploying a GRE tunnel in two of our sites. This is the diagram.
SiteA is using 4506 and SiteB and C are using 6509 in hybrid mode. The broken lines are 100Mbps Metro Ethernet link.
Each Site has its own ISP. We wanted to create a GRE tunnel between SiteA and SiteC because we don't want to failover the internet connection of SiteA to SiteB during SiteA ISP downtime. We want to fail it over to SiteC. However, after creating the GRE tunnel between SiteA and C, the tunnel interface keeps on going up and down. I already filtered the loopback addresses used as source/destination to not cross the GRE tunnel to prevent recursive routing. We are using EIGRP as the routing protocol. Also, the Microsoft Exchange traffic and other traffic that SiteC is hosting became inaccessible from SiteA.
I tried to check the link above but unfortunately, I can only go for 3rd solution. The 4500 and 6500 doesn't have the ip tcp adjust-mss command. So I just increased the interface IP MTU to 1500 as recommended by Cisco. I also keep on receiving this output from the 6500 switch.
Oct 7 13:51:37.311: IP: recv fragment from 10.196.255.3 offset 0 bytes
Oct 7 13:51:37.315: IP: recv fragment from 10.196.255.3 offset 1456 bytes
We have already tried building GRE tunnel between one of our sites to another site in a different country but we are using E1 connection and I was able to use the ip tcp adjust-mss command coz we used a router. I suspect there's a problem with MTU size.
Please help me understand what to do next. Thanks in advance.