I'm running a pair of 6509s with a single SUP2/PFC2/MSFC2 each, IOS (tm) c6sup2_rp Software (c6sup2_rp-JK2SV-M), Version 12.1(27b)E4, RELEASE SOFTWARE (fc3).
I'm running BGP to two external ISPs and to each other, and OSPF for the loopback addresses between them. I'm encountering a sporadic problem where sometimes I see exactly 50% packetloss (every other packet) to random connected routes. There seems to be no rhyme or reason to why this starts to happen, and I'm not finding anything in the logs either. I've checked the ARP table, CEF, routes, etc, with no success.
When this problem is demonstrating itself, I can reach all addresses on that interface from the router itself, but if I try to reach them from another interface on the router, I get exactly 50% loss to ALL addresses on that particular interface, including the router's IP address on that interface, but see no problems at all on similarly configured ports on the same module. (The 50% loss occurs with all TCP, UDP and ICMP traffic. I've also removed the ip icmp rate-limit to rule that out as well.)
The most odd thing is that if I add a default route, the problem goes away, even if I remove the default route immediately afterward! Have I encountered an IOS bug? Any suggestions of things to look at would be greatly appreciated.