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3845 router failing to look up routes from routing table correctly!

Hi all,

 

Got a really strange issue which I am wondering if someone can point me in the right direction for. Facts of the issue:

- Some customers, all with a specific ISP, cannot access a hosted service we host internally

- External user can reach service but never gets a response - hence focussing on reachability of their public IP

- Cisco 3845 router used, peers with service provider over BGP - receives full internet routing tables

- All affected customers receive dynamic IP addresses within the same /10 public IP range

 

I received an example IP address and when doing "show ip route x.x.x.x" for this IP, the router responds "Subnet not in table". When trying to traceroute to the IP, the router doesnt even go to the next hop. However, the subnet is definitely in the routing table with the correct next hop (and, for complete information, is also within the BGP updates). Before anyone asks the question :) - yes I have definitely verified that the hosts are within this subnet.

 

If I put a static /32 route in for this specific IP address, everything works fine - then fails again once its taken out.

 

I literally cannot understand why the router is not correctly performing the lookup for the hosts within this subnet. I can understand a lot of potential reasons why the BGP received route wouldnt be placed in the routing table, but that is not the case here. 

Some other factors (if applicable):

- Nothing showing in the logs

- Plenty memory available (despite the high number of routes)

- Plenty CPU resource available

- No default route is ran

 

I am going to restart the router and really expect this to resolve the issue (would log a TAC but this one is a bit time precious) - but it is frankly doing my head in and I assume I am missing something!

 

Any help or guidance would be appreciated!

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