Asymetric routing

[Donachie]

Not sure if this is the right forum or this -

Hav an issue where we are mid-migration on our network. Changing the topology and IP scheme. A number of quick fixes have been put in place to solve various problems.

Problem I have now is that from one site back to the head office I am experiencing asymetric routing. Will this be causing nbt connections to fail? I cannot se reources over the network from site to site - although icmp works. If this is causing the problem thats fine - as I say we are mid migration. If not I need to find out what is...

 

[ADB100]

It is unlikely Asymetric routing is causing NBT connections to fail as the hosts themselves don't care how the packets are routed. Do you have any firewalls or routers doing packet inspection along the traffic paths as asymetric routing will not work over these.

Andy

 

[watchguardmonkey]

are you using any routing protocols like RIP, OSPF etc between the sites?

 

[Donachie]

thanks - we do have a firewall in between the connections - it is set to allow the connections. however the traffic only passes through it one way - on the way back it is just routed.

 

[ADB100]

If the forward path goes through a firewall, but the return path goes another way then this is a probable cause for your problems. At the site where it's forward path is not firewalled connections from here will start but the return traffic will go via the firewall. The firewall will not have seen the initial SYN (for TCP connections) so won't have any knowledge of the conversation and will (probably) drop the packet depending on its configuration.
Remember that NetBIOS and Windows networksing relies on multiple conversations (both UDP & TCP) so you are likely to have certain flows working and others being dropped depending on who and where they were started from.

What state is your routing in and how long is this going to take you to put right?

Andy

 

[Donachie]

thanks for that - I thought this was what was causing it. The migration will last for another few months yet. As a workaround I am going to staically route the box that is having the problem so the return path is the same as the outwards.

Thanks for all your help guys.