Issues with routing

[phenomlab]

@mathourthy said in Issues with routing:

I feel embarassed to tell you that the previous firewall was a Cisco PIX

I won’t judge you - don’t worry. At least it’s gone now. The reason as to why this wasn’t an issue before is pretty obvious now. The PIX firewall range did not offer IP Routing (until firmware v7.x which wasn’t compatible with all of the range, so may not have worked depending on the model) or Layer 3 technology, so you would have had to have a router in place to send traffic destined for other networks elsewhere.

This now explains the issue you are experiencing because you effectively have two routing devices on the same subnet, and as they are both capable of ARP resolution, the incorrect one is sending the response back. After 30 seconds, because no valid response was received, the exchange is discarded.

What we need to do here is to alleviate this issue - this is also known as “black hole” routing meaning that packets are never delivered and simply disappear. As you have a Cisco 3750 switch, you do the following (based on the “Manchester” office)

• Create a new VLAN on the Cisco switch, but have it so it is layer 2. For example

conf t
int vlan 900
no ip address
description ***** MPLS Router *****
int gi0/1
switchport mode access
switchport access vlan 900
description ***** MPLS Router *****
int gi0/2
switchport mode access
switchport access vlan 900
description ***** MPLS Router *****

By specifying no ip address on the new VLAN, we effectively make it Layer 2 so it cannot route any traffic itself.

• Change the IP address of the router so that it is in a completely different subnet - for example, 172.16.0.0/24, so you would use 172.16.0.1 255.255.255.0. This will of course attract downtime, so do it when there are no users around.
• Connect the router to port gi0/2 on the switch
• Change the default gateway on your DHCP (I’m assuming) clients so that the firewall each each location becomes the default gateway - in the case of Manchester, that will be 192.168.11.254.
• On the firewall, create a new interface and identify is as “MPLS”. Give it an IP address - for example, 172.16.0.254 255.255.255.0 and connect that into port gi0/1 on the switch
• On the router, create a last resort route of 0.0.0.0 0.0.0.0 172.16.0.254 so that it sends any traffic it cannot resolve back to the firewall
• On the firewall, create static routes for 192.168.12.0 255.255.255.0 172.16.0.1 and 192.168.10.0 255.255.255.0 172.16.0.1 so that any traffic looking to reach these subnets via the new firewall default gateway will be passed to the MPLS router and vice-versa
• On the firewall, crate policies between the LAN and this new “MPLS” interface and vice-versa so traffic can pass between the segments

Taking this route means the below

• You remove the MPLS router from the same network as the firewall - effectively, placing it behind the firewall and out of the direct reach of connected clients. This way, the only “router” that can respond is the firewall itself which is what we want.
• The firewall becomes the default gateway for all connected devices in each office network

Obviously, you’ll need to do some repatching in all offices to support this new layout.

What you will land up with once all sites are completed is the below

Hopefully this makes sense.

[mathourthy]

@phenomlab OMG thanks so much for this, and yeah - seeing it in picture format, this does make a lot of sense. I’ll get onto this now as everyone finished at 5 (same for London and the manufacturing plants here and Bristol). Are you available here this evening (I’m assuming you’re in the UK?) in case I run into any issues? I know it’s an ask, but I feel like I’m close to getting this fixed now.

I’m going to get the IT helpdesk guys in London and Bristol to come back into the office so we can do these all at once. Wish me luck!!!

[phenomlab]

@mathourthy Yes, I’ll be here if you need me. Once you’ve moved the first site, you should still be able to ping the other locations as normal

[mathourthy]

@phenomlab Thanks. Just finished in Manchester, and everything seems to work as it did before - I also have had a connection open to a PC (RDP) in Bristol which has been connected without breaks for about 20 minutes. I had to revamp the config you provided because those switch ports were already in use, but everything else is good to go.

I created the firewall rules on each firewall (in each location) and we are passing traffic with no restrictions. Just waiting for the guys in London and Bristol to finish up and then we’ll perform more tests.

[phenomlab]

@mathourthy sounds positive so far

What sizes are your MPLS circuits and internet circuits?

[mathourthy]

@phenomlab see below (these are for both internet and MPLS)

London = 100Mbps
Manchester = 100Mbps
Bristol = 100Mbps

All are provided from the same vendor. Why ?

[phenomlab]

@mathourthy Because you could easily dump the MPLS and use IPSec tunnels between the sites. This not only constitutes a saving in terms of no longer needing the MPLS, but traffic passing across these links would be encrypted by default, which isn’t the case with MPLS networks.

The upside of MPLS is that they are 100% private networks, and are not exposed to the internet. This makes them pretty secure by default. In contrast, the downside of IPSec VPN tunnels is that they are public, and exposed to the internet, so could easily be the target of a DDoS attack.

You could also consider SD-WAN as an alternative technology.

[mathourthy]

@phenomlab Yeah, that’s a good point. To be honest, I’ve just taken over the IT stuff here, and still working my way through. I’ll need to have a look at the MPLS contract to see when it comes up for renewal, but thanks for the tip.

[mathourthy]

My guy in Bristol is having issues so need to get on a call with him. I’ll be back as soon as I can.

EDIT - seems the port was dodgy on his switch. We’ve changed it for another one and assigned it the same VLAN. he has an RDP session open to a spare machine here in Manchester.

[mathourthy]

@phenomlab some good news. Since making the changes in Manchester and Bristol, it seems that both sides are now working without any breaks (and have been for the past 20+ minutes). I’m just waiting for the changes to be made in London, so so far so good.

[phenomlab]

@mathourthy great! Thanks for the update.

[mathourthy]

@phenomlab Ok, just heard back from my guy in London. He’s finished his end, and…

IT WORKS !!! Thank you so much for everything. You are an absolute LEGEND, and a genius to boot.

[phenomlab]

@mathourthy Glad to hear it’s sorted. Let me know if you get any more issues.