Multicast forwarding through VLANs for KVM Over Ip

[elimco.jttocinog]

I'm currently designing a terminal-server architecture, using KVM-Over-IP technology (I'm using Gefen devices) where I have two VLANs:

• One VLAN for the screens (KVM receivers) at 192.168.240.0/24
• One VLAN for the servers (KVM senders) at 192.168.241.0/24

I'm using a Cisco 3750G Layer 3 Switch. I've configured all the VLANs with their gateways (.1) and there's connectivity between the devices, I ca ping each other, etc. The configuration is as follows (this is an excerpt, the rest are the default values)

ip routing
ip multicast-routing distributed
!

interface GigabitEthernet1/0/1
switchport access vlan 2
!
interface GigabitEthernet1/0/2
switchport access vlan 10
!
interface Vlan2
ip address 192.168.240.1 255.255.255.0
ip pim sparse-dense-mode
!
interface Vlan10
ip address 192.168.241.1 255.255.255.0
ip pim sparse-dense-mode
!

Now, the KVM devices (these Gefen DVI KVM devices) cannot auto discover other KVM devices in different VLANs. That's to be expected, because broadcast messages do not cross VLAN boundaries. After further investigation, looks like these devices use multicast groups to annonunce themselves, using standard TCP connections afterwards to stablish the video transmission.

In order to fix the problem, I've enabled multicast-routing distributed and also tried activating sparse-dense mode. Using wireshark with port mirroring I can see some Membership Report messages from the devices within a VLAN, and PIM protocol messages coming from the VLAN interface (.1), but the devices fail to see each other still.

So, from scratch, how should I configure the 3750G so multicast traffic is shared between VLANs? I have also checked MVR to no avail.

Thanks in advance.

[Reza Sharifi]

For test, can you try putting the servers and KVM devices in the same vlan and run multicast?

[elimco.jttocinog]

Yes, if both the sender and the receiver are in the same vlan, they discover each other. In fact, there's a curious situation: the sender is at 192.168.240.2 / 24, and the receiver is at 192.168.241.2 / 24. When they're connected in ports with the same VLAN, they discover each other (the receiver shows the proper IP of the sender) but understandably the video connection doesn't get stablished because they're in different subnetworks. Now, if I quickly connect them back to their original ports, the video connection gets stablished, as the "discovery" step has already been fulfilled (the receiver knows the IP of the sender) and they're in their original VLANs, so they can communicate through VLAN routing.

[paul driver]

Hello

As you are using sparse-dense mode pim should fall back to dense mode if no Rp is found,

1) do you have ip routng enabled on the L3 switch
2) is ip pim dm-fallback disabled

sh ip mroute
sh ip pim interface count

res

Paul

[chrihussey · Accepted solution]

Just a thought.

What is the discovery multicast group? If it is 224.0.0.x. that is only specific to the local LAN segment with a TTL of 1. In these cases multicast routing won't route it.

If it is a different multicast group, still check the TTL in the packets. It still may be 1.

[elimco.jttocinog]

Oops, I'm afraid it's right that. The packets I want forwarded seem to belong to mDNS, whose IP address in fact starts with 224.0.0.x. I've been reading and looks like the 3850 series do have some kind of "service forwarding", but the 3750 doesn't.

I'm going to try to use another computer with several NICs to act as a mDNS forwarder, too bad this can't be done directly in the switch.

Thanks!

[chrihussey]

Great. Thanks for the rating.