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Community Member

ISL 1900 switch and 1750 router broadcast issue

I have a segment of my network that is very chatty using netbeui for keepalives for redundancy. I have 1900 switches and 1750 routers. This broadcast traffic that is hitting the rest of my network crushes it. I understand that ISL routes in between VLANS, but what if I don't want my VLANS to talk to each other, but still back to headquarters. Is that possible without using ISL? I want to take one port and make a two VLANS, one for that port and one for the rest of network. These are remote sites and have data that needs to make it's way back to HQ. These two network portions do not need to talk. Is this possible. Thanks in advance

Community Member

Re: ISL 1900 switch and 1750 router broadcast issue

ISL does not route between VLANs. ISL and IEEE 802.1q are trunking protocols that prepend LAN frame with a VLAN tag that allows a physical port to carry traffic across it for multiple VLANs. The tag keeps the traffic seperate. Trunks run between switchs and/or routers that can read ISL. Inter-VLAN traffic requires a router. Netbeui is also not a routable protocol.

Your VLANs are not talking to each other unless you have the bridged somehow (via cross-over cable or bridged interfaces on your 1750). I suspect what is happening is that the broadcasts on VLAN A (for example) are such that they are interrupting all devices that have are on VLAN A. This includes switches with ISL trunks.

Check to see if you have automatic VTP pruning enabled. This removes a VLAN from a trunk if there are not active ports in that VLAN on the downstream switch. Best solution is to manually prune the VLAN from the trunk.

Since Netbeui is not routable, you could also be bridging multiple interfaces (to pass netbeui) on your router together which will lead to poor performance as well.

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