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Helmeczi Zoltan

Netbios queries accross vlans

Hi all,

My question and problem is the following:

In our network there are vlans and in the windows explorer we want to browse all the PCs and access the shares in the whole microsoft windows network.

We have a 3550 switch for routing etc. Within the vlans everything works fine. But i cant browse the neighborhoods (pc-s) or acces the shares in the other vlan. i can ping the devices accross valns and in the windows explorer i can access the shares if i type ip addresses ( \\ ) but its dont work with netbios names ( \\pcname ) and no network neighborhood browsing.

Please give advices what i have to set on the switch or what i have to set for this from windows side. Up to now i set the ip helper address for vlan iterface.But seems like it is not enough, even though with ip helper address the forwarding of netbios ports 137 138 139 are enabled.


Vlan1 interface

ip adress

ip helper address

ip directed broadcast

Vlan2 interface

ip adress

ip helper address

ip directed broadcast

Rising star

For starters, the helper address you have configured is your subnet broadcast address, so that's not really going to work.

As far as Windows goes, do you have a Domain setup, or is this all on a workgroup? Also, can you ping a host in VLAN2 from a host in VLAN?

Marvin Rhoads
VIP Community Legend

In addition to not using the subnet broadcast as the "ip helper-address" as John suggested (you would normally use the SVI - same address you have defined in the preceding line), you should also have have ip forward-protocol enabled gloablly. Please reference this guide.

Hope this helps.


by default the udp ports for netbios are forwarded with the ip helper-address and it can be the broadcast address and it should work.



Don't forget to rate helpful posts.

Cadet, you bring up a very valid point. I would assume that if he has his helper address pointed to the subnet broadcast address, all the devices in that subnet range would receive the Broadcast turned unicast and the DHCP server would respond.

Rising star

tahe a look at ip forward protocol. This brings back bad memories of supporting OS/2

ip forward-protocol

To specify which protocols and ports the router forwards when forwarding broadcast packets, use the ip forward-protocol command in global configuration mode. To remove a protocol or port, use the no form of this command.

ip forward-protocol {udp [port] | nd | sdns}

no ip forward-protocol {udp [port | nd | sdns}

Syntax Description


Forwards User Datagram Protocol (UDP) packets. See the "Usage  Guidelines" section for a list of port numbers forwarded by default.


(Optional) Destination port that controls which UDP services are forwarded.


Forwards Network Disk (ND) packets. This protocol is used by older diskless Sun workstations.


Secure Data Network Service.



Command Modes

Global configuration

Command History



This command was introduced.

Usage Guidelines

Enabling a helper address or UDP flooding on an interface causes the  Cisco IOS software to forward particular broadcast packets. You can use  the ip forward-protocol command to specify exactly which types of broadcast packets you would  like to have forwarded. A number of commonly forwarded applications are  enabled by default. Enabling forwarding for some ports [for example,  Routing Information Protocol (RIP)] may be hazardous to your network.

If you use the ip forward-protocol command, specifying only UDP without the port enables forwarding and flooding on the default ports.

One common application that requires helper addresses is Dynamic Host  Configuration Protocol (DHCP). DHCP is defined in RFC 1531. DHCP  protocol information is carried inside of BOOTP packets. To enable BOOTP  broadcast forwarding for a set of clients, configure a helper address  on the router interface closest to the client. The helper address should  specify the address of the DHCP server. If you have multiple servers,  you can configure one helper address for each server. Because BOOTP  packets are forwarded by default, DHCP information can now be forwarded  by the software. The DHCP server now receives broadcasts from the DHCP  clients.

If an IP helper address is defined, UDP forwarding is enabled on default  ports. If UDP flooding is configured, UDP flooding is enabled on the  default ports.

If a helper address is specified and UDP forwarding is enabled,  broadcast packets destined to the following port numbers are forwarded  by default:

Trivial File Transfer Protocol (TFTP) (port 69)

Domain Naming System (port 53)

Time service (port 37)

NetBIOS Name Server (port 137)

NetBIOS Datagram Server (port 138)

Boot Protocol (BOOTP) client and server packets (ports 67 and 68)

TACACS service (port 49)

IEN-116 Name Service (port 42)