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IP helper for Juniper switches

Posted on 2014-11-16
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Last Modified: 2014-12-02
Can the experts help me to understand how IP helper works for the Juniper switch? I have the below config snapshot. I understand for vlan.3. But I am not sure about ge-0/0/28. Does it mean that everything going through that port with vlan 10 will get a DHCP?

forwarding-options {
    helpers {
        bootp {
            server 192.168.1.230;
            server 192.168.1.231;
            interface {
                vlan.3 {
                    server 192.168.1.230;
                    server 192.168.1.231;  }
                ge-0/0/28 {
                   server 192.168.1.230;
                    server 192.168.1.231;
                }
...
vlans {
   users1 {
        vlan-id 10;
        interface { 
                ge-0/0/28; 
                ge-0/0/29;}
        l3-interface vlan.0;
      }
      users2 {
        vlan-id 25;                               
        l3-interface vlan.3;
...

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Question by:leblanc
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Expert Comment

by:vivigatt
ID: 40447212
I can't remember for sure if the syntax is correct (It's been a long time since I did not configure a JunOS device).
If the syntax is correct, it would mean that every broadcast packet on UDP port 68 (bootp/dhcp client) that the interfaces vlan3 and ge-0/0/28 are forwarded as Unicast to server 192.168.1.230 and server 192.168.1.231. 2 DHCP servers, that may be quite unusual, except if there are a DHCP and a PXE server, or if there are relationships (failover, split-scopes...) between the 2 servers
Now I am not too sure about your config, especially the "default servers" which seem to be the same as the specific servers for all the defined interfaces.

If applicable, you may use a J-Web configuration editor in order to configure dhcp relay, as described here:
http://www.juniper.net/techpubs/software/junos-es/junos-es92/junos-es-admin-guide/configuring-the-device-as-a-bootpdhcp-relay-agent.html
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Expert Comment

by:pergr
ID: 40447233
Note that "vlan.3" is the L3 interface (in cisco language SVI), so you need to look for that under:

interfaces {
     vlan   {
            unit 3   {

This is the default gateway in vlan "users2"/vlan-id 25, and I am not sure if any physical interfaces are actually connected to that vlan.

Also, not that this just means the switch is forwarding the DHCP packets to the DHCP server, so it is up to the server if the PCs will get IP address or not.

You could also configure the switch itself as a DHCP server.
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Author Comment

by:leblanc
ID: 40448444
So what ge-0/0/28 { server 192.168.1.230; server 192.168.1.231;} means? Thx
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Accepted Solution

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vivigatt earned 500 total points
ID: 40448779
It means that all packets that this interface (this port on the switch if you want) receives and that are sent on UDP Port 68 to broadcast address, such as the first packet a DHCP client sends to initiate a DHCP transaction, will be forwarded to 192.168.1.230 and 192.168.2.231, as Unicast packets.
Check this small Wikipedia section in DHCP article:
http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol#DHCP_relaying
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