Ip helper-address command explanation and configuration, and some important details and one additional helper cmd!

This will be a quicky I hope, pretty straight forward topic, but I always say that so lets get started.

The IP helper-address is a command issued at the interface level, on the interface of incoming broadcasts, and its purpose is to forward UDP broadcasts on (as routers DO NOT forward broadcasts) to other routers in Unicast format.

The command itself is fairly simple:

R1(config-if)#ip helper-address ?
  A.B.C.D  IP destination address
  global   Helper-address is global
  vrf      VRF name for helper-address (if different from interface VRF)

R1(config-if)#ip helper-address 172.12.123.2 ?
  redundancy  defines VRG group name
  <cr>

R1(config-if)#ip helper-address 172.12.123.2
R1(config-if)#

In red is what we are focusing on, configuring an IP address, though it is worth noting that you can assign VRF instances for it as well as a global command.

Now the global command was not covered in my materials thus far, but from what I researched (fairly quickly), it works in conjunction with VRF to allow ip helper-address to work with VRF – For the CCNP ROUTE these very well could be unnecessary to know but I wanted to point them out.

Now, back to business.

This command will primarily be used to forward BOOTP (DHCP) requests from host machines onto Domain Controllers running a DHCP server, but it is also VERY important for Cisco Voice as VOIP phones sometimes need a helper-address to reach their TFTP server to pull down their settings. Wanted to drop that little bit of real world knowledge.

By default, the “ip helper-address x.x.x.x” command only forward 8 UDP port #’s:

  • TIME (TimeServer) = 37  *** Note this is not NTP and has nothing to do with NTP ***
  • TACACS = 49
  • DNS = 53
  • BOOTP (DHCP SERVER) = 67
  • BOOTP (DHCP CLIENT) = 68
  • TFTP (Trivial File Transfer Protocol) = 69
  • NETBIOS Name Service = 137
  • NETBIOS Datagram Service = 138

So we get kind of a win with memorizing these with 67-69 being in sequential order, but the others if you don’t work with them will be good to commit to memory. This command is obviously very oriented on reaching server types, and is mainly used for DHCP / TFTP / DNS so I would really burn those port numbers into your mind.

Remember, it is configured on the interface of the incoming broadcasts, and can be verified with “sh IP int s0/0” and it is becoming so clearly important to remember that “sh ip int” and “sh int” give completely different output so I would recommend really committing the output to memory (as with everything else… of course):

R1#sh ip int s0/0
Serial0/0 is up, line protocol is up
  Internet address is 172.12.123.1/24
  Broadcast address is 255.255.255.255
  Address determined by non-volatile memory
  MTU is 1500 bytes
  Helper address is 172.12.123.2
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is disabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP fast switching on the same interface is enabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF Fast switching turbo vector
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  WCCP Redirect outbound is disabled
  WCCP Redirect inbound is disabled
  WCCP Redirect exclude is disabled
R1#

I was going to stop at the helper address, but I wanted to exemplify all the different information you can get from “sh ip int x/x” vs “sh int x/x” so really know the differences for exam day!

So we are unable to add protocols to be forwarded via “ip helper-address” (also note the hyphen is between helper-address), so we will need another command:

R1(config)#ip forward-protocol

“ip forward-protocol” comes to save the day, and I want to walk through the output modifiers for clarity on maybe a couple things:

R1(config)#ip forward-protocol ?
  nd             Sun’s Network Disk protocol
  sdns           Network Security Protocol
  spanning-tree  Use transparent bridging to flood UDP broadcasts
  turbo-flood    Fast flooding of UDP broadcasts
  udp            Packets to a specific UDP port

R1(config)#ip forward-protocol udp ?
  <0-65535>      Port number
  biff           Biff (mail notification, comsat, 512)
  bootpc         Bootstrap Protocol (BOOTP) client (68)
  bootps         Bootstrap Protocol (BOOTP) server (67)
  discard        Discard (9)
  dnsix          DNSIX security protocol auditing (195)
  domain         Domain Name Service (DNS, 53)
  echo           Echo (7)
  isakmp         Internet Security Association and Key Management Protocol
                 (500)
  mobile-ip      Mobile IP registration (434)
  nameserver     IEN116 name service (obsolete, 42)
  netbios-dgm    NetBios datagram service (138)
  netbios-ns     NetBios name service (137)
  netbios-ss     NetBios session service (139)
  non500-isakmp  Internet Security Association and Key Management Protocol
                 (4500)
  ntp            Network Time Protocol (123)
  pim-auto-rp    PIM Auto-RP (496)
  rip            Routing Information Protocol (router, in.routed, 520)
  snmp           Simple Network Management Protocol (161)
  snmptrap       SNMP Traps (162)

R1(config)#ip forward-protocol udp ntp ?
  <cr>

R1(config)#ip forward-protocol udp ntp
R1(config)#

A few things to note with “ip forward-protocol …” command:

  • It is configured on the global configuration level, not the interface
  • It also defines in the first ? that there is no TCP options available
  • It does not require an IP address to send it to, however it will forward it

Really the main difference that I have found between the two protocols is in terms of vlans, that “ip helper-address” will forward broadcasts onto its same vlan that the interface is part of that it’s configured on, while “ip forward-protocol” will forward to all attached vlans.

So the helper-address is a more focused command, while the forward-protocol is a bit more general in allowing traffic to flow around.

That is actually it for this, I have one more lab on quieting down “sh ip packet” so you can run it on a router without pegging the CPU, but I think it would take voodoo for that to work and then it is on to the massive topic of BGP!

So, I will wrap up the security stuff with that, and then it is going to be some epic labbing from what I understand of BGP – It’s going to be like catching King Kong in a net 🙂 Can’t wait!

3 thoughts on “Ip helper-address command explanation and configuration, and some important details and one additional helper cmd!

  1. You really make it appear so easy with your presentation but I to find this
    topic to be actually one thing which I believe I
    might never understand. It seems too complex and very huge for me.

    I’m looking ahead for your subsequent put up, I will try to get the cling of it!

    Like

    1. If your Phone System server is at a central office, or more commonly Layer 3 switches acting as little routers on the LAN, this will forward the DHCP / TFTP from VOIP phones to that server if congiured on the receiving interface of the broadcast “ip helper-address (server IP address)” as router the will not forward broadcasts without this or “ip forward-protocol …” I believe it is.

      So if you want broadcasts to forward on your router, this is your command, just be sure it configured on the inbound interface of the broadcast segment that needs forwarding.

      Or build a GRE tunnel and put that traffic into an IPSec tunnel with a site to site VPN to also accomplish this goal, hope to get a post up tonight with a lab of configuring / troubleshooting / the usual 🙂

      Like

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