                Readme for revised router functions of the ELSA MicroLink Cable modem


1. Short description

From firmware version 1.51 an ELSA MicroLink Cable modem operated as a router uses

        - TWO cable-side IP addresses (formerly: ONE cable -side IP address)
        - TWO MAC addresses (formerly: ONE MAC address)

The two IP addresses have the following tasks:

        - Cable IP address: used for connecting the modem to the DOCSIS-IP network. 
	  The device is configured over this network.
        - Router IP address: used for connecting the cable modem operated as a router 
 	  to the Internet. Data is transmitted over this network.


See point 7 for a detailed description of the significance of the two cable-side 
IP addresses.



2. What must be considered when upgrading to firmware 1.51 and higher?

If only the cable modems and computers in your network whose MAC addresses have been 
explicitly entered on the DHCP server have IP addresses assigned by the DHCP server, 
you must now also enter the second, previously unused MAC address at the DHCP server. 
This ensures that the second IP address, which is required in router operation with 
all firmware versions from 1.51, will also be assigned to the cable modem.



3. From which address range should the second IP address be assigned to the cable modem?

Case 1: If the IP address range is not subdivided in your network and your cable modems 
and computers receive addresses from the same IP address range, the second IP address 
must also be assigned from this range.

Case 2: If your network is divided into two IP networks - one IP address range for 
the cable modems and another address range for the computers, the second IP address 
must be assigned from the address range for computers.

See point 7 for a detailed description of the two cases.


4. What changes for the first IP address?

Nothing changes for this address. Any settings made in the DHCP server can remain as is.


5. How do you determine the second MAC address?

To be able to assign the second IP address to your cable modem in router mode with 
a firmware version above 1.51, you may have to enter the second MAC address at the 
DHCP server (see section 2.). This second MAC address can be determined as follows:

a: with LANconfig:       Interfaces->Network: 2nd Hardware Address
b: with Telnet via LAN:  dir /Setup/2nd Hardware
c: with SNMP:            Get-Request at Object se-second-hardware-addr
                        (private ELSA MIB, SNMP-Object-Id
                        1.3.6.1.4.1.2356.100.1.1.2.22)

CAUTION: There is a MAC address sticker on the bottom of the cable modem. There may be 
one or two MAC addresses printed on this sticker. If there is only one MAC address there, 
the cable modem will automatically generate a second MAC address. This MAC address is 
guaranteed to be unique. If the cable modem cannot generate the second MAC address, 
which happens in rare cases, the router mode will be blocked for security reasons. 
Please contact us if this occurs and provide the first MAC address of your cable modem.



6. Fields and options of the DHCP negotiation of the router IP address

During negotiation of the router IP address, the internal DHCP client acts like that 
of a computer. The following fields and options must be included in the DHCP response 
of your DHCP server:

1) Fields:

YIaddr:                         IP address to be assigned
GIaddr:                         IP address of a DHCP Relay Agent. However, 
                                this is only required if the DHCP server and
                                cable modem are in different networks.
2) Options:

Subnetmask (option 1):          IP network mask
Router (option 3):              IP address(es) of one or more routers
                                that act as a gateway for the cable modem 
                                during routing of the data packets.
Domain Name Server (option 6):  IP address(es) of one or more
                                Domain Name servers.
Lease Time (option 51):         Maximum period after which the cable modem
                                requests its IP address again.


The values assigned during this DHCP negotiation (addresses etc.) can be read out at 
the cable modem as follows:

a: with Telnet via LAN:  dir/Status/DHCP-client-status/Router-DHCP-client
b: with SNMP:            private ELSA-MIB, tree st-DHCP-router-client-status,
                        SNMP-Object-Id 1.3.6.4.1.2356.100.1.1.1.20.2



7. How are the two cable-side IP addresses used internally?

Case 1: using ONE IP network for cable IP addresses and
router IP addresses


Example:   
      	      |                                  |
                          |          |
              |           |          |           |
                          |   DOCSIS |
              |           |   CMTS   |           |
                          +----------+
              |                                  |
                           Cable-IP
              |            123.1.2.3             |
                          +----------+
              |           |  DOCSIS  |           |
                          |  Bridge  |
              |           |          |           |
                          |Router-IP |
              |           |123.1.2.4 |           |
                          | +------+ |              public network
              + - - - - - | |      | | - - - - - +  e.g. 123.1.x.x
                          | |CM-   | |
                          | |Router| |
                          | |      | |
              + - - - - - | +------+ | - - - - - +  private network
                          |  LAN-IP  |              e.g. 172.18.x.x
              |           |172.18.0.1|           |
                          +----------+
              |                                  |


In this case the cable IP address and the router IP address are in the same IP network 
123.x.x.x. Because this network can be reached in the Internet, not only the router but 
also the DOCSIS interface of the cable modem and the CMTS are in the Internet.



Case 2: Use of TWO different IP networks for cable IP addresses and router IP addresses


Example:
              |                                  |
                  + - - - |          | - - - +
              |           |          |           |
                  |       |   DOCSIS |       |
              |           |   CMTS   |           |
                  |       +----------+       |
              |                                  |
                  |                          |      private network
              |             Cable-IP             |  e.g. 10.x.x.x
                  |         10.1.2.3         |<------------------
              |           +----------+           |
                  + - - - |  DOCSIS  | - - - +
              |           |  Bridge  |           |
                          |          |
              |           |Router-IP |           |
                          |123.1.2.4 |
              |           | +------+ |           |  public network
              + - - - - - | |      | | - - - - - +  e.g. 123.1.x.x
                          | |CM-   | |
                          | |Router| |
                          | |      | |
              + - - - - - | +------+ | - - - - - +  private network
                          |  LAN-IP  |           |  e.g. 172.18.x.x
              |           |172.18.0.1|
                          +----------+           |
              |
                                                 |

In this case the two cable-side IP addresses are in different IP networks. The cable IP 
address is in the network 10.x.x.x. This network is only valid in the DOCSIS range and 
therefore must be a private network that cannot be reached from the Internet. The 
router IP address is in network 123.x.x.x. These addresses are available in the Internet.


The cable IP address is used to link the cable modem to the DOCSIS system. The cable 
modem is configured and controlled via this IP address.

The router IP address is either directly in the Internet (see Figure) or in a network 
that is connected to the Internet by NAT/PAT.

The router can only be operated in case 1 with firmware versions lower than 1.51. With 
firmware versions 1.51 or higher the router can now be operated in case 1 and also in
Case 2.

ELSA recommends case 2. The use of a private IP address for the cable IP address, 
Which is therefore not visible in the Internet, increases the security of the DOCSIS 
system. Unauthorized access from the Internet to the configuration of the cable modem 
and of the CMTS become more difficult. This case is preferred by network providers 
with standard cable modems as DOCSIS bridges. With the new router design ELSA offers 
the option of operating the router cable modem also in this safer mode.
