DHCP
- Gateway address
Although the computers in the LAN can use ICMP redirects to learn which
router is the correct virtual router, it is still advisable to designate
the correct router as gateway directly during the DHCP negotiation. This
allows the assigning gateway address to be determined as follows:
- If a gateway is explicitly defined for the interface in the DHCP module,
then only this will be assigned.
- If no explicit gateway is set, then the default route is looked up
in the routing table. If the default route exists and is connected to
a virtual router which is directly linked to the interface through which
the DHCP request is received, then the virtual router's address is assigned
as gateway.
- If other remote sites are linked to virtual routers, then these will
not be assigned via DHCP since there can only be one default gateway.
A host can only learn the corresponding routers via ICMP redirects.
- Otherwise, the address corresponding to the address pool or interface
(intranet or DMZ) will be assigned.
If more than one virtual router is connected by the default route, then
the address of the router with the highest priority will be assigned.
This allows for automatic load balancing through the selection of the
DHCP server by the respective client. The DHCP server is to be activated
on all routers involved in load balancing. All routers then define many
virtual routers, each with different priorities. If the client randomly
selects a DHCP server from those that answer, then it will also be randomly
assigned a virtual router.
Example with two routers
LANCOM A defines the following virtual routers:
Router ID
|
Virt. address
|
Prio
|
B Prio
|
Peer
|
1
|
10.0.0.1
|
100
|
50
|
INTERNET
|
2
|
10.0.0.2
|
60
|
50
|
INTERNET
|
and, correspondingly LANCOM B:
Router ID
|
Virt. address
|
Prio
|
B Prio
|
Peer
|
1
|
10.0.0.1
|
60
|
30
|
INTERNET
|
2
|
10.0.0.2
|
100
|
30
|
INTERNET
|
Depending on whether it chooses LANCOM A or LANCOM B, a DHCP client
will now be assigned 10.0.0.1 or 10.0.0.2 as gateway and is initially
distributed on both LANCOM devices.
Using this example, it becomes clear how load balancing can be combined
with backup. If LANCOM A falls into backup mode, then LANCOM B will
become the master for all clients. If LANCOM B fails, then LANCOM
A will become the master for all clients and will attempt to establish
its backup. If this fails, then it is LANCOM B's turn again (this signals
the end of the backup chain).
- Further addresses
If the DHCP server is to assign explicit addresses for certain services
which the LANCOM provides, such as DNS and NBNS server, then either
the configured addresses or the real addresses are assigned to the respective
interfaces. Assigning a virtual router violates the RFC which prohibits
a virtual router from offering other services (a device may only react
to a virtual address when it is also the "owner" of this address, i.e.
when the address is also the real interface address. At the same time,
this means that DNS and NBNS must receive special treatment.