Lucent Technologies 6000 - Manual (Page 352)

8-4 MAX 6000/3000 Network Configuration Guide
Configuring OSPF Routing
OSPF overview
Figure 8-2. Designated and Backup Designated Routers
To reduce the number of adjacencies each router must form, OSPF calls one of the routers the
Designated Router. A Designated Router is elected as routers are forming adjacencies, and
then all other routers establish adjacencies only with the designated router. This simplifies the
routing table update procedure and reduces the number of link-state records in the database.
The Designated Router also plays other important roles in reducing the overhead of OSPF
link-state procedures. For example, other routers send LSAs to only the Designated Router by
using the All-Designated-Routers multicast address of 224.0.0.6.
To prevent the Designated Router from becoming a serious liability to the network if it fails,
OSPF elects a Backup Designated Router at the same time. Other routers maintain adjacencies
with both the Designated Router and its backup router, but the backup router leaves as many of
the processing tasks as possible to the Designated Router. If the Designated Router fails, the
backup immediately becomes the Designated Router and a new backup is elected.
The administrator chooses which router is to be the Designated Router on the basis of the
processing power, speed, and memory of the system, and then assigns priorities to other routers
on the network in case the Backup Designated Router is also down at the same time.
Note: The MAX can function as a Designated Router (DR) or Backup Designated Router
(BDR). However, many sites choose to assign a LAN-based router for these roles in order to
dedicate the MAX to WAN processing.
Configurable metrics
The administrator assigns a cost to the output side of each router interface. The lower the cost,
the more likely the interface is to be used to forward data traffic. Costs can also be associated
with the externally derived routing data.
You can also use the OSPF cost for preferred path selection. If two paths to a destination have
equal costs, you can assign a higher cost to one of the paths, to configure it as a backup to be
used only when the primary path is not available.
Figure 8-3 shows how costs direct traffic over high-speed links. For example, if Router-2 in
Figure 8-3 receives packets destined for Host B, it routes them through Router-1, across two
T1 links (Cost=20), rather than across one 56Kbps B-channel to Router-3 (Cost=240).
Backup
Designated
Router (BDR)
Designated
Router (DR)
Router-2
MAX
Router-1