Nortel Optical Multiservice Edge 6130, Руководство по планированию, Страница: 288 / 292

10-102 Appendix A: Data communications planning
Optical Multiservice Edge 6130 NT6Q92MA Rel 1.0 Iss 1 Standard September 2006
Figure 10-29
Route summarization
.
• Supernetting Supernetting comes from the introduction of CIDR. Address
space is allocated without class as a contiguous number of class C
addresses. For example, an allocation of addresses in the range 196.0.16.0
to 196.0.31.255 could be represented by 196.0.16/20. Such an entry in a
routing table is referred to as a supernet. Subnetting is used to achieve route
summarization and cannot be used with routing protocols such as RIP
which categorize IP addresses by class.
• Network reachability There is no path limit with OSPF.
• Bandwidth considerations OSPF just sends out link state updates when
they occur, with a maximum interval of 30 minutes.
• Route selection OSPF determines a best route using the concept of cost.
Each interface configured with OSPF has a metric parameter, whose value
has been derived from the formula 10
8
/interface speed (however, it is at the
discretion of the engineer to change it, if required—for example when the
link is busy). According to the formula, some values are:
— Ethernet 100BaseTX: 1
— Ethernet 10BaseT: 10
— Serial at 2 Mbit/s: 48
— Serial at 64 kbit/s: 1562
Route preference
All routing protocols are assigned a preference which allows the router to
select routes when different protocols each report a path to the same network.
It could be considered as a measure of believability. The exact ranking of
protocols depends on the router manufacturer, but link state routing protocols
(for example, OSPF) are considered more reliable than distance vector
Area Border
Router
140.100.10.0/24
140.100.20.0/24
140.100.30.0/24
140.100.40.0/24
Route summarization
140.100.0.0/16 140.150.10.0/24
Route summarization allows the right hand networks to be
advertised using one update
140.150.10.0/24