Information Technology Reference
In-Depth Information
10
8
/ BW, where BW is the bandwidth of the interface expressed as a full integer of bits
per second (bps). If the result is smaller than 1, the cost is set to 1. A 10BASE-T (10 Mbps
= 10
7
bps) interface has a cost of 10
8
/ 10
7
= 10. OSPF performs a summation of the costs
to reach a destination; the lowest cost is the preferred path. Table11-2 shows some sample
interface metrics.
Ta b l e 1 1- 2
OSPF Interface Costs
Interface Type
OSPF Cost
10 Gigabit Ethernet
.01 => 1
Gigabit Ethernet
.1 => 1
OC-3 (155 Mbps)
.64516 => 1
Fast Ethernet
10
8
/10
8
= 1
DS-3 (45 Mbps)
2
Ethernet
10
8
/10
7
= 10
T1
64
512 kbps
195
256 kbps
390
The default reference bandwidth used to calculate OSPF costs is 10
8
(cost = 10
8
/ BW).
Notice that for technologies that support speeds greater than 100 Mbps, the default met-
ric gets set to 1 without regard for the network's different capabilities (speed).
Because OSPF was developed prior to high-speed WAN and LAN technologies, the de-
fault metric for 100 Mbps was 1. Cisco provides a method to modify the default reference
bandwidth. The cost metric can be modified on every interface.
OSPFv2 Adjacencies and Hello Timers
OSPF uses Hello packets for neighbor discovery. The default Hello interval is 10 seconds
(30 seconds for nonbroadcast multiaccess [NBMA] networks). For point-to-point net-
works the Hello interval is 10 seconds. Hellos are multicast to 224.0.0.5 (ALLSPFRouters).
Hello packets include such information as the router ID, area ID, authentication, and
router priority.
After two routers exchange Hello packets and set two-way communication, they establish
adjacencies.
Figure 11-1 shows a point-to-point network and an NBMA network.
For point-to-point net works, valid neighbors always become adjacent and communic ate
using multicast address 224.0.0.5. For broadcast (Ethernet) and NBMA networks (Frame
