IP Link-State Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 374

OSPF Interface Costs Table 8-1

Interface Type OSPF Cost

Gigabit Ethernet .1 → 1

OC-3 (155 Mbps) .64516 → 1

10 8 / 10 8 = 1 Fast Ethernet

DS-3 (45 Mbps) 2

10 8 / 10 7 =10 Ethernet

T1 64

512 KB 195

256 KB 390

In Layer-2 technologies, which support speeds greater than 100 Mbps, the default metric is set
to 1 without regard to the different capabilities (speed) of the network.
The default reference bandwidth that calculates OSPF costs is 10 8 (Cost = 10 8 / BW). You can
change the reference bandwidth by using the following command:

ospf auto-cost reference-bandwidth ref-bw
The ref-bw is the reference bandwidth, in megabits per second. To change the reference
bandwidth to 1 Gbps, use the following commands:

router ospf 10
ospf auto-cost reference-bandwidth 1000
Change the interface cost by using the interface command ip ospf cost value . The value
specified with this command overrides the cost resulting from the ospf auto-cost command.
You can change the default cost for a T1 serial interface from the default (64) to 50 by using the
following commands:

interface serial 0
ip ospf cost 50

OSPF 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). Hellos are multicasted to 224.0.0.5
(ALLSPFROuters). OSPF neighboring routers become adjacent when the parameters in their
Hello packets match, the routers see their own address listed in a Hello packet from their
neighbor (indicating bidirectional communication), they exchange link-state advertisements
(LSAs), and their databases are synchronized.

For point-to-point (p2p) networks, valid neighbors always become adjacent and communicate
by using multicast 224.0.0.5. For broadcast (such as Ethernet) and NBMA (such as Frame