OSPF - Routing Protocols and Concepts - page 499

Hello Protocol

Every OSPF message includes the header, as shown in Figure 11-2. Also shown in the figure are the
fields of the OSPF Hello packet. Fill in the missing field contents.

Figure 11-2 OSPF Message Format (Answer)

Data Link Frame
Header

IP Packet
Header

OSPF Packet
Header

OSPF Packet Type-Specific
Data Hello Packet

0 7 8 15 16 23 24 31

Bits

Version Type = 1 Packet Length

Router ID
Area ID

OSPF
Packet
Headers

Checksum AuType

Authentication
Authentication
Network Mask

Hello Interval Option Router Priority

OSPF
Hello
Packets

Router Dead Interval
Designated Router (DR)
Backup Designated Router (BDR)
List of Neighbor(s)

Receiving a Hello packet on an interface confirms for a router that there is another OSPF router on
this link. OSPF then establishes adjacency with the neighbor.

Before two routers can form an OSPF neighbor adjacency , they must agree on three values: Hello
interval, Dead interval, and network type. The OSPF Hello interval indicates how often an OSPF
router transmits its Hello packets. By default, OSPF Hello packets are sent every 10 seconds on multi-
access and point-to-point segments and every 30 seconds on nonbroadcast multiaccess (NBMA) seg-
ments (Frame Relay, X.25, ATM).

In most cases, OSPF Hello packets are sent as multicast to an address reserved for ALLSPFRouters at
224.0.0.5 .

The Dead interval is the period, expressed in seconds, that the router will wait to receive a Hello
packet before declaring the neighbor “down.” Cisco uses a default of four times the Hello interval. For
multiaccess and point-to-point segments, this period is 40 seconds. For NBMA networks, the Dead
interval is 120 seconds.

If the Dead interval expires before the routers receive a Hello packet, OSPF will remove that neighbor
from its link-state database.

To reduce the amount of OSPF traffic on multiaccess networks, OSPF elects a designated router ( DR )
and backup designated router ( BDR ). The DR is responsible for updating all other OSPF routers
(called DRothers ) when a change occurs in the multiaccess network. The BDR monitors the DR and
takes over as the DR if the current DR fails.

OSPF Link-State Updates

Link-state updates (LSU) are the packets used for OSPF routing updates. An LSU packet can contain
11 different types of link - state advertisements ( LSA ).