Although this is an OSPF book, many different protocols use a concept of Hello packets just like OSPF, for example EIGRP. Therefore, understanding the rationale behind the use
and implementation of Hello is important. Specifically in OSPF, the Hello protocol is used for the following purposes:
• To ensure that communication between neighbors is bidirectional (two-way)
• To discover, establish, and maintain neighbor relationships
• To elect the DR and BDR on broadcast and NBMA networks
• To verify that neighboring OSPF routers are operational (that is, to act as a keepalive mechanism)
Figure 3-22 demonstrates how OSPF routers transmit Hello packets into the network to discover their neighbors.
Figure 3-22 Hello Protocol Operation
Unless otherwise configured, Hello packets default to a transmission time of once every 10 seconds or 30 seconds for nonbroadcast multiaccess (NBMA). When a new OSPF router is introduced, the operation of the Hello protocol is as follows:
1 The OSPF router sends out the Hello packet as a multicast.
2 The Hello packet is received by the new OSPF router.
3 All OSPF routers on that segment receive and process the Hello packet. Only the new OSPF router replies with its own multicast Hello packet.
Designating OSPF's Hello packets as multicast was a coherent protocol design decision by the OSPF Working Group. Multicasting provides the OSPF with the capability to forward a single packet, which is processed only by certain interfaces on the subnet or wire. Specifically, only those interfaces that have the OSPF routing process enabled are capable; therefore, those network interface cards (NICs) are "listening" for Hello packets with the OSPF multicast addresses in them. Interfaces that are not running OSPF ignore the multicast packet.
124 Chapter 3: OSPF Communication
At the data link layer of the OSI reference model, the IP multicast address is mapped to a Layer 2 multicast address. For example, on Ethernet, the last 23 bits of the IP multicast address are added to the Ethernet multicast header of 0100.5e. Therefore, the AllSPFRouters multicast IP address maps to a MAC address of 0100.5e00.0005. In the event that a broadcast subnet does not support multicasting, the AllSPFRouters address is subsequently mapped into the Layer 2 broadcast address (ffff.ffff.ffff). The OSPF Hello protocol operates with some variation, depending on the type of network in use.
TIP The Hello interval can be manipulated with the ip ospf hello-interval seconds interface configuration command, although there is rarely a good reason for changing this parameter. Also, the OSPF dead interval = 4 x ospf hello interval is good to know for the CCIE lab. Use care when changing the Hello interval because neighbor relationships might never come up. Two routers that cycle through various OSPF states frequently have mismatched Hello/dead intervals, as shown in the following example you can check timer values: HAL9000#show ip ospf interface ethernet 0 Ethernet0 is up, line protocol is up
Internet Address 192.168.254.253/24, Area 0
Process ID 100, Router ID 10.10.10.10, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 10.10.10.10, Interface address 192.168.254.253 No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:03 Neighbor Count is 0, Adjacent neighbor count is 0 Suppress hello for 0 neighbor(s) HAL9000#
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