Ospf Packets - Avaya ERS 8800 series Configuration Manual

the designated PollInterval, for example, every 120 seconds. Otherwise, neighbors send hello
packets at the designated HelloInterval, for example, every 10 seconds.
Forming adjacencies
In an NBMA network, as in a broadcast network, all routers become adjacent to the DR and
the BDR. The adjacencies form after the router priorities are assigned, the neighbors are
configured, and the network DR is elected.
OSPF and IP
OSPF runs on top of IP, which means that nodes send an OSPF packet with an IP data packet
header. The protocol field value in the IP header is 89, which identifies it as OSPF and
distinguishes it from other packets that use an IP header.
An OSPF route advertisement expresses a destination as an IP address and a variable-
length mask. The address and the mask indicate the range of destinations to which the
advertisement applies.
OSPF can specify a range of networks and can send one summary advertisement that
represents multiple destinations. For example, a summary advertisement for the destination
128.185.0.0 with a mask of 255.255.0.0 describes a single route to destinations 128.185.0.0
to 128.185.255.255.

OSPF packets

All OSPF packets start with a 24-octet header containing information about the OSPF version,
the packet type and length, the ID of the router transmitting the packet, and the ID of the OSPF
area from which the packet is sent. An OSPF packet is one of the following types:
• Hello packets
Hello packets transmit between neighbors and are never forwarded. The Hello Protocol
requires routers to send hello packets to neighbors at predefined hello intervals. If the
router receives no hello packets within the specified dead interval, the neighbor router
declares the other router dead.
• Database description (DD) packets
OSPF exchanges DD packets when a link is first established between neighboring routers
that synchronize the link-state databases.
• Link-state request packets
Link-state request packets describe one or more link-state advertisements that a router
requests from a neighbor. Routers send link-state requests if the information received in
DD packets from a neighbor is not consistent with the router's link-state database.
• Link-state update packets
Configuration — IPv6 Routing
Routing
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