Only TOS X paths to the forwarding address are examined, and the AS boundary router must be advertising a TOS X route to the destination. These parameters need only be configured in those routers that are themselves eligible to become Designated Router (i.e., those router’s whose Router Priority for the network is non-zero): List of all other attached routers The list of all other routers attached to the non-broadcast network. This list is sent to the neighbor in Database Description packets. The OSPF Options field is contained in OSPF Hello packets, Database Description packets and in OSPF link state advertisements. This length includes the standard OSPF header. Thus if the OSPF router and another router have multiple attached networks in common, multiple conversations ensue, each described by a unique neighbor data structure. In the second stage, leaves are added to the tree by considering the links to stub networks. The E-bit should be set if and only if the attached area is capable of processing AS external advertisements (i.e., it is not a stub area). Virtual links cannot be configured through stub areas (see Section 3.6). The virtual link is treated as if it were an unnumbered point-to- point network (belonging to the backbone) joining the two area border routers.
9.5.1. Sending Hello packets on non-broadcast networks Static configuration information is necessary in order for the Hello Protocol to function on non-broadcast networks (see Section C.5). On broadcast networks and physical point-to-point networks, Hello packets are sent every HelloInterval seconds to the IP multicast address AllSPFRouters. On unnumbered point-to-point networks. Although a virtual link acts like an unnumbered point-to-point link, it does have an associated IP interface address. On virtual links, Hello packets are sent as unicasts (addressed directly to the other end of the virtual link) every HelloInterval seconds. Otherwise, Hello Packets are sent every HelloInterval seconds. If the neighbor is in state Down, Hello Packets are sent every PollInterval seconds. The format of an Hello packet is detailed in Section A.3.2. Each packet type is then described in a succeeding section. The Hello Packet also indicates how often a neighbor must be heard from to remain active (RouterDeadInterval). Both HelloInterval and RouterDeadInterval must be the same for all routers attached to a common network. If the E-bit is set incorrectly the neighboring routers will refuse to accept the Hello Packet (see Section 10.5). The rest of the Hello Packet’s Options field should be set to zero.
When sending Hello packets periodically to any neighbor, the interval between Hello Packets is determined by the neighbor’s state. OSPF protocol packets should be given precedence over regular IP data traffic, in both sending and receiving. 9.5. Sending Hello packets Hello packets are sent out each functioning router interface. A router’s hello-sending behavior varies depending on whether the router itself is eligible to become Designated Router. Only those routers on the list that have not declared themselves to be Designated Router are eligible to become Backup Designated Router. In order to ensure two-way communication between adjacent routers, the Hello packet contains the list of all routers from which Hello Packets have been seen recently. The Hello Packet contains the router’s Router Priority (used in choosing the Designated Router), and the interval between Hello Packets sent out the interface (HelloInterval). Each conversation is bound to a particular OSPF router interface, and is identified either by the neighboring router’s OSPF Router ID or by its Neighbor IP address (see below). OSPF is IP protocol number 89. This number has been registered with the Network Information Center. 10. The Neighbor Data Structure An OSPF router converses with its neighboring routers.
4) If Router X is now newly the Designated Router or newly the Backup Designated Router, or is now no longer the Designated Router or no longer the Backup Designated Router, repeat steps 2 and 3, and then proceed to step 5. For example, if Router X is now the Designated Router, when step 2 is repeated X will no longer be eligible for Backup Designated Router election. If the router is eligible to become Designated Router, it must periodically send Hello Packets to all neighbors that are also eligible. For example, a class A network would have the mask 0xff000000. Attached Router The Router IDs of each of the routers attached to the network. 13.7. Receiving link state acknowledgments Many consistency checks have been made on a received Link State Acknowledgment packet before it is handed to the flooding procedure. The lookup in this case is still well defined, since no two network links advertisements can have the same Link State ID. No two subnets have the exact same IP subnet address
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