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OSPF Demand Circuits and BackupBackup with OSPF can be difficult, especially with dial-on-demand backup such as ISDN. OSPF becomes difficult in the backup mode because of area connectivity. The key to how you control backup interfaces depends on what OSPF area the backup interface is in. Because of the different area properties, such as what LSA get flooded to what areas, the area selection for backup is important. Where the router is located in the network, the area type, and how it connects to Area 0 will also influence what area the backup interface goes in. Instead of going over the countless possible examples for demand circuits and OSPF backup, we instead cover configuration guidelines and issues. For specific lab examples and more information on the backup interface and OSPF demand circuits, see Chapter 7, "WAN Protocols and Technologies: Integrated Services Digital Network (ISDN)." The sections that follow cover the configuration guidelines for OSPF demand circuits and backup. Adhere to OSPF Design RulesWhen the backup link becomes active, the network still must comply with all OSPF design guidelines. That is, Area 0 cannot be partitioned, virtual links cannot be run through stub areas, all areas must connect to Area 0, and so on. The network topology will change when in backup mode, but it still must adhere to the same OSPF guidelines. OSPF Demand CircuitsRFC 1793 outlines the original standard for OSPF demand circuits. Essentially, OSPF demand circuits will spoof hellos, 224.0.0.5, from activating dial-on-demand circuits. It attempts to control LSA flooding by exchanging LSA information only the first time that the circuit is activated and by setting the LSA DoNotAge bit. If the dial link is in Area 0, or if the OSPF internetwork has external LSAs or LSA Type 5s or is in an NSSA area with LSA Type 7s, a demand circuit will not operate properly. Type 5 and Type 7 LSAs or NSSA LSAs force a DDR link, such as ISDN, to constantly dial. Most networks have some form of redistribution and almost always have LSA Type 5s floating around. Only a stub area used in conjunction with demand circuit will prevent the DDR link from constant dialing because of LSA Type 5s. To configure a demand circuit, use the following three-step process:
Area 0 GuidelinesThe backup interface should go into Area 0 only if the primary interface is in Area 0. If dynamic routing is to occur, use the backup interface command or dialer watch commands . Remember, all LSAs will be flooded into Area 0. This type of constant LSA flooding to Area 0 will cause constant dialing of the interface. In cases such as this, the interface must be forced not to dial with additional configuration techniques. |
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