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Sidewinder G2 6.1.2 Administration Guide - Glossary of Technical ...

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Figure 238: Three OSPF<br />

protocol phases<br />

Appendix C: Configuring Dynamic Routing with OSPF<br />

Overview <strong>of</strong> OSPF routing<br />

At this point, all routers should have a full database. Each database contains<br />

consistent (not identical) information about the network. Based upon this<br />

information, routes are calculated via the "Dijkstra" algorithm. This algorithm<br />

generates the set <strong>of</strong> shortest routes needed to traverse the network. These<br />

routes are then enabled for use by IP.<br />

All OSPF routers on a network do not exchange OSPF data—this limits<br />

network overhead. Instead, they communicate with the DR (and BDR), which<br />

are then responsible for updating all other routers on the network. Election <strong>of</strong><br />

the DR is based upon the priority <strong>of</strong> that router.<br />

OSPF multicasts using the AllSPFRouters (224.0.0.5) and AllDRouters<br />

(224.0.0.6) addresses. The Designated Router (DR) and Backup Designated<br />

Router (BDR) receive packets on the second address.<br />

Important: Since the <strong>Sidewinder</strong> <strong>G2</strong> performs many other functions, Secure<br />

Computing Corporation recommend that customers should not configure the<br />

<strong>Sidewinder</strong> <strong>G2</strong> to become DR (or BDR) unless forced to by network topology.<br />

OSPF routing<br />

OSPF router<br />

R<br />

OSPF router<br />

OSPF router<br />

1 Exchange hello messages to discover neighbor OSPF<br />

routers<br />

2 Exchange Link state databases<br />

3 Exchange Link state advertisements<br />

OSPF is considered an Interior Gateway Protocol (IGP). An IGP limits the<br />

exchange <strong>of</strong> routes to a "domain <strong>of</strong> control," known as an Autonomous System<br />

(AS). An AS is a large network (an ISP for example) created under a central<br />

authority running a consistent routing policy, policies that include different<br />

routing protocols. RIP (both V1 and V2), IS-IS, EIGRP (a proprietary Cisco<br />

protocol), are all IGPs.<br />

Exterior Gateway Protocols, such as EGP and Boundary Gateway Protocols<br />

(BGP), communicate routing information between Autonomous Systems.<br />

Routers on the "edge" <strong>of</strong> the AS generate "special" LSAs (AS-External-LSAs)<br />

for the rest <strong>of</strong> the AS. There's also a mechanism (forwarding address) so that<br />

an OSPF router can "point over there" for a route. This feature allows a<br />

customer to introduce static routes for their network from a central router.<br />

603<br />

R<br />

R

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