background image
Troubleshooting OSPF
345
Troubleshooting OSPF
O
SPF differs from IGRP and Enhanced IGRP because it is a pure link-
state routing technology. Also, it is an open standard routing protocol,
which means that it was not developed solely by Cisco. OSPF was designed
and developed by the IETF to provide a scalable, quickly converging, and
efficient routing protocol that can be used by all routing equipment. Com-
plete details for OSPF are found in RFC2178.
Areas are used within OSPF to define a group of routers and networks
belonging to the same OSPF session. Links connect routers, and the infor-
mation about each link is defined by its link state. On each broadcast or
multi-access network segment, two routers must be assigned the responsibil-
ities of designated router (DR) and backup designated router (BDR).
Like EIGRP, OSPF maintains three databases: adjacency, topology, and
route. The adjacency database is similar to the neighbor database used by
EIGRP. It contains all information about OSPF neighbors and the links con-
necting them. The topology database maintains all route information. The
best routes from the topology database are placed in the route database, or
route table.
Neighbor and Adjacency Formation
The Hello protocol is used to establish peering sessions between routers.
Hello packets are multicast out every interface. The information that is mul-
ticast includes the router ID, timing intervals, existing neighbors, area iden-
tification, router priority, designated and backup router information,
authentication password, and stub area information. All this information is
used when establishing new peers. Descriptions of each element can be found
in Table 6.9.
T A B L E 6 . 9
OSPF Multicast Information
Information Description
Router ID
Highest active IP address on the router.
Time intervals
Intervals between Hello packets, and the allowed
dead time interval.
Existing neighbors
Addresses for any existing OSPF neighbors.
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com