Ospf Neighbors; Ospf Network Types - Foundry Networks NetIron M2404C User Manual

Foundry NetIron M2404C and M2404F Metro Access Switches
Table 1: LSA Type Names and Numbers
LSA Number
1
2
3,4
5

OSPF Neighbors

When routers running OSPF initialize, they attempt to locate neighboring routers to exchange
Link-State Advertisements (LSAs) from which routing tables are constructed. Routers form
adjacencies with neighboring routers before exchanging this routing information. Details such as
subnet address, OSPF area number, network type, and authentication passwords are all checked
before an adjacency is formed between neighbors. On broadcast or point-to-point segments,
neighbor discovery is done dynamically through the OSPF multicast, 224.0.0.5, using the OSPF
"Hello" protocol. On NBMA (Non-Broadcast Multiple Access) networks neighbors must be
configured manually before the Hello protocol will initialize in unicast fashion and begin the
adjacency forming process.

OSPF Network Types

OSPF has defined standards for communicating across a diverse set of network media.
Broadcast
The Broadcast OSPF network type typically runs on multi-access broadcast IP interfaces such as
Ethernet, Token Ring, or FDDI. With OSPF Broadcast, a Designated Router (DR) and Backup
Designated Router (BDR) are elected dynamically on a broadcast segment with which all other
routers form adjacencies, to exchange link-state information. Election criteria include router ID,
loopback IP interface presence, and router IP interface priority values. All these criteria can be
manually configured to influence the selection process. It is the responsibility of the DR and BDR
to collect link state information from all routers on the broadcast segment, and then compile and
distribute the resulting area map back to each. This precludes all routers on a common segment
from exchanging link state information with every other router on a segment and reduces the
amount of traffic on a broadcast segment. It is important to understand that there is a singular
DR/BDR on every broadcast segment in an OSPF network and not just one per area.
Point-to-point (Pt-to-Pt)
The Point-to-Point OSPF network type is typically implemented across dedicated WAN circuits
such as T-1 links or on frame relay point-to-point sub-interfaces. No designated router is elected on
Open Shortest Path First (OSPF)
LSA Type LSA Description
Originated by all routers. A router-LSA describes the
Router-LSAs
collected states of the router IP interfaces to an area.
Network-LSAs Contains the list of routers connected to the network.
Summary-LSAs Originated by area border routers, and flooded throughout
the LSAs associated area. Each summary-LSA describes
a route to a destination outside the area, yet still inside the
AS (i.e., an inter-area route).
Type 3 summary-LSAs describe routes to networks.
Type 4 summary-LSAs describe routes to AS-boundary
routers.
AS-external-LSAs Originated by AS boundary routers and flooded
throughout the AS. Each AS-external-LSA describes a
route to a destination in another Autonomous System.
Default routes for the AS can also be described by AS-
external-LSAs.
© 2008 Foundry Networks, Inc
Configuring Advanced Routing Information (Rev. 03)
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