IP Link-State Routing Protocols - CCIE Routing and Switching Exam Certification Guide - page 375

Relay) networks, all routers become adjacent to the designated and backup designated router
(DR and BDR) but not to each other. The DR concept is discussed later in this chapter, in the
“Designated Router” section.

On OSPF point-to-multipoint (p2mp) networks, it might be necessary to configure the set of
neighbors that are directly reachable over the p2mp network. Each neighbor is identified by its
IP address on the p2mp network. DRs are not elected on p2mp networks, so the DR eligibility
of configured neighbors is undefined. Communication is similar to p2p networks that use
multicast 224.0.0.5.

For OSPF virtual links, OSPF packets are unicast. Virtual links are discussed in this chapter.

Certain parameters in the Hello packets that are exchanged between neighbors must match
before the neighbors can become adjacent. The interface that receives the Hello packets checks
the network mask, the Hello interval, and the router dead timer to determine if they match its
own values. Interfaces on p2p links or virtual links ignore the value of the network mask. If
these values don't match, the Hello packet is dropped.

Another parameter that must match is indicated by an option bit, which identifies how the area
handles the external routes. This is the E-bit. An interface belongs to a single OSPF area. The
E-bit in a Hello packet that is received by an interface must match the receiving area's external
route capabilities. If external routes are not flooded into and throughout this area (this is a stub
area), the E-bit must not be set in the received Hello packets; otherwise, the E-bit must be set.
Stub areas are discussed later in this chapter, in the “OSPF Stub Area Types” section.

LSA Types

OSPF routers generate LSAs that are flooded throughout an area or the entire autonomous
system (AS). OSPF defines different LSA types for router, network, summary, external, and so
on. Understanding of the LSA types help with other OSPF concepts. Table 8-2 describes the
major LSA types.

Major LSA Types Table 8-2

Type Code Type Description

1 Router LSA Produced by every router and includes all the router links,

interfaces, state of links, and cost. This LSA type is
flooded within a single area.

2 Network LSA Produced by every DR on every broadcast or NBMA

network. It lists all the routers in the multiaccess network.
This LSA type is contained within an area.

3 Summary LSA for ABRs Produced by area border routers (ABRs). It is sent into an

area to advertise destinations outside the area.

continues