After the DR and BDR have been
elected, the routers are considered to be in the Exstart state, as shown
in the Figure
. They are ready to discover the link-state information about the
internetwork and create their link-state databases.
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Lab
Activity |
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In this lab, you
will learn how to observe the DR and BDR selection
process.
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The process used to discover the network routes is called the exchange
process, and is performed to advance the routers to a Full state of
communication. After adjacent routers are in a Full state, they do not modify
the exchange protocol unless the Full state changes. The exchange process
operates as follows:
- In the Exstart state,
the DR and BDR establish adjacencies with
each router in the network. During this process, a master-slave relationship
is created between each router and its DR/BDR. The router that has the higher
router ID acts as the master.
Note that link-state information is exchanged and synchronized only between
the DR/BDR and the routers to which they have established adjacencies. Having
the DR represent the network in this capacity reduces the amount of routing
update traffic.
- The master and slave routers exchange one or more database description
packets (DDPs or DBDs). This is referred to as the Exchange state.

A DDP includes an LSA (link-state advertisement) that describes the
router's link-state database. Each database description LSA includes a variety
of information such as link-state type, the address of the advertising router,
the cost of the link, and the sequence number. The sequence number is a
router's way of determining the order in which the link-state information was
received. The initial sequence number used by the adjacent routers to start
keeping track of the LSA sequence is defined by the master.
- When the slave router receives the DDP, it does the following:
- Acknowledges the receipt of the DDP by echoing the link-state entry
sequence numbers in a link-state acknowledgment (LSAck) packet, as shown
in the figure..
- Compares the information it received with the information it has.
Remember, the initial entries put into the link-state database are from
the adjacencies database. If the DDP has a more up-to-date link-state
entry, then the slave router sends a link-state request (LSR) to the
master router.
- The master router responds with the complete information about the
requested entry in a link-state update (LSU) packet. Again, the slave
router sends an LSAck when the LSU is received. The process of sending
LSRs is referred to as the Loading state.

- All routers add the new link-state entries into their link-state
database.
- After all LSRs have been updated for a given router, adjacent routers
are considered synchronized in a Full state.
The routers must be in the Full
state before they can route traffic. At this point, the routers should all
have identical link-state databases.
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Lab
Activity |
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In this lab,
you will learn how to force the DR and BDR election process. |
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