OSPF DR and BDR
In Chapter 6, “Enhanced IGRP (EIGRP) and Open Shortest Path
First (OSPF),” of the Sybex
CCNA Study Guide
CCNA Study Guide
Deluxe Editions, I discussed EIGRP and OSPF
in detail. But I’d really like to expand the section on designated routers and backup designated
routers. I’d also like to delve deeper into verifying the election process and provide you with a
hands-on lab to help you understand that process even better!
To start with, I need to make sure you fully understand the terms
because they’re really crucial to the DR and BDR election process. The election process happens
when a broadcast or nonbroadcast multi-access network is connected together. (Think Ethernet
or Frame Relay.)
Finally, I’m going to end this OSPF update section with a hands-on lab that’s critically
important for you to understand the OSPF designated router (DR) and backup designated
router (BDR) election process.
Routers that share a common segment become neighbors on that segment. These neighbors
are elected via the Hello protocol. Hello packets are sent periodically out of each interface
using IP multicast.
Two routers won’t become neighbors unless they agree on the following:
The idea here is that the two routers interfaces have to belong to the same area on a
particular segment. And of course, those interfaces have to belong to the same subnet.
OSPF allows for the configuration of a password for a specific area. Although
authentication between routers isn’t required, you have the option to set it if you need to do so.
Also, keep in mind that in order for routers to become neighbors, they need to have the same
password on a segment if you’re using authentication.
Hello and Dead Intervals
OSPF exchanges Hello packets on each segment. This is a keepalive
system used by router