Cisco Systems IOS XR Neighbors and Adjacency for OSPF, Designated Router (DR) for OSPF

Implementing OSPF on Cisco IOS XR Software

Information About Implementing OSPF on Cisco IOS XR Software

RC-178

Cisco IOS XR Routing Configuration Guide

Authentication can be specified for an entire process or area, or on an interface or a virtual link. An

interface or virtual link can be configured for only one type of authentication, not both. Authentication

configured for an interface or virtual link overrides authentication configured for the area or process.

If you intend for all interfaces in an area to use the same type of authentication, you can configure fewer

commands if you use the authentication command in the area configuration submode (and specify the

message-digest keyword if you want the entire area to use MD5 authentication). This strategy requires

fewer commands than specifying authentication for each interface.

To support the changing of an MD5 key in an operational network without disrupting OSPF adjacencies

(and hence the topology), a key rollover mechanism is supported. As a network administrator configures

the new key into the multiple networking devices that communicate, some time exists when different

devices are using both a new key and an old key. If an interface is configured with a new key, the software

sends two copies of the same packet, each authenticated by the old key and new key. The software tracks

which devices start using the new key, and the software stops sending duplicate packets after it detects

that all of its neighbors are using the new key. The software then discards the old key. The network

administrator must then remove the old key from each the configuration file of each router.

Neighbors and Adjacency for OSPF

Routers that share a segment (Layer 2 link between two interfaces) become neighbors on that segment.

OSPF uses the hello protocol as a neighbor discovery and keep alive mechanism. The hello protocol

involves receiving and periodically sending hello packets out each interface. The hello packets list all

known OSPF neighbors on the interface. Routers become neighbors when they see themselves listed in

the hello packet of the neighbor. After two routers are neighbors, they may proceed to exchange and

synchronize their databases, which creates an adjacency. On broadcast and NBMA networks all

neighboring routers have an adjacency.

Designated Router (DR) for OSPF

On point-to-point and point-to-multipoint networks, the CiscoIOS XR software floods routing updates

to immediate neighbors. No DR or backup DR (BDR) exists; all routing information is flooded to each

router.

On broadcast or NBMA segments only, OSPF minimizes the amount of information being exchanged on

a segment by choosing one router to be a DR and one router to be a BDR. Thus, the routers on the

segment have a central point of contact for information exchange. Instead of each router exchanging

routing updates with every other router on the segment, each router exchanges information with the DR

and BDR. The DR and BDR relay the information to the other routers. On broadcast network segments

the number of OSPF packets is further reduced by the DR and BDR sending such OSPF updates to a

multicast IP address that all OSPF routers on the network segment are listening on.

The software looks at the priority of the routers on the segment to determine which routers are the DR

and BDR. The router with the highest priority is elected the DR. If there is a tie, then the router with the

higher router ID takes precedence. After the DR is elected, the BDR is elected the same way. A router

with a router priority set to zero is ineligible to become the DR or BDR.