Dell BigIron RX Series, Configuration Manual

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682 BigIron RX Series Configuration Guide
53-1002253-01
Overview of OSPF (Open Shortest Path First)
25
NOTE
By default, the Brocade router ID is the IP address configured on the lowest numbered loopback
interface. If the device does not have a loopback interface, the default router ID is the lowest
numbered IP address configured on the device. For more information or to change the router ID,
refer to
“Changing the router ID” on page 182.
When multiple routers on the same network are declaring themselves as DRs, then both priority
and router ID are used to select the designated router and backup designated routers.
When only one router on the network claims the DR role despite neighboring routers with higher
priorities or router IDs, this router remains the DR. This is also true for BDRs.
The DR and BDR election process is performed when one of the following events occurs:
•
an interface is in a waiting state and the wait time expires
•
an interface is in a waiting state and a hello packet is received that addresses the BDR
•
a change in the neighbor state occurs, such as:
•
a neighbor state transitions from ATTEMPT state to a higher state
•
communication to a neighbor is lost
•
a neighbor declares itself to be the DR or BDR for the first time
OSPF RFC 1583 and 2328 compliance
Brocade routers are configured, by default, to be compliant with the RFC 1583 OSPF V2
specification. Brocade routers can also be configured to operate with the latest OSPF standard,
RFC 2328.
NOTE
For details on how to configure the system to operate with the RFC 2328, refer to “Modify OSPF
standard compliance setting” on page 718.
Reduction of equivalent AS external LSAs
An OSPF ASBR uses AS External link advertisements (AS External LSAs) to originate advertisements
of a route learned from another routing domain, such as a BGP4 or RIP domain. The ASBR
advertises the route to the external domain by flooding AS External LSAs to all the other OSPF
routers (except those inside stub networks) within the local OSPF Autonomous System (AS).
In some cases, multiple ASBRs in an AS can originate equivalent LSAs. The LSAs are equivalent
when they have the same cost, the same next hop, and the same destination. The device optimizes
OSPF by eliminating duplicate AS External LSAs in this case. The device with the lower router ID
flushes the duplicate External LSAs from its database and thus does not flood the duplicate
External LSAs into the OSPF AS. AS External LSA reduction therefore reduces the size of the
device’s link state database. The AS External LSA reduction is described in RFC 2328
Figure 104 shows an example of the AS External LSA reduction feature. In this example, Routers D
and E are OSPF ASBRs, and thus communicate route information between the OSPF AS, which
contains Routers A, B, and C, and another routing domain, which contains Router F. The other
routing domain is running another routing protocol, such as BGP4 or RIP. Routers D, E, and F,
therefore, are each running both OSPF and either BGP4 or RIP.