Ospf Rfc 1583 And 2178 Compliance; Reduction Of Equivalent As External Lsas - Foundry Networks FESX Manual

NOTE: By default, the Foundry router ID is the IP address configured on the lowest numbered loopback
interface. If the Layer 3 Switch 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, see "Changing the Router ID"
on page 16-23.
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 2 or higher
• communication to a neighbor is lost
• a neighbor declares itself to be the DR or BDR for the first time

OSPF RFC 1583 and 2178 Compliance

Foundry routers are configured, by default, to be compliant with the RFC 1583 OSPF V2 specification. Foundry
routers can also be configured to operate with the latest OSPF standard, RFC 2178.
NOTE: For details on how to configure the system to operate with the RFC 2178, see "Modify OSPF Standard
Compliance Setting" on page 20-35.

Reduction of Equivalent AS External LSAs

An OSPF ASBR uses AS External link advertisements (AS External LSAs) to originate advertisements of a route
to 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. Foundry devices optimize OSPF by eliminating
duplicate AS External LSAs in this case. The Layer 3 Switch 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 Layer 3 Switch's link state database.
This enhancement implements the portion of RFC 2328 that describes AS External LSA reduction. This
enhancement is enabled by default, requires no configuration, and cannot be disabled.
Figure 20.4 shows an example of the AS External LSA reduction feature. In this example, Foundry Layer 3
Switches 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.
December 2005 © Foundry Networks, Inc.
Configuring OSPF
20 - 5