Algorithm For As External Lsa Reduction - Brocade Communications Systems ICX 7250 Series Configuration Manual

Algorithm for AS external LSA reduction

Algorithm for AS external LSA reduction
The AS external LSA reduction example shows the normal AS External LSA reduction feature. The behavior changes under the following
conditions:
• There is one ASBR advertising (originating) a route to the external destination, but one of the following happens:
– A second ASBR comes on-line
– A second ASBR that is already on-line begins advertising an equivalent route to the same destination.
In either case above, the router with the higher router ID floods the AS External LSAs and the other router flushes its equivalent AS
External LSAs. For example, if Router D is offline, Router E is the only source for a route to the external routing domain. When Router D
comes on-line, it takes over flooding of the AS External LSAs to Router F, while Router E flushes its equivalent AS External LSAs to
Router F.
• One of the ASBRs starts advertising a route that is no longer equivalent to the route the other ASBR is advertising. In this case,
the ASBRs each flood AS External LSAs. Since the LSAs either no longer have the same cost or no longer have the same next-
hop router, the LSAs are no longer equivalent, and the LSA reduction feature no longer applies.
• The ASBR with the higher router ID becomes unavailable or is reconfigured so that it is no longer an ASBR. In this case, the
other ASBR floods the AS External LSAs. For example, if Router D goes off-line, then Router E starts flooding the AS with AS
External LSAs for the route to Router F.
OSPFv2 areas
Backbone area
The backbone area (also known as area 0 or area 0.0.0.0) forms the core of OSPFv2 and OSPFv3 networks. All other areas are
connected to it, and inter-area routing happens by way of routers connected to the backbone area and to their own associated areas.
The backbone area is the logical and physical structure for the OSPF domain and is attached to all non-zero areas in the OSPF domain.
The backbone area is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous,
but it does not need to be physically contiguous; backbone connectivity can be established and maintained through the configuration of
virtual links.
Area types
OSPFv2 areas can be normal, a stub area, a totally stubby area (TSA), or a not-so-stubby area (NSSA).
• Normal: OSPFv2 devices within a normal area can send and receive external link-state advertisements (LSAs).
• Stub: OSPFv2 devices within a stub area cannot send or receive external LSAs. In addition, OSPFv2 devices in a stub area
must use a default route to the area's Area Border Router (ABR) to send traffic out of the area.
• NSSA: The Autonomous System Boundary Router (ASBR) of an NSSA can import external route information into the area.
– ASBRs redistribute (import) external routes into the NSSA as type 7 LSAs. Type 7 External LSAs are a special type of LSA
generated only by ASBRs within an NSSA, and are flooded to all the routers within only that NSSA.
– ABRs translate type 7 LSAs into type 5 External LSAs, which can then be flooded throughout the autonomous system.
The NSSA translator converts a type 7 LSA to a type 5 LSA if F-bit and P-bit are set and there is a reachable forwarding
address. You can configure summary-addresses on the ABR of an NSSA so that the ABR converts multiple type 7
external LSAs received from the NSSA into a single type 5 external LSA.
216
Brocade FastIron Layer 3 Routing Configuration Guide
53-1003903-04