Table of Contents
Advertisement
OSPF area , and is transferred among area border layer3 switches; AS external LSA is generated by layer3
switches on external border of AS, and is trans ferred throughout the AS.
As to autonomous systems mainly advertises exterior link-state, OSPF allow some areas to be configured as
STUB areas to reduce the size of the topology database. Type4 LSA (ASBR summary LSA) and type5 LSA
(AS external LSA) are not allowed to flood into/through S TUB areas. STUB areas must use the default routes,
the layer3 switches on S TUB area edge advertise the default routes to STUB areas by type 3 summary LSA,
those default routes only floods inside S TUB area and will not get out of S TUB area. Each S TUB area has a
corresponding default route, the route from a S TUB area to AS exterior destination must rely on the default
route of that area.
The following simply outlines the route calculation process of OSPF protocol:
1） Each OSPF-enabled layer3 switch maintains a database (LS database) describing the link-state of
the topology structure of the whole autonomous system. Each layer3 switch generates a link-state
advertisement according to its surrounding network topology structure (router LSA ), and sends the
LSA to other layer3 switches through link-state update (LS U) packets. Thus each layer3 switches
receives LSAs from other layer3 switches, and all LSAs are combined to the link-state database.
2） Since a LSA is the description of the network topology structure around a layer3 switch, the LS
database is the description of the network topology structure of the whole network. The layer3
switches can easily create a weighted vector map according to the LS database. Obviously, all layer3
switches in the same autonomous system will have the same network topology map.
3） Each layer3 switch uses the shortest path first (SPF) algorithm to calculate a tree of shortest path
rooted by itself. The tree provides the route to all the nodes in the autonomous system, leaf nodes
consist of the exterior route information. The exterior route can be marked by the layer3 switch
broadcast it, so that additional information about the aut onomous system can be recorded. As a
result, the route table of each layer3 switch is different.
OSPF protocol is developed by the IE TF, the OSPF v2 widely used now is fulfilled according to the content
described in RFC2328.
37.2 OSPF Configuration Task List
The OSPF configuration for XGS3 series switches may be different from the configuration procedure to
switches of the other manufacturers. It is a two-step process:
1、
Enable OSPF in the Global Mode; 2、Configure OSPF area for the interfaces. The configuration task
list is as follows:
1.
Enable OSPF protocol (required)
(1)
Enable/disable OSPF protocol (required)
(2)
Configure the ID number of the layer3 switch running OSPF (optional)
(3)
Configure the network scope for running OSPF (optional)
(4)
Configure the area for the interface (required)
2.
Configure OSPF protocol parameters (optional)
(1)
Configure OSPF packet sending mechanism parameters
1)
Configure OSPF packet verification
37-3
Advertisement
Table of Contents
Troubleshooting
