Enterasys ssr-glx19-02 Guia Do Utilizador

OSPF will add the internal cost to the autonomous system border router to the external 
metric. Type 2 ASEs are used for Exterior Gateway Protocols (EGP) that have metrics not 
comparable to OSPF metrics. In this case, only the internal OSPF cost to the autonomous 
system border router is used in the routing decision.

From the topology database, each router constructs a tree of the shortest paths with itself 
as the root. This shortest-path tree gives the route to each destination in the autonomous 
system. Externally derived routing information appears on the tree as leaves. The link-
state advertisement format distinguishes between information acquired from external 
sources and information acquired from internal routers, so there is no ambiguity about the 
source or reliability of routes. Externally derived routing information (for example, routes 
learned from EGP) is passed transparently through the autonomous system and is kept 
separate from OSPF’s internally derived data. Each external route can also be tagged by 
the advertising router, enabling a passing of additional information between routers on 
the borders of the autonomous system.

OSPF intra-area and inter-area routes are always imported into the SSR routing database 
with a preference of 10. It would be a violation of the protocol if an OSPF router did not 
participate fully in the area's OSPF, so it is not possible to override this. Although it is 
possible to give other routes lower preference values explicitly, it is ill-advised to do so.

Hardware multicast capabilities are also used where possible to deliver link-status 
messages. OSPF areas are connected by the backbone area, the area with identifier 0.0.0.0. 
All areas must be logically contiguous and the backbone is no exception. To permit 
maximum flexibility, OSPF allows the configuration of virtual links to enable the 
backbone area to appear contiguous despite the physical reality.

All routers in an area must agree on that area's parameters. A separate copy of the link-
state algorithm is run for each area. Because of this, most configuration parameters are 
defined on a per area basis. All routers belonging to an area must agree on that area's 
configuration. Erroneous configuration will lead to adjacencies not forming between 
neighbors, and routing information might not flow, or even loop.

You can use CoreWatch to set global OSPF parameters to start or stop OSPF on the SSR 
and specify how often autonomous-system export link-state advertisements will be 
generated and flooded into OSPF. 

To set the SSR’s global OSPF parameters:

1.

Start Configuration Expert if you have not already done so.

2.

Open the configuration file you want to modify and then double-click that file’s 
Routing Configuration object.

3.

Double-click the IP Routing Configuration object.