INE Workbook Vol 1 OSPF

After completing the OSPF section, I realized again that I am pretty well versed in all things OSPF (excluding MPLS which comes later). As such, I've simply collected a few notes below.

• On a broadcast/non-broadcast OSPF interface, the next-hop is not changed. Pay attention to layer 3 to layer 2 F/R mappings.
• Use 'show ip ospf data router 150.1.5.5 self-originate' to show the costs for different neighbors. This can be useful when verifying 'neighbor x.x.x.x cost y' statements under the OSPF process.
• Additionally, 'show ip ospf data summ x.x.x.x' can be used to show the costs in routing to different neighbors for one route.
• Found an important explanation. Cisco utilizes OSPF transit capability by default. What this means is that if a lower cost route exists through a non-zero area, the route will be sent through a transit area and not directly to area 0. You can disable this with 'no capability transit'
• When configuring a virtual link, the VL takes on the cost metrics of the transit interface between the two routers.

Here is how to progress through the OSPF data show commands. 

• 'show ip ospf data (type of LSA) x.x.x.x'. A forward address of 0.0.0.0 means the router must now compute the metric towards the advertising router.
• 'show ip ospf data router x.x.x.x' where x.x.x.x is the advertising router.
• 'show ip ospf data router x.x.x.x' where x.x.x.x is the DR router.
• 'show ip ospf data router y.y.y.y self-originate' where y.y.y.y is the router-id. This will show the links to another routers with cost and interface/router-id addresses.This is a result of the SPF calculation.

When originating type-5 external routes, the forward address is 0.0.0.0, thus the need for the Type-4 LSA. This works differently in NSSA originated external routes where the inter-area forward address is the advertising router.

When multiple ABRs connect the NSSA to area 0, the router with the highest router-id will translate the type-7 to type-5 LSAs.

The 'area 3 nssa no-redistribution' is locally significant and only prevents the router this is configured on from originating type-7 LSAs into the area. This can be useful in conjunction with default-information-originate or no-summary if there is only one exit out of the area. This will still allow routers within the NSSA to redistribe external routes.

When using the distance command, the source is the originator of the route, and not where you learned the route from.

When using a route-map in conjunction with a distribute list, you can match both the route (match ip add ROUTE-ACL) and desination (match ip next-hop ROUTER-ACL).You can also match the outgoing interface of the route, the route source(router-id) and of course, route type, metrics and tag.

Well, that's all for now. BGP is up next, hopefully I will have some time to begin that tomorrow. As always, if anyone has any questions or comments, please let me know. I would love to hear them!