Think about a bunch of routers on the same Ethernet segment flooding all of their LSAs to each other who it turn flood the LSAs to all of the other routers. If every router had an adjacency with every other router that would equate to a lot of adjacencies which would then equate to unnecessary flooding of router LSAs. To solve this problem, multi-access networks elect a DR that is responsible for representing the the entire multi-access network and all connected routers as well as to mange the flood process within the multi-access network.
Each router on the network will form an adjacency with both the elected DR on a segment as well as the elected BDR. Taking a look at a subsection of our lab topology, we can see this in action…. While in reality, each of my segments is a multi-access network Ethernet when not manually configured to be point to point and will elect a DR and a BDR, this section is truly a multi-access network. Here, router1, router2, and switch1 share a common subnet.
If we look at the neighbors from the perspective of each device we can get some insight on the DR and BDR roles…. I was initially confused on the matter so I posted a question about it at network-forum. Deadcow and mellowd took a look at it for me and eventually we determined that it was spanning-tree causing the problem. Adding in a 4th device temporarily we can see this in action…. This is achieved by neighboring routers telling each other about all of the other LSAs that it knows about.
The confusion comes in when we start talking about LSAs. These LSAs are only flooded within the area which they are originated. The LSA contains information about the network including attached routers and is flooded only within the area in which they are originated. This essentially tells all of the routers inside a specific area what other prefixes are available through a specific ABR. Additionally, this LSA type is used to send prefixes available within a specific area into the backbone area 0.
The destination is not actually a prefix, but rather a specific host address of an ASBR. Never implemented.
Opaque 9,10,11 — Used to carry other information that may or may not be used by the actual OSPF routers. Area types Now that we know the different kinds of LSAs, we can talk about specific area types. If you need to traverse the ABR, why would all of the routers in the stub area need the specific external routes to tell them to talk to the ABR?
Why not just give them one default route to the ABR? The rules for stub areas are…. After all, if you are using a 0s route through an ABR to get out of the area anyway, why do you need the network summaries from other areas?
This would allow a stub area to learn routes from an external source and then advertise them into the OSPF domain. The lower the cost , the better the route. Two methods are available to change the OSPF cost. One method is to run the ospf cost command to directly change the interface cost , and the other is to change the bandwidth reference value to indirectly change the interface cost.
Two routers speaking OSPF to each other exchange information about the routes they know about and the cost for them to get there. When many OSPF routers are part of the same network, information about all of the routes in a network are learned by all of the OSPF routers within that network—technically called an area.
Since Cisco IOS release What this means is that it prefers N1 routes before E1 and N2 over E2 routes. To specify the cost of sending a packet on an interface, use the ip ospf cost command. To reset the path cost to the default, use the no form of this command.
The idea behind the OSPF areas is to lower the amount of routing traffic and lower the processing of your core routers. OSPF is link state routing protocol and you can tune a link state routing protocol to converge fast.
OSPF is an open standard, classless, link-state routing protocol. OSPF has an unlimited hop count. An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification.
A router within an area must maintain a topological database for the area to which it belongs. The RIP metric is hop count, which is simply a number of routers between the source and destination. Best Regards, Seunghyun Lee. Intra-area routes. Inter-area routes. External Type-1 routes.
External Type-2 routes. B If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
C If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths. Cheers, Benjamin. I am sorry that I have to choose only one answer. I would like to thank you again for your great answers.
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