OSPF routing protocol is an open, using the Dijkstra algorithm to calculate the route short. First generates a tree, and then the resulting table with the best roads. Is a link state protocol.
These are its main features:
- has a hierarchical design can be divided into areas.
- Minimizes routing update traffic.
- Supports VLSM (variable-sized subnets). Unlimited
- hops or jumps.
- Swing links of equal cost only (EIGRP can not be the same).
Every design consists of an OSPF area 0 or area backbone. All other areas must be connected with it, physically or logically ( virtual link) . A router that connects one with the backbone area, within an autonomous system (AS) , called Area Border Router (ABR). at least one of the interface must be an ABR connected to area 0.
OSPF can connect multiple AS, the router that connects called Autonomous System Boundary Router (ASBR).
Here are some concepts that uses OSPF:
- Router ID: IP address that identifies the router. Ip is the highest of all configured interfaces. Router
- neighbor for two routers are neighbors must share the area and intervals Hello and Dead . If authentication is enabled must also share the same password. Router
- adjacent relationship between two routers can exchange routing updates. Ie between a router and DR and BDR. Hello
- protocol: can maintain relationships with neighboring routers. These packets and LSA build and maintain database Topological . Link state
- advertisement (LSA): packages containing the link status and routing information shared.
- Neighborship database: list of OSPF routers to which Hello packets are sent. It contains details such as the router ID or state. Topological
- database: LSA contains information received from an area. It is used as input to the Dijkstra algorithm to find the best route.
- Designated Router (DR) router is chosen in an area responsible for informing the other routers on the network changes by LSAs.
- Backup Designated Router (BDR) as a fallback DR.
tree Shortest Path First
Within an area each router calculates the best routes to Topological database. If the router has interfaces in different areas, build a tree for each.
The key criterion used in selecting routes, is the potential cost to a network. The cost is allocated to each outgoing interface listed in the tree. The cost a full path is the sum of the costs of the interface along the path. Cisco uses: 10 ^ 8/ancho band. Ex: Fast Ethernet (100 Mbps), would cost 1. We can change the cost of an interface with the command ip ospf cost .
If you use routers from different manufacturers will use other metrics to calculate the cost and we will adjust.
DR and BDR Election
The idea is that all the routers to communicate with a central router, rather than with everyone else. The DR and BDR election is done via the Hello protocol (This process will only occur in networks broadcast or non-broadcast multi-access , not dpi).
The DR is the router configured with a higher priority. If everyone has the same will elect the router ID higher. As the router ID is the highest ip interface. We can configure a loopback ip to change it. You can also set a priority of 0, and the router will not intervene in the process.
Configuring OSPF
To activate the OSPF routing process:
router (config) # router ospf process_id
The process_id is a local value, it has to be equal with other routers. This is because there may be multiple instances of OSPF running independent.
The following areas have been set. These are indicated by identifying the interfaces and area of \u200b\u200bresidence. OSPF uses wildcards in the configuration.
router (config-router) # network 10.0.0.0 0.255.255.255. area 0
A 0 in the wildcard indicates that coincide exactly with the network and 1 means it does not matter. Ex: 0.255.255.255 1.0.0.0 gives the range: 1.0.0.0-1.255.255.255.
Tip: to know the wildcard for a network, look no Mask subnet, calculate the size of the block and subtract one. Ex: 192.168.10.64/28 is a block size of 2 ^ 4 = 16. Wildcard = 0.0.0.15
Configuring a loopback interface
try to be logical or virtual interfaces, and take part in the election of the router ID. Simply:
router (config) # int loopback 0
router (config-if) # ip 172.16.10.1 255 255 255 255
After this we reset the router to change the router ID.
Verifying OSPF
To view the routing table (OSPF entries to O) using sh ip route . To view information on the processes sh ip ospf OSPF .
To view the database Topological use the command sh ip ospf database. With sh ip interface show ospf OSPF interface information (cost, area, router ID, timers, etc).
command sh ip ospf neighbor allows us to see a table with neighbors and what the DR and BDR. Finally sh ip protocols dumps information from routing protocols configured on the router.
Debug OSPF
With ip ospf packet degub see the packets sent and received on the router. To see only Hello there command ip ospf hello debuff.
Author: David Gonzalez Perez
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