OSPF - Virtual Links

When you are using OSPF as your routing protocol there is one thing that you must consider before the actual network implementation. An OSPF network must contain an area 0 and might contain different areas. All OSPF areas must be connected to area 0, what we called backbone area. Usually, when our network expends, we might end up having non-zero area connected to a non-zero area and this of course won't allow communication between the areas. The solution to this case is to use a virtual link to bridge across an intermediate area. Let's see an example.

Topology

R2(config)# router ospf 1

R2(config)# router-id 2.2.2.2

R2(config)# network 10.23.23.0 0.0.0.3 area 23

R3(config)# router ospf 1

R3(config)# router-id 3.3.3.3

R3(config)# network 10.23.23.0 0.0.0.3 area 23

R3(config)# network 10.34.34.0 0.0.0.3 area 34

R4(config)# router ospf 1

R4(config)# router-id 4.4.4.4

R4(config)# network 10.34.34.0 0.0.0.3 area 34

R4(config)# network 10.46.46.0 0.0.0.3 area 0

R6(config)# router ospf 1

R6(config)# router-id 6.6.6.6

R6(config)# network 10.46.46.0 0.0.0.3 area 0

As you can see the area 23 is not connected to area 0 and R4 won't have the route 10.23.23.0 in its routing table. To solve this issue we have to configure a virtual-link between R3 and R4 (transit area).

R3(config)# router ospf 1

R3(config)# area 34 virtual-link 4.4.4.4

R4(config)# router ospf 1

R4(config)# area 34 virtual-link 3.3.3.3

After the virtual-link configuration, check the R4's routing table and you will see an IA route to 10.23.23.0 network. You can also verify with the show ip ospf virtual-links command.

Route Redistribution

This is the concept when routing information must be exchanged between different protocols or routing domains. Only routes that are in the routing table and learned via the specified protocol are redistributed. 

Redistribution Characteristics

RIP - Default metric is Infinity. Metric must be set, except when redistributing static or connected routes, which have a metric of 1.

OSPF - Default metric is 20. Can specify the metric type; the default is E2. Must use subnets keyword or only classful networks are redistributed.

EIGRP - Default metric is Infinity. Metric must be set, except when redistributing static or connected routes, which get their metric from the interface. Metric value is "bandwidth, delay, reliability, load, MTU." Redistributed routes have a higher administrative distance than internal ones.

Static/Connected - To include local networks not running the routing protocol, you must redistribute connected interfaces. You can also redistribute static routes into a dynamic protocol.

BGP - Metric (MED) is set to IGP metric value.

Command:

Router(config-router)# redistribute {route-source} [metric metric] [route-map tag]

OSPF: Establishing Neighbors and Exchanging Routes

Step #1: Down state: OSPF process not yet started. No Hello packet sent.
Step #2: Init state: Hello packets have been sent to all OSPF interfaces.
Step #3: Two-way state: Router has received a Hello packet from another router that contains its own router ID in the neighbor lost. All parameters match, so routers can become neighbors.
Step #4: Exstart state: If routers become adjacent (exchange routes), they determine which one starts the exchange process.
Step #5: Exchange state: Routers exchange DBDs listing the LSAs in their LSD by RID and sequence number.
Step #6: Loading state: Each router compares the DBD received to the contents of its LS database. It then sends a LSR for missing or outdated LSAs. Each router responds to its neighbor's LSR with a Link State Update. Each LSU is acknowledged.
Step #7: Full state: The LSDB has been synchronized with the adjacent neighbor.

OSPF Packets

Hello: Identifies neighbors and serves as a keepalive.
Link State Request (LSR): Request for a Link State Update (LSU). Contains the type of LSU requested and the ID of the router requesting it.
Database Description (DBD): A summary of the LSDB, including the RID and sequence number of each LSA in the LSDB.
Link State Update (LSU): Contains a full LSA entry. An LSA includes topology information; for example, the RID of this router and the RID and cost to each neighbor. One LSU can contain multiple LSAs.
Link State Acknowledgment (LSAck): Acknowledges all other OSPF packets (except Hellos).

CCNP ROUTE - EIGRP Characteristics

EIGRP used to be a Cisco-proprietary protocol which means it could be only used with cisco routers. Cisco however, in order to help companies operate in multi-vendor environment, opened up EIGRP as an open standard protocol. Some of its characteristics are:
1. Support for variable-length subnet masking (VLSM).
2. Fast converge after topology changes through its backup routes.
3. It sends routing updates if and only if there is a change in the network topology.
4. Its updates contain only the routes that have been changed and not the entire routing table.
5. Support Appletalk, IP, and IPX.
6. Using packets multicast to 224.0.0.10 to discover neighbors.
7. The updates go only to the routers that actually need them.

CCNP ROUTE - Administrative Distance (AD)

In some cases, usually when we are talking for WAN topology, there is a huge chance to have more than one path from the source to destination. Router has to decide which path is the best, that will be shown in its routing table. To make this decision a router goes through a four-step process. First, the route with the longest prefix length takes the place of the best route. Second, if both routes have the same prefix length, then the router with the lowest administrative distance is used. Third, if the previous two steps are exactly the same, then the route with the lowest metric becomes the best route. Fourth, if again, all the previous steps are exactly the same, then all of these routes will be used in load balancing based on which protocol is used. 

Administrative Distance Metric
Directly Connected--- 0
Static Route--- 1
EIGRP Summary--- 5
Internal EIGRP--- 90
OSPF--- 110
RIP--- 120
External EIGRP--- 170
Internal BGP--- 200