Information can be injected into BGP dynamically or statically. Dynamically injected routes come and go from the BGP routing table, depending on the status of the networks they identify. Statically injected routes are constantly maintained by the BGP routing tables, regardless of the status of the networks they identify. Each method has its pros and cons.
Dynamic route injection into BGP
Dynamically injected information can be further divided into purely dynamic, in which all the IGP routes are redistributed into BGP (via the redistribute BGP subcommand), and semidynamic, in which only certain IGP routes are to be injected into BGP (via the network BGP subcommand).
What dynamic redistribution offers is ease of configuration: All internal IGP routes will dynamically flow into BGP, regardless of what particular protocols are being used.
Another method of injecting information into BGP that isn’t quite as dynamic is to manually specify a subset of IGP networks to be advertised by individually listing them for injection into BGP by using the network BGP subcommand. This method is less than completely dynamic, because a list of all the prefixes that need to be advertised must be maintained in the router.
BGP assumes that networks defined with the network command are existing networks and will try to verify that by checking in the IP routing table. If BGP does not find an exact match for these networks, they will not be advertised. YES! If the network is not in routing table exactly and I mean exactly also with the mask then it wont be propagated into the BGP UPDATE.
Injecting routes into BGP via the network command offers more-controlled route advertisement. Injecting IGPs into BGP via redistribution might result in the side effect of leaking unwanted or faulty information into BGP. See the next section to know more about this.
Injection Faulty or Unwanted Information
Redistributing the whole IGP into BGP could result in some unwanted information being leaked into BGP. Such information could be private addresses, or illegal (unregistered) internal addresses that are supposed to be used within the AS only. Other information could include routes that have a prefix length that does not comply with the provider’s aggregation policies; a host route with a prefix length of /32 is an example. Careful filtering is required in order to guard against undesired consequences. Another issue with injecting the IGP into the BGP table is the possibility of unstable routes.
Injecting the IGP routes into BGP dynamically or semidynamically is based on the dependency of the BGP routes on the IGP routes. Although you could argue that this is good because it reflects the actual status of networks, it can have drawbacks as well. The IGP route you advertise will translate into a BGP route. If that route goes down, a WITHDRAWN message will be sent via BGP, requesting peers to remove that route from their BGP routing tables. A route constantly going up and down in your AS has the effect of being constantly sent and withdrawn by other ASs. The example of one fluctuating route is very simplistic; imagine having hundreds of these routes fluctuating in hundreds of ASs. Internet stability will be affected very negatively. Route dampening features was created to mitigate this.
Injecting Information Statically into BGP
Today, injecting information statically into BGP has proven to be the most effective method of ensuring route stability. Of course, this method also has drawbacks. To statically inject information into BGP, IGP routes (or aggregates) that need to be advertised to other peers are manually defined as static routes. This ensures that these routes will never disappear from the IP routing table and hence will always be advertised. Because administrators are often uncomfortable advertising routes to networks that might be down or unreachable, the appropriateness of injecting information statically depends on the particular situation.