The BGP topology table is also referred to as the BGP RIB holds all the Network Layer Reachability Information (NLRI) learned by BGP in addition to the associated Path Attributes for each prefix.
NLRIs are simply an IP prefix and prefix length.
Path Attributes include metric variables such as Weight, Local Preference, AS Path and so on…
Routes are injected into the BGP RIP via:
- network command
- topology changes through update messages
- redistribution from another routing protocol
The BGP network command looks for a route in the router’s current IP routing table that exactly matches the parameters of the network command, if the IP route exists put the equivalent NLRI into the local BGP table.
Redistributing can inject static, connected, and IGP learned routes into the BGP RIB. BGP does not use the concept of calculating a metric for each alternate route to a specific prefix. As a result there is no requirement to set a metric upon redistribution into BGP.
If there is a metric applied in the redistribution command it is injected into BGP with the Multi-Exit Discriminator (MED) value changed.
Auto Summary and its Impact on redistributed routes and the network command:
If any subnets of a classful network would be redistributed, do not redistribute, instead redistribute a route for the classful network.
If a network command lists a classful network number with or without a classful subnet mask, and any subnets of the classful network exist, inject a route for the classful network.