Examine the following output.
You are investigating why router RouterA does not include the 172.16.0.0 network in its BGP advertisements. The output contains portions of RouterA’s configuration and IP routing table.
Which statement correctly identifies the reason why this subnet does not appear in the BGP advertisements?
- BGP synchronization is enabled by default.
- The no auto-summary command was used.
- The 172.16.31.0/24 network was learned through BGP.
- The 10.1.2.3 IP address was not defined as a BGP neighbor.
The no auto-summary command affects how a network identified by using the network command is advertised. The way the router in the exhibit is configured, it will not announce the 172.16.31.0/24 subnet. The BGP router will announce the classful address 172.16.0.0/16 when the routing table, maintained by the IGP, contains an exact match to the network command.
The network command directly affects what network is advertised in BGP. If the command does not also include a network mask, and if auto-summary is enabled, then the classful address 172.16.0.0 is advertised any time that the router learns about a subnet of 172.16.0.0 via its IGP such as OSPF or EIGRP. The routing table does contain an entry for the 172.16.31.0/24 subnet that was learned through the IGP. However, auto-summary is disabled with the no auto-summary command. Therefore, only networks in the routing table that are an exact match to the network commands are advertised.
If the configuration goal is to announce the 172.16.0.0/16 network any time one of its subnets is learned, such as 172.16.31.0/24, then you should enable auto-summary. If the goal is to announce only the 172.16.31.0/24 subnet learned through the IGP, then you should alter the network command’s IP address and include the subnet mask.
The BGP synchronization rule specifies that networks will not be advertised or used via iBGP unless it also has been learned through an IGP. If synchronization is disabled, iGBP will advertise a network without also learning it through an IGP. It does not affect the summarization of routes.
The 172.16.31.0 network was learned through an IGP session with router 10.1.2.3.
Layer 3 Technologies
Configure and verify eBGP (IPv4 and IPv6 address families)