When you notice that only classful networks are being advertised in your BGP configuration, the culprit is often the auto-summary feature. Auto-summary, enabled by default, aggregates subnet routes to their classful boundaries, which may lead to unintended summarization of your specific routes.
Symptom:
Only classful networks are advertised, or BGP ignores subnet-specific advertisements.
Root Cause:
Auto-summary is enabled by default, causing BGP to summarize routes to their classful boundaries. For example, a subnet like 50.50.50.0 might be summarized to 50.0.0.0/8.
Solution:
Disable auto-summary to allow BGP to advertise the full subnet, preserving the intended network granularity.
Router0(config)# router bgp 5110 Router0(config-router)# network 50.50.50.0
Issue: BGP automatically summarizes the subnet to 50.0.0.0/8, which is not desired.
Router0(config-router)# no auto-summary
Disabling auto-summary ensures that the specific subnet (50.50.50.0/24) is advertised as intended, maintaining the proper routing information for your network.
By disabling auto-summary, you retain control over how your routes are advertised, ensuring that subnet-specific routes are not inadvertently summarized to classful boundaries. This practice is crucial in networks where precise routing details are required.