BGP Issue: Sync Fix

When BGP routes are not being advertised to eBGP peers, the likely culprit is the synchronization rule. By default, BGP will only advertise routes to eBGP peers if those routes are also present in the IGP. This behavior is meant to ensure that the routes are truly reachable. However, in many modern networks, this strict requirement can be unnecessary.

Key Points:

• Symptom:
  BGP routes are not being advertised to eBGP peers.

• Root Cause:
  The BGP synchronization rule is enabled, which prevents BGP from advertising routes unless they exist in the IGP.

• Solution:
  Disable the synchronization feature with the command no synchronization unless you specifically require strict IGP validation.

Example Scenario:

Without Disabling Synchronization:

Router0(config)# router bgp 5110 
Router0(config-router)# network 50.50.50.0 mask 255.255.255.0 
! Note: Without "no synchronization" 
!       BGP will wait for the route to be present 
!       in the IGP before advertising.

Corrected Configuration:

Router0(config-router)# no synchronization

Explanation:

• What Happens:
  When synchronization is enabled, BGP will only advertise a route if that same route is already installed in the IGP. This can lead to situations where BGP routes remain unadvertised, even though they are correctly configured within BGP.

• Why Disable It:
  In many modern networks, the IGP and BGP operate in separate domains or the IGP does not need to verify every BGP route for reachability. Disabling synchronization allows BGP to advertise routes immediately, relying on the inherent capabilities of the BGP process and underlying network reachability.

• When to Keep It Enabled:
  If you require strict validation that routes are present in the IGP before they are advertised (for instance, in some specific network designs or legacy environments), you may choose to leave synchronization enabled. However, this is rarely necessary in most contemporary setups.

By disabling synchronization, you ensure that BGP is free to advertise its routes to eBGP peers without waiting for corresponding IGP routes. This streamlines the routing process and prevents potential delays in route advertisement.