Henry's Points

When we show ip bgp and observe some routes are not best route (not greater sign in the front), we should first check if next hop is reachable, then, we should check if iBGP synchronization is enabled.

BGP will put learned prefix to Adj-RIB-in. All learned route will pass Input Policy Engine. A path selection algorithm is then performed to determine the best path for each prefix. The resulting best paths are stored in the local BGP RIB (Loc-RIB) and then are submittted to the local IP routing table (IP-RIB) for installation consideration. To make next hop reachable, we can either change next hop by next-hop-self when eBGP peer advertise the prefix to its neighbor, or redistribute BGP to IGP, which we don’t recommend because two manys prefixes injected in to IGP.

If iBGP synchronization is enabled, the router will not use or advertise the prefixes learned from iBGP neighbor until the matching prefixes learned from an IGP. Prefix synchronization between iBGP and IGP to prevent routing loops and black-holing of traffic. Prefix synchronization means that a prefix learned via iBGP is not included in the best-path selection unless that same prefix exists in the IGP. If the IGP is OSPF, the router ID for the prefix in the IGP must also match the router ID of the BGP peer advertising the prefix. Synchronization may be disabled if this AS does not provide transit for another AS or if all routers in the transit path are running BGP. It is generally a good practice to disable prefix synchronization.

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