If the BGP process fails to create an IP routing table entry, all traffic
destined for missing IP subnets in the routing table will be dropped. This is
generic behavior of hop-by-hop IP packet forwarding done by routers.
Problems in this section assume that the BGP table has all of the updates
for IP prefixes, but that BGP is not installing them into the IP routing table,
whether the prefixes are IBGP-learned or EBGP-learned routes.
The most
common causes of IBGP-learned routes not getting installed in the IP routing
table are:
- IBGP routes are not synchronized.
- The BGP next hop is not reachable.
The most common causes of EBGP-learned routes not getting installed in
the IP routing table are:
- BGP hop is not reachable in the case of multihop EBGP.
- The multiexit routes are dampened.
- The BGP multiexit discriminator (MED) value is infinite.
The following sections will examine two of these problems, the BGP next
hop is not reachable and BGP routes being dampened in more detail.