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| 5.9 |  | Troubleshooting BGP |
| 5.9.3 |  | Troubleshooting routes not being installed in the IP routing table |

A common problem is with BGP routes not getting installed into the IP routing table. If a router must forward an IP packet by looking at the IP destination address of the IP packet, the router must have an IP routing table entry for the subnet of the IP destination address.

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 routes are dampened.
- The BGP next hop is not reachable in the case of multihop EBGP.
- The 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.