Content

| 5.3 |  | Common IGP Routing Protocol Issues, Causes, and Solutions |
| 5.3.4 |  | Distribute-list in/out blocking |

A distribute list is a filtering mechanism for routing updates. Distribute lists are not supported for OSPF or IS-IS. Routers running link state protocols determine their routes based on information in their link state database, rather than the advertised route entries received from its neighbors.

Route filters have no effect on link state advertisements or the link state database. This is because a basic requirement of link state routing protocols is that routers in an area must have identical link state databases.

In addition, it is important to remember that distribute lists do not permit or deny the actual packets from entering the routers, only which routing updates a router will send or receive. A distribute list calls on an access list and checks which networks are supposed to be permitted. If the access list does not contain the network, it will automatically be denied. A distribute list can be applied on incoming routing updates or outgoing routing updates.

This example shows two routers running IGRP between each other. A distribute-list in is configured, but because the access list does not contain the permit statement for 131.108.0.0, router R2 is not installing this route in the routing table.

Debugs and Verification
Figure shows the current configuration of router R2. In the access list configuration, the network 131.108.0.0 is not explicitly permitted (and, therefore denied), so the router is not installing any subnets of the 131.108.0.0 network. In this case, the access list was incorrectly configured with the wrong network, 131.107.0.0.

Solution
Figure shows the new configuration of router R2 with the correct access list.

Figure shows that router R2 is learning IGRP routes after the configuration change.