| 2.3 |  | Securing the Perimeter Router |
| 2.3.3 |  | Routing protocol authentication and update filtering |

| An unprotected router or routing domain is an easy target for any network-savvy adversary. For example, an attacker who sends false routing update packets to an unprotected router can easily corrupt its routing table. This enables the attacker to re-route network traffic as desired. The key to preventing this type of an attack is to protect the routing tables from unauthorized and malicious changes.

There are two basic approaches available for protecting routing table integrity:

- Use only static routes: This may work in small networks, but is unsuitable for large networks .
- Authenticate route table updates: By using routing protocols with authentication, network administrators can deter attacks based on unauthorized routing changes. Authenticated router updates ensure that the update messages come from legitimate sources. Bogus messages are automatically discarded.

Another attack involves preventing router update messages from being sent or received, which will result in bringing down parts of a network. To resist such attacks and recover from them quickly, routers need rapid convergence and backup routes.

Routing protocol authentication is vulnerable to eavesdropping and spoofing of routing updates. Message Digest 5 (MD5) authentication of routing protocol updates prevents the introduction of unauthorized or false routing messages from unknown sources.

Cisco IOS software supports the use of MD5 authentication of routing protocol updates for the following protocols:

- OSPF
- RIPv2
- Enhanced IGRP
- BGP

The key-string command defines the MD5 key that is used to create the message digest, or hash, that is exchanged with the opposite router. It is possible to specify the time period during which the key can be received and sent with the accept-lifetime and send-lifetime commands.

Static routes are manually configured on the router as the sole path to a given destination. In one sense, static routes are very secure. They are not vulnerable to spoofing attacks because they do not deal with router update packets. However, exclusively using static routes will make network administration extremely difficult. Also, configuring a large network to use only static routes can make the availability of large segments of the network subject to single points of failure. Static routes cannot handle events such as router failures. However, a dynamic routing protocol, such as OSPF, can correctly reroute traffic in the case of a router failure.

Passive interfaces
The passive-interface command is used to prevent other routers on the network from learning about routes dynamically. It can also be used to keep any unnecessary parties from learning about the existence of certain routes or routing protocols used. It is typically used when the wildcard specification on the network router configuration command configures more interfaces than desirable.

| Lab Activity Lab Exercise: Configure Routing Authentication and Filtering In this lab, the student will demonstrate the use of authentication and filters to control route updates from neighbor routers.