ASDM Book 1: Cisco ASA Series General Operations ASDM Configuration Guide, 7.2

Configuring a Static Route

Static routing algorithms are basically table mappings established by the network administrator before the beginning of routing. These mappings do not change unless the network administrator alters them. Algorithms that use static routes are simple to design and work well in environments where network traffic is relatively predictable and where network design is relatively simple. Because of this fact, static routing systems cannot react to network changes.

Static routes remain in the routing table even if the specified gateway becomes unavailable. If the specified gateway becomes unavailable, you need to remove the static route from the routing table manually. However, static routes are removed from the routing table if the specified interface goes down, and are reinstated when the interface comes back up.

Note If you create a static route with an administrative distance greater than the administrative distance of the routing protocol running on the ASA, then a route to the specified destination discovered by the routing protocol takes precedence over the static route. The static route is used only if the dynamically discovered route is removed from the routing table.

You can define up to three equal cost routes to the same destination per interface. Equal-cost multi-path (ECMP) is not supported across multiple interfaces. With ECMP, the traffic is not necessarily divided evenly between the routes; traffic is distributed among the specified gateways based on an algorithm that hashes the source and destination IP addresses.

Static null0 Route Configuration

Typically ACLs are used for traffic filtering and they enable you to filter packets based on the information contained in their headers. In packet filtering, the ASA firewall examines packet headers to make a filtering decision, thus adding some overhead to the processing of the packets and affecting performance.

Static null 0 routing is a complementary solution to filtering. A static null0 route is used to forward unwanted or undesirable traffic into a black hole. The null interface null0, is used to create the black hole. Static routes are created for destinations that are not desirable, and the static route configuration points to the null interface. Any traffic that has a destination address that has a best match of the black hole static route is automatically dropped. Unlike with ACLs static null0 routes do not cause any performance degradation.

The static null0 route configuration is used to prevent routing loops. BGP leverages the static null0 configuration for Remotely Triggered Black Hole routing.

For example:

To configure a static route, choose one of the following:

• Adding or Editing a Static Route
• Configuring Static Route Tracking
• Deleting Static Routes

Adding or Editing a Static Route

To add or edit a static route in ASDM, perform the following steps:

Step 1 In the main ASDM window, choose Configuration > Device Setup > Routing > Static Routes.

Step 2 Choose which route to filter by clicking one of the following radio buttons:

• Both (filters both IPv4 and IPv6)
• IPv4 only
• IPv6 only

By default, the Both radio button is selected, and both IPv4 and IPv6 addresses appear in the pane. To limit your viewed choices to routes configured with IPv4 addresses, click the IPv4 radio button. To limit your viewed choices to routes configured with IPv6 addresses, click the IPv6 radio button.

Step 3 Click Add or Edit.

The Add or Edit Static Route dialog box appears.

Step 4 From the Interface drop-down list, choose the internal or external network interface name enabled in the Interface field:

• management (internal interface)
• outside (external interface)

Step 5 In the IP Address field, type an internal or external network IP address for the destination network.

For IPv4 addresses, enter 0.0.0.0 to specify a default route. The 0.0.0.0 IP address can be abbreviated as 0. Optionally, click the ellipsis to browse for an address.

For IPv6 addresses, enter two colons (::) to specify a default route. Optionally, click the ellipsis to browse for an address.

Step 6 In the Gateway IP field, enter the IP address of the gateway router, which is the next hop address for this route.

To enter a default route, set the IP address and mask to 0.0.0.0, or the shortened form of 0.

Optionally, click the ellipsis to browse for an address.

Note If an IP address from one ASA interface is used as the gateway IP address, the ASA will ARP the designated IP address in the packet instead of ARPing the gateway IP address.

The addresses you specify for the static route are the addresses that are in the packet before entering the ASA and performing NAT.

Step 7 Choose the netmask from the drop-down list for the destination network. Depending upon which route you chose to filter (IPv4, IPv6, or both), do one of the following:

• For IPv4 static routes (or for both IPv4 and IPv6 static routes), enter the network mask address that applies to the IP address. Enter 0.0.0.0 to specify a default route. The 0.0.0.0 netmask can be abbreviated as 0.
• For IPv6 static routes only, enter a prefix length.

Step 8 In the Metric field, type the metric, or administrative distance.

The metric or distance is the administrative distance for the route. The default is 1 if you do not specify a value. Administrative distance is a parameter used to compare routes among different routing protocols. The default administrative distance for static routes is 1, giving it precedence over routes discovered by dynamic routing protocols, but not directly connected routes.

The default administrative distance for routes discovered by OSPF is 110. If a static route has the same administrative distance as a dynamic route, the static routes take precedence. Connected routes always take precedence over static or dynamically discovered routes.

Step 9 (Optional) In the Options area, choose one of the following options for a static route:

• None to have no options specified for the static route. This setting is the default.
• Tunneled to specify the route as the default tunnel gateway for VPN traffic. This setting is used for the default route only. You can configure only one tunneled route per device. The tunneled option is not supported in transparent mode.
• Tracked to specify that the route is tracked. The tracking object ID and the address of the tracking target also appear. The tracked option is supported in single, routed mode only. Specify the following settings for the tracked option:

– In the Track ID field, enter a unique identifier for the route tracking process.

– In the Track IP Address/DNS Name field, enter the IP address or hostname of the target being tracked. Typically, this would be the IP address of the next hop gateway for the route, but it could be any network object available from that interface.

– In the SLA ID field, enter a unique identifier for the SLA monitoring process.

Note The Tracked option is not supported for IPv6.

Step 10 (Optional) Click Monitoring Options.

The Route Monitoring Options dialog box appears. From here, you change the following tracking object monitoring properties:

• Frequency, which allows you to modify how often, in seconds, the ASA should test for the presence of the tracking target. Valid values range from 1 to 604800 seconds. The default value is 60 seconds.
• Threshold, which allows you to enter the amount of time, in milliseconds, that indicates an over-threshold event. This value cannot be more than the timeout value.
• Timeout, which allows you to modify the amount of time, in milliseconds, that the route monitoring operation should wait for a response from the request packets. Valid values range from 0 to 604800000 milliseconds. The default value is 5000 milliseconds.
• Data Size, which allows you to modify the size of data payload to use in the echo request packets. The default value is 28. Valid values range from 0 to 16384.

Note This setting specifies the size of the payload only; it does not specify the size of the entire packet.

• ToS, which allows you to choose a value for the type of service byte in the IP header of the echo request. Valid values are from 0 to 255. The default value is 0.
• Number of Packets, which allows you to choose the number of echo requests to send for each test. Valid values range from 1 to 100. The default value is 1.

Step 11 Click OK.

Step 12 Click Apply to save the configuration.

The added or edited route information appears in the Static Routes pane. The monitoring process begins as soon as you save the newly configured route.

Configuring Static Route Tracking

To configure tracking for a static route, perform the following steps:

Note Static route tracking is available for IPv4 routes only.

Step 1 Choose a target of interest. Make sure that the target responds to echo requests.

Step 2 Open the Static Routes pane by choosing Configuration > Device Setup > Routing > Static Routes.

Step 3 Click Add to configure a static route that is to be used based on the availability of your selected target of interest. You must enter the Interface, IP Address, Mask, Gateway, and Metric settings for this route.

Step 4 Click the Tracked radio button in the Options area for this route.

Step 5 Configure the tracking properties. You must enter a unique Track ID, a unique SLA ID, and the IP address of your target of interest.

Step 6 (Optional) To configure the monitoring properties, click Monitoring Options in the Add Static Route dialog box.

Step 7 Click OK to save your changes.

The monitoring process begins as soon as you save the tracked route.

Step 8 Create a secondary route by repeating Steps 1 through 7.

The secondary route is a static route to the same destination as the tracked route, but through a different interface or gateway. You must assign this route a higher administrative distance (metric) than your tracked route.

Step 9 Click OK to save your changes.

Deleting Static Routes

To delete a static route, perform the following steps:

Step 1 Choose Configuration > Device Setup > Routing > Static Routes.

Step 2 On the Static Routes pane, choose which route to delete.

By default, the Both radio button is checked, and both IPv4 and IPv6 addresses appear in the pane.

• To limit your viewed choices to routes configured with IPv4 addresses, click the IPv4 radio button.
• To limit your viewed choices to routes configured with IPv6 addresses, click the IPv6 radio button.

Step 3 Click Delete.

The deleted route is removed from list of routes on in the main Static Routes pane.

Step 4 Click Apply to save the changes to your configuration.

Configuring a Default Static Route

A default route identifies the gateway IP address to which the ASA sends all IP packets for which it does not have a learned or static route. A default static route is simply a static route with 0.0.0.0/0 as the destination IP address. Routes that identify a specific destination take precedence over the default route.

Note In Versions 7.0(1) and later, if you have two default routes configured on different interfaces that have different metrics, the connection to the ASA that is made from the higher metric interface fails, but connections to the ASA from the lower metric interface succeed as expected.

You can define up to three equal cost default route entries per device. Defining more than one equal cost default route entry causes the traffic sent to the default route to be distributed among the specified gateways. When defining more than one default route, you must specify the same interface for each entry.

If you attempt to define more than three equal cost default routes or a default route with a different interface than a previously defined default route, you receive the following message:

You can define a separate default route for tunneled traffic along with the standard default route. When you create a default route with the tunneled option, all traffic from a tunnel terminating on the ASA that cannot be routed using learned or static routes is sent to this route. For traffic emerging from a tunnel, this route overrides any other configured or learned default routes.

Limitations on Configuring a Default Static Route

The following restrictions apply to default routes with the tunneled option:

• Do not enable unicast RPF ( ip verify reverse-path command) on the egress interface of a tunneled route, because this setting causes the session to fail.
• Do not enable TCP intercept on the egress interface of the tunneled route, because this setting causes the session to fail.
• Do not use the VoIP inspection engines (CTIQBE, H.323, GTP, MGCP, RTSP, SIP, SKINNY), the DNS inspect engine, or the DCE RPC inspection engine with tunneled routes, because these inspection engines ignore the tunneled route.
• You cannot define more than one default route with the tunneled option.
• ECMP for tunneled traffic is not supported.

To add or edit a tunneled default static route in ASDM, perform the following steps:

Step 1 On the main ASDM window, choose Configuration > Device Setup > Routing > Static Routes.

Step 2 Click Add or Edit.

Step 3 In the Options area, choose Tunneled.

Step 4 Click OK.

Configuring IPv6 Default and Static Routes

The ASA automatically routes IPv6 traffic between directly connected hosts if the interfaces to which the hosts are attached are enabled for IPv6 and the IPv6 ACLs allow the traffic.

To add or edit a default static route in ASDM, perform the following steps:

Step 1 In the main ASDM window, choose Configuration > Device Setup > Routing > Static Routes.

Step 2 Click the IPv6 only radio button.

Step 3 Click Add or Edit.

Step 4 Click OK.