Cisco ASA 5500 Series Configuration Guide using the CLI, 8.4 and 8.6

Configuring Static and Default Routes

---

This chapter describes how to configure static and default routes on the ASA and includes the following sections:

• Information About Static and Default Routes
• Licensing Requirements for Static and Default Routes
• Guidelines and Limitations
• Configuring Static and Default Routes
• Monitoring a Static or Default Route
• Configuration Examples for Static or Default Routes
• Feature History for Static and Default Routes

Information About Static and Default Routes

To route traffic to a nonconnected host or network, you must define a static route to the host or network or, at a minimum, a default route for any networks to which the ASA is not directly connected; for example, when there is a router between a network and the ASA.

Without a static or default route defined, traffic to nonconnected hosts or networks generates the following syslog message:

Multiple context mode does not support dynamic routing,

You might want to use static routes in single context mode in the following cases:

• Your networks use a different router discovery protocol from EIGRP, RIP, or OSPF.
• Your network is small and you can easily manage static routes.
• You do not want the traffic or CPU overhead associated with routing protocols.

The simplest option is to configure a default route to send all traffic to an upstream router, relying on the router to route the traffic for you. However, in some cases the default gateway might not be able to reach the destination network, so you must also configure more specific static routes. For example, if the default gateway is outside, then the default route cannot direct traffic to any inside networks that are not directly connected to the ASA.

In transparent firewall mode, for traffic that originates on the ASA and is destined for a nondirectly connected network, you need to configure either a default route or static routes so the ASA knows out of which interface to send traffic. Traffic that originates on the ASA might include communications to a syslog server, Websense or N2H2 server, or AAA server. If you have servers that cannot all be reached through a single default route, then you must configure static routes. Additionally, the ASA supports up to three equal cost routes on the same interface for load balancing.

Licensing Requirements for Static and Default Routes

The following table shows the licensing requirements for this feature:

Guidelines and Limitations

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent firewall mode.

IPv6 Guidelines

Supports IPv6.

Failover Guidelines

Supports stateful failover of dynamic routing protocols.

Additional Guidelines

IPv6 static routes are not supported in transparent mode in ASDM.

Configuring Static and Default Routes

This section explains how to configure a static route and a static default route, and includes the following topics:

• Configuring a Static Route
• Configuring a Default Static Route
• Configuring IPv6 Default and Static Routes

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) routing 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.

To configure a static route, see the following section:

• Adding or Editing a Static Route

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:

“ERROR: Cannot add route entry, possible conflict with existing routes.”

 

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, enter the following command:

 

Command

Purpose

route if_name 0.0.0.0 0.0.0.0 gateway_ip [ distance | tunneled ]   hostname(config)# route outside 0 0 192.168.2.4 tunneled

Enables you to add a static route. The dest_ip and mask arguments indicate the IP address for the destination network and the gateway_ip argument is the address of the next hop router. The addresses you specify for the static route are the addresses that are in the packet before entering the ASA and performing NAT. The distance argument 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 connect 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.

---

Tip You can enter 0 0 instead of 0.0.0.0 0.0.0.0 for the destination network address and mask, as shown in the following example:

hostname(config)# route outside 0 0 192.168.1 1

---

Detailed Steps

 

	Command	Purpose
Step 1	ipv6 route if_name ::/0 next_hop_ipv6_addr   hostname(config) # ipv6 route inside 7fff::0/32 3FFE:1100:0:CC00::1	Adds a default IPv6 route. The example routes packets for network 7fff::0/32 to a networking device on the inside interface at 3FFE:1100:0:CC00::1 The address ::/0 is the IPv6 equivalent of any.
Step 2	ipv6 route if_name destination next_hop_ipv6_addr [ admin_distance ]   hostname(config)# ipv6 route i nside 7fff::0/32 3FFE:1100:0:CC00::1 [110]	Adds an IPv6 static route to the IPv6 routing table. The example routes packets for network 7fff::0/32 to a networking device on the inside interface at 3FFE:1100:0:CC00::1, and with an administrative distance of 110.

 

---

Note The ipv6 route command works the same way as the route command, which is used to define IPv4 static routes.

---

Monitoring a Static or Default Route

One of the problems with static routes is that there is no inherent mechanism for determining if the route is up or down. They remain in the routing table even if the next hop gateway becomes unavailable. Static routes are only removed from the routing table if the associated interface on the ASA goes down.

The static route tracking feature provides a method for tracking the availability of a static route and installing a backup route if the primary route should fail. For example, you can define a default route to an ISP gateway and a backup default route to a secondary ISP in case the primary ISP becomes unavailable.

The ASA implements this feature by associating a static route with a monitoring target that you define, and monitors the target using ICMP echo requests. If an echo reply is not received within a specified time period, the object is considered down and the associated route is removed from the routing table. A previously configured backup route is used in place of the removed route.

When selecting a monitoring target, you need to make sure that it can respond to ICMP echo requests. The target can be any network object that you choose, but you should consider using the following:

• The ISP gateway (for dual ISP support) address
• The next hop gateway address (if you are concerned about the availability of the gateway)
• A server on the target network, such as a AAA server, that the ASA needs to communicate with
• A persistent network object on the destination network

Note A desktop or notebook computer that may be shut down at night is not a good choice.

You can configure static route tracking for statically defined routes or default routes obtained through DHCP or PPPoE. You can only enable PPPoE clients on multiple interfaces with route tracking configured.

To configure static route tracking, perform the following steps:

Configuration Examples for Static or Default Routes

The following example shows how to create a static route that sends all traffic destined for 10.1.1.0/24 to the router 10.1.2.45, which is connected to the inside interface, defines three equal cost static routes that direct traffic to three different gateways on the outside interface, and adds a default route for tunneled traffic. The ASA then distributes the traffic among the specified gateways:

Unencrypted traffic received by the ASA for which there is no static or learned route is distributed among the gateways with the IP addresses 192.168.2.1, 192.168.2.2, and 192.168.2.3. Encrypted traffic received by the ASA for which there is no static or learned route is passed to the gateway with the IP address 192.168.2.4.

The following example creates a static route that sends all traffic destined for 10.1.1.0/24 to the router (10.1.2.45) connected to the inside interface:

Feature History for Static and Default Routes

Table 22-1 lists each feature change and the platform release in which it was implemented.