Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.7.xE and IOS 15.2(3)Ex

How Unicast RPF Works

When Unicast RPF is enabled on an interface, the switch examines all packets received as input on that interface to make sure that the source address and source interface appear in the routing table and match the interface on which the packet was received. This ability to look backwards is available only when Cisco Express Forwarding (CEF) is enabled on the switch, because the lookup relies on the presence of the Forwarding Information Base (FIB). CEF generates the FIB as part of its operation.

Note Unicast RPF is an input function and is applied only on the input interface of a switch at the upstream end of a connection.

Unicast RPF checks to see if any packet received at a switch interface arrives on the best return path (return route) to the source of the packet. Unicast RPF does this by doing a reverse lookup in the CEF table. If the packet was received from one of the best reverse path routes, the packet is forwarded as normal. If there is no reverse path route on the same interface from which the packet was received, it might mean that the source address was modified. If Unicast RPF does not find a reverse path for the packet, the packet is dropped.

Note With Unicast RPF, all equal-cost “best” return paths are considered valid. This means that Unicast RPF works in cases where multiple return paths exist, provided that each path is equal to the others in terms of the routing cost (number of hops, weights, and so on) and as long as the route is in the FIB. Unicast RPF also functions where EIGRP variants are being used and unequal candidate paths back to the source IP address exist.

When a packet is received at the interface where Unicast RPF and ACLs have been configured, the following actions occur:

Step 1 Input ACLs configured on the inbound interface are checked.

Step 2 Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it does by doing a reverse lookup in the FIB table.

Step 3 CEF table (FIB) lookup is carried out for packet forwarding.

Step 4 Output ACLs are checked on the outbound interface.

Step 5 The packet is forwarded.

This section provides information about Unicast RPF enhancements:

• Access control lists and logging
• Per-interface statistics

Figure 36-1 illustrates how Unicast RPF and CEF work together to validate IP source addresses by verifying packet return paths. In this example, a customer has sent a packet having a source address of 192.168.1.1 from interface Gigabit Ethernet 1/1. Unicast RPF checks the FIB to see if 192.168.1.1 has a path to Gigabit Ethernet 1/1. If there is a matching path, the packet is forwarded. If there is no matching path, the packet is dropped.

Figure 36-1 Unicast RPF Validating IP Source Addresses

Figure 36-2 illustrates how Unicast RPF drops packets that fail validation. In this example, a customer has sent a packet having a source address of 209.165.200.225, which is received at interface Gigabit Ethernet 1/1. Unicast RPF checks the FIB to see if 209.165.200.225 has a return path to Gigabit Ethernet 1/1. If there is a matching path, the packet is forwarded. There is no reverse entry in the routing table that routes the customer packet back to source address 209.165.200.225 on interface Gigabit Ethernet 1/1, and so the packet is dropped.

Figure 36-2 Unicast RPF Dropping Packets That Fail Verification

Implementing Unicast RPF

Unicast RPF has several key implementation principles:

• The packet must be received at an interface that has the best return path (route) to the packet source (a process called symmetric routing). There must be a route in the FIB matching the route to the receiving interface. Adding a route in the FIB is done with a static route, network statement, or dynamic routing. (ACLs permit the use of Unicast RPF when packets will arrive by specific, less optimal asymmetric input paths.)
• IP source addresses at the receiving interface must match the routing entry for the interface.
• Unicast RPF is an input function and is applied only on the input interface of a switch at the upstream end of a connection.

Given these implementation principles, Unicast RPF becomes a tool that network administrators can use not only for their customers but also for their downstream network or ISP, even if the downstream network or ISP has other connections to the Internet.

Caution Using optional BGP attributes such as weight and local preference, you can modify the best path back to the source address. Modification affects the operation of Unicast RPF.

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This section provides information about the implementation of Unicast RPF:

• Security Policy and Unicast RPF
• Where to Use Unicast RPF
• Routing Table Requirements
• Where Not to Use Unicast RPF
• Unicast RPF with BOOTP and DHCP

Security Policy and Unicast RPF

Consider the following points in determining your policy for deploying Unicast RPF:

• Unicast RPF must be applied at the interface downstream from the larger portion of the network, preferably at the edges of your network.
• The farther downstream you apply Unicast RPF, the finer the granularity you have in mitigating address spoofing and in identifying the sources of spoofed addresses. For example, applying Unicast RPF on an aggregation switch helps mitigate attacks from many downstream networks or clients and is simple to administer, but it does not help identify the source of the attack. Applying Unicast RPF at the network access server helps limit the scope of the attack and trace the source of the attack; however, deploying Unicast RPF across many sites does add to the administration cost of operating the network.
• The more entities that deploy Unicast RPF across Internet, intranet, and extranet resources, the better the chances of mitigating large-scale network disruptions throughout the Internet community, and the better the chances of tracing the source of an attack.
• Unicast RPF will not inspect IP packets encapsulated in tunnels, such as GRE, LT2P, or PPTP. Unicast RPF must be configured at a home gateway so that Unicast RPF processes network traffic only after the tunneling and encryption layers have been stripped off the packets.

Where to Use Unicast RPF

Unicast RPF can be used in any single-homed environment where there is essentially only one access point out of the network; that is, one upstream connection. Networks having one access point offer the best example of symmetric routing, which means that the interface where a packet enters the network is also the best return path to the source of the IP packet. Unicast RPF is best used at the network perimeter for Internet, intranet, or extranet environments, or in ISP environments for customer network terminations.

Enterprise Networks with a Single Connection to an ISP

In enterprise networks, one objective of using Unicast RPF for filtering traffic at the input interface (a process called ingress filtering) is for protection from malformed packets arriving from the Internet. Traditionally, local networks with one connection to the Internet use ACLs at the receiving interface to prevent spoofed packets from the Internet from entering their local network.

ACLs work well for many single-homed customers; however, there are trade-offs when ACLs are used as ingress filters, including two commonly referenced limitations:

• Packet per second (PPS) performance at very high packet rates

Note This restriction applies only to software packet forwarding. Hardware packet forwarding is the same on both ACL and uRPF.

• Maintenance of the ACL (whenever new addresses are added to the network)

Unicast RPF is one tool that addresses both of these limitations. With Unicast RPF, ingress filtering is done at CEF PPS rates. This processing speed makes a difference when the link is more than 1 Mbps. Additionally, since Unicast RPF uses the FIB, no ACL maintenance is necessary, and thus the administration overhead of traditional ACLs is reduced. The following figure and example demonstrate how Unicast RPF is configured for ingress filtering.

Figure 36-3 illustrates an enterprise network that has a single link to an upstream ISP. In this example, Unicast RPF is applied at interface Gigabit Ethernet 1/1 on the Enterprise switch for protection from malformed packets arriving from the Internet. Unicast RPF is also applied at interface
Gigabit Ethernet 2/1 on the ISP switch for protection from malformed packets arriving from the enterprise network.

Figure 36-3 Enterprise Network Using Unicast RPF for Ingress Filtering

Using the topography in Figure 36-3, a typical configuration (assuming that CEF is turned on) on the ISP switch appears as follows:

The gateway switch configuration of the enterprise network (assuming that CEF is turned on) appears as follows:

Unicast RPF works with a single default route. No additional routes or routing protocols exist. Network 192.168.10.0/22 is a connected network. Packets arriving from the Internet with a source address in the range 192.168.10.0/22 are dropped by Unicast RPF.

Routing Table Requirements

To work correctly, Unicast RPF needs proper information in the CEF tables. This requirement does not mean that the switch must have the entire Internet routing table. The amount of routing information needed in the CEF tables depends on where Unicast RPF is configured and what functions the switch performs in the network. For example, in an ISP environment, a switch that is a leased-line aggregation switch for customers needs only the information based on the static routes redistributed into the IGP or IBGP (depending on which technique is used in the network). Unicast RPF is configured on the customer interfaces, creating the requirement for minimal routing information. In another scenario, a single-homed ISP could place Unicast RPF on the gateway link to the Internet. The full Internet routing table is required. Requiring the full routing table helps protect the ISP from external DoS attacks that use addresses that are not in the Internet routing table.

Where Not to Use Unicast RPF

Do not use Unicast RPF on interfaces that are internal to the network. Internal interfaces are likely to have routing asymmetry (see Figure 36-4), meaning multiple routes to the source of a packet. Apply Unicast RPF only where there is natural or configured symmetry. Provided administrators carefully plan which interfaces they activate Unicast RPF on, routing asymmetry is not a serious problem.

For example, switches at the edge of the network of an ISP are more likely to have symmetrical reverse paths than switches that are in the core of the ISP network. Switches that are in the core of the ISP network have no guarantee that the best forwarding path out of the switch is the path selected for packets returning to the switch. We do not recommend that you apply Unicast RPF where there is a chance of asymmetric routing, unless you use ACLs to allow the switch to accept incoming packets. ACLs permit the use of Unicast RPF when packets will arrive by specific, less optimal asymmetric input paths. However, it is simplest to place Unicast RPF only at the edge of a network or, for an ISP, at the customer edge of the network.

Figure 36-4 illustrates how Unicast RPF can block legitimate traffic in an asymmetrical routing environment.

Figure 36-4 Unicast RPF Blocking Traffic in an Asymmetrical Routing Environment

Unicast RPF with BOOTP and DHCP

Unicast RPF will allow packets with 0.0.0.0 source and 255.255.255.255 destination to pass so that Bootstrap Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP) functions work properly.

Restrictions

Restrictions for applying Unicast RPF to multihomed clients include the following:

• Clients should not be multihomed to the same switch because action defeats the purpose of building a redundant service for the client.
• Customers must ensure that the packets flowing up the link (out to the Internet) match the route advertised out the link. Otherwise, Unicast RPF filters those packets as malformed packets.

Limitation

Unicast loose mode is not supported.

Related Features and Technologies

For more information about Unicast RPF-related features and technologies, review the following:

• Unicast RPF requires Cisco express forwarding (CEF) to function properly on the switch. For more information about CEF, refer to the Cisco IOS Switching Services Configuration Guide.
• Unicast RPF can be more effective at mitigating spoofing attacks when combined with a policy of ingress and egress filtering using Cisco IOS access control lists (ACLs).

– Ingress filtering applies filters to traffic received at a network interface from either internal or external networks. With ingress filtering, packets that arrive from other networks or the Internet and that have a source address that matches a local network, private, or broadcast address are dropped. In ISP environments, for example, ingress filtering can apply to traffic received at the switch from either the client (customer) or the Internet.

– Egress filtering applies filters to traffic exiting a network interface (the sending interface). By filtering packets on switches that connect your network to the Internet or to other networks, you can permit only packets with valid source IP addresses to leave your network.

For more information on network filtering, refer to RFC 2267 and to the Cisco IOS IP Configuration Guide.

Prerequisites to Configuring Unicast RPF

Prior to configuring Unicast RPF, configure ACLs:

• Configure standard or extended ACLs to mitigate transmission of invalid IP addresses (perform egress filtering). Permit only valid source addresses to leave your network and get onto the Internet. Prevent all other source addresses from leaving your network for the Internet.
• Configure standard or extended ACLs entries to drop (deny) packets that have invalid source IP addresses (perform ingress filtering). Invalid source IP addresses include the following types:

– Reserved addresses

– Loopback addresses

– Private addresses (RFC 1918, Address Allocation for Private Internets)

– Broadcast addresses (including multicast addresses)

– Source addresses that fall outside the range of valid addresses associated with the protected network

Configuring Unicast RPF

Unicast RPF is an input-side function that is enabled on an interface operates on IP packets received by the switch.

To configure Unicast RPF, perform the following task:

Verifying Unicast RPF

To verify that Unicast RPF is operational, use the show cef interface command. The following example shows that Unicast RPF is enabled at interface Gigabit Ethernet 3/1: