Cisco ASA 5500 Series Configuration Guide using the CLI, 8.2
Information About Routing
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Information About Routing

Table Of Contents

Information About Routing

Information About Routing

Switching

Path Determination

Supported RouteTypes

How Routing Behaves Within the Adaptive Security Appliance

Egress Interface Selection Process

Next Hop Selection Process

Supported Internet Protocols for Routing

Information About the Routing Table

Displaying the Routing Table

How the Routing Table is Populated

Backup Routes

How Forwarding Decisions are Made

Dynamic Routing and Failover

Information About IPv6 Support

Features that Support IPv6

IPv6-Enabled Commands

IPv6 Command Guidelines in Transparent Firewall Mode

Entering IPv6 Addresses in Commands


Information About Routing


This chapter describes underlying concepts of how routing behaves on the ASA, and the routing protocols that are supported. Subsequent chapters address each specific routing protocol in more detail.

This chapter includes the following sections:

Information About Routing

How Routing Behaves Within the Adaptive Security Appliance

Supported Internet Protocols for Routing

Information About the Routing Table

Information About IPv6 Support

Information About Routing

Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing involves two basic activities: determining optimal routing paths and transporting information groups (typically called packets) through an internetwork. In the context of the routing process, the latter of these is referred to as packet switching. Although packet switching is relatively straightforward, path determination can be very complex.

Switching

Switching algorithms is relatively simple; it is the same for most routing protocols. In most cases, a host determines that it must send a packet to another host. Having acquired a router's address by some means, the source host sends a packet addressed specifically to a router's physical (Media Access Control [MAC]-layer) address, this time with the protocol (network layer) address of the destination host.

As it examines the packet's destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet, it typically drops the packet. If the router knows how to forward the packet, however, it changes the destination physical address to that of the next hop and transmits the packet.

The next hop may be the ultimate destination host. If not, the next hop is usually another router, which executes the same switching decision process. As the packet moves through the internetwork, its physical address changes, but its protocol address remains constant.

Path Determination

Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used.

Routing algorithms fill routing tables with a variety of information. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing the "next hop" on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop.

Routing tables also can contain other information, such as data about the desirability of a path. Routers compare metrics to determine optimal routes, and these metrics differ depending on the design of the routing algorithm used.

Routers communicate with one another and maintain their routing tables through the transmission of a variety of messages. The routing update message is one such message that generally consists of all or a portion of a routing table. By analyzing routing updates from all other routers, a router can build a detailed picture of network topology. A link-state advertisement, another example of a message sent between routers, informs other routers of the state of the sender's links. Link information also can be used to build a complete picture of network topology to enable routers to determine optimal routes to network destinations.


Note Asymetric routing is not supported on the ASA.


Supported RouteTypes

There are several types of route types that a router can use, Listed below are the route types that the ASA uses.

Static Versus Dynamic

Single-Path Versus Multipath

Flat Versus Hierarchical

Link-State Versus Distance Vector

Static Versus Dynamic

Static routing algorithms are hardly algorithms at all, but are 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 static routing systems cannot react to network changes, they generally are considered unsuitable for today's large, constantly changing networks. Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly.

Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of last resort (a router to which all unroutable packets are sent), for example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.

Single-Path Versus Multipath

Some sophisticated routing protocols support multiple paths to the same destination. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines. The advantages of multipath algorithms are obvious: They can provide substantially better throughput and reliability. This is generally called load sharing.

Flat Versus Hierarchical

Some routing algorithms operate in a flat space, while others use routing hierarchies. In a flat routing system, the routers are peers of all others. In a hierarchical routing system, some routers form what amounts to a routing backbone. Packets from nonbackbone routers travel to the backbone routers, where they are sent through the backbone until they reach the general area of the destination. At this point, they travel from the last backbone router through one or more nonbackbone routers to the final destination.

Routing systems often designate logical groups of nodes, called domains, autonomous systems, or areas. In hierarchical systems, some routers in a domain can communicate with routers in other domains, while others can communicate only with routers within their domain. In very large networks, additional hierarchical levels may exist, with routers at the highest hierarchical level forming the routing backbone.

The primary advantage of hierarchical routing is that it mimics the organization of most companies and therefore supports their traffic patterns well. Most network communication occurs within small company groups (domains). Because intradomain routers need to know only about other routers within their domain, their routing algorithms can be simplified, and, depending on the routing algorithm being used, routing update traffic can be reduced accordingly.

Link-State Versus Distance Vector

Link-state algorithms (also known as shortest path first algorithms) flood routing information to all nodes in the internetwork. Each router, however, sends only the portion of the routing table that describes the state of its own links. In link-state algorithms, each router builds a picture of the entire network in its routing tables. Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table, but only to its neighbors. In essence, link-state algorithms send small updates everywhere, while distance vector algorithms send larger updates only to neighboring routers. Distance vector algorithms know only about their neighbors. Typically, this type of algorithmn is used in conjunction with OSPF routing protocols.

How Routing Behaves Within the Adaptive Security Appliance

The ASA uses both routing table and XLATE tables for routing decisions. To handle destination IP translated traffic, that is, untranslated traffic, the ASA searches for existing XLATE, or static translation to select the egress interface. The selection process is as follows:

Egress Interface Selection Process

1. If destination IP translating XLATE already exists, the egress interface for the packet is determined from the XLATE table, but not from the routing table.

2. If destination IP translating XLATE does not exist, but a matching static translation exists, then the egress interface is determined from the static route and an XLATE is created, and the routing table is not used.

3. If destination IP translating XLATE does not exist and no matching static translation exists, the packet is not destination IP translated. The ASA processes this packet by looking up the route to select egress interface, then source IP translation is performed (if necessary).

For regular dynamic outbound NAT, initial outgoing packets are routed using the route table and then creating the XLATE. Incoming return packets are forwarded using existing XLATE only. For static NAT, destination translated incoming packets are always forwarded using existing XLATE or static translation rules.

Next Hop Selection Process

After selecting egress interface using any method described above, an additional route lookup is performed to find out suitable next hop(s) that belong to previously selected egress interface. If there are no routes in routing table that explicitly belong to selected interface, the packet is dropped with level 6 error message 110001 "no route to host", even if there is another route for a given destination network that belongs to different egress interface. If the route that belongs to selected egress interface is found, the packet is forwarded to corresponding next hop.

Load sharing on the ASA is possible only for multiple next-hops available using single egress interface. Load sharing cannot share multiple egress interfaces.

If dynamic routing is in use on ASA and route table changes after XLATE creation, for example route flap, then destination translated traffic is still forwarded using old XLATE, not via route table, until XLATE times out. It may be either forwarded to wrong interface or dropped with message 110001 "no route to host" if old route was removed from the old interface and attached to another one by routing process.

The same problem may happen when there is no route flaps on the ASA itself, but some routing process is flapping around it, sending source translated packets that belong to the same flow through the ASA using different interfaces. Destination translated return packets may be forwarded back using the wrong egress interface.

This issue has a high probability in same security traffic configuration, where virtually any traffic may be either source-translated or destination-translated, depending on direction of initial packet in the flow. When this issue occurs after a route flap, it can be resolved manually by using the clear xlate command, or automatically resolved by an XLATE timeout. XLATE timeout may be decreased if necessary. To ensure that this rarely happens, make sure that there is no route flaps on ASA and around it. That is, ensure that destination translated packets that belong to the same flow are always forwarded the same way through the ASA.

Supported Internet Protocols for Routing

The ASA supports several internet protocols for routing. Each protocol is briefly desribed in this section.

Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced IGRP provides compatibility and seamless interoperation with IGRP routers. An automatic-redistribution mechanism allows IGRP routes to be imported into Enhanced IGRP, and vice versa, so it is possible to add Enhanced IGRP gradually into an existing IGRP network.

For more infomation on configuring EIGRP, see the chapter `Configuring EIGRP'.

Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF) is a routing protocol developed for Internet Protocol (IP) networks by the interior gateway protocol (IGP) working group of the Internet Engineering Task Force (IETF). OSPF uses a link-state algorithm in order to build and calculate the shortest path to all known destinations. Each router in an OSPF area contains an identical link-state database, which is a list of each of the router usable interfaces and reachable neighbors

For more infomation on configuring OSPF, see the chapter `Configuring OSPF'.

Routing Information Protocol

The Routing Information Protocol (RIP) is a distance-vector protocol that uses hop count as its metric. RIP is widely used for routing traffic in the global Internet and is an interior gateway protocol (IGP), which means that it performs routing within a single autonomous system.

For more infomation on configuring RIP, see the chapter `Configuring RIP'.

Information About the Routing Table

This section contains the following topics:

Displaying the Routing Table

How the Routing Table is Populated

How Forwarding Decisions are Made

Displaying the Routing Table

To view the entries in the routing table, enter the following command:

hostname# show route
 
   
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
 
   
Gateway of last resort is 10.86.194.1 to network 0.0.0.0
 
   
S    10.1.1.0 255.255.255.0 [3/0] via 10.86.194.1, outside
C    10.86.194.0 255.255.254.0 is directly connected, outside
S*   0.0.0.0 0.0.0.0 [1/0] via 10.86.194.1, outside
 
   

On the ASA 5505 ASA, the following route is also shown. It is the internal loopback interface, which is used by the VPN hardware client feature for individual user authentication.

C 127.1.0.0 255.255.0.0 is directly connected, _internal_loopback 
 
   

How the Routing Table is Populated

The ASA routing table can be populated by statically defined routes, directly connected routes, and routes discovered by the RIP, EIGRP, and OSPF routing protocols. Because the ASA can run multiple routing protocols in addition to having static and connected routed in the routing table, it is possible that the same route is discovered or entered in more than one manner. When two routes to the same destination are put into the routing table, the one that remains in the routing table is determined as follows:

If the two routes have different network prefix lengths (network masks), then both routes are considered unique and are entered in to the routing table. The packet forwarding logic then determines which of the two to use.

For example, if the RIP and OSPF processes discovered the following routes:

RIP: 192.168.32.0/24

OSPF: 192.168.32.0/19

Even though OSPF routes have the better administrative distance, both routes are installed in the routing table because each of these routes has a different prefix length (subnet mask). They are considered different destinations and the packet forwarding logic determine which route to use.

If the ASA learns about multiple paths to the same destination from a single routing protocol, such as RIP, the route with the better metric (as determined by the routing protocol) is entered into the routing table.

Metrics are values associated with specific routes, ranking them from most preferred to least preferred. The parameters used to determine the metrics differ for different routing protocols. The path with the lowest metric is selected as the optimal path and installed in the routing table. If there are multiple paths to the same destination with equal metrics, load balancing is done on these equal cost paths.

If the ASA learns about a destination from more than one routing protocol, the administrative distances of the routes are compared and the routes with lower administrative distance is entered into the routing table.

You can change the administrative distances for routes discovered by or redistributed into a routing protocol. If two routes from two different routing protocols have the same administrative distance, then the route with the lower default administrative distance is entered into the routing table. In the case of EIGRP and OSPF routes, if the EIGRP route and the OSPF route have the same administrative distance, then the EIGRP route is chosen by default.

Administrative distance is a route parameter that the ASA uses to select the best path when there are two or more different routes to the same destination from two different routing protocols. Because the routing protocols have metrics based on algorithms that are different from the other protocols, it is not always possible to determine the "best path" for two routes to the same destination that were generated by different routing protocols.

Each routing protocol is prioritized using an administrative distance value. Table 18-1 shows the default administrative distance values for the routing protocols supported by the ASA.

Table 18-1 Default Administrative Distance for Supported Routing Protocols

Route Source
Default Administrative Distance

Connected interface

0

Static route

1

EIGRP Summary Route

5

Internal EIGRP

90

OSPF

110

RIP

120

EIGRP external route

170

Unknown

255


The smaller the administrative distance value, the more preference is given to the protocol. For example, if the ASA receives a route to a certain network from both an OSPF routing process (default administrative distance - 110) and a RIP routing process (default administrative distance - 120), the ASA chooses the OSPF route because OSPF has a higher preference. This means the router adds the OSPF version of the route to the routing table.

In the above example, if the source of the OSPF-derived route was lost (for example, due to a power shutdown), the ASA would then use the RIP-derived route until the OSPF-derived route reappears.

The administrative distance is a local setting. For example, if you use the distance-ospf command to change the administrative distance of routes obtained through OSPF, that change would only affect the routing table for the ASA the command was entered on. The administrative distance is not advertised in routing updates.

Administrative distance does not affect the routing process. The OSPF and RIP routing processes only advertise the routes that have been discovered by the routing process or redistributed into the routing process. For example, the RIP routing process advertises RIP routes, even if routes discovered by the OSPF routing process are used in the ASA routing table.

Backup Routes

A backup route is registered when the initial attempt to install the route in the routing table fails because another route was installed instead. If the route that was installed in the routing table fails, the routing table maintenance process calls each routing protocol process that has registered a backup route and requests them to reinstall the route in the routing table. If there are multiple protocols with registered backup routes for the failed route, the preferred route is chosen based on administrative distance.

Because of this process, you can create "floating" static routes that are installed in the routing table when the route discovered by a dynamic routing protocol fails. A floating static route is simply a static route configured with a greater administrative distance than the dynamic routing protocols running on the ASA. When the corresponding route discover by a dynamic routing process fails, the static route is installed in the routing table.

How Forwarding Decisions are Made

Forwarding decisions are made as follows:

If the destination does not match an entry in the routing table, the packet is forwarded through the interface specified for the default route. If a default route has not been configured, the packet is discarded.

If the destination matches a single entry in the routing table, the packet is forwarded through the interface associated with that route.

If the destination matches more than one entry in the routing table, and the entries all have the same network prefix length, the packets for that destination are distributed among the interfaces associated with that route.

If the destination matches more than one entry in the routing table, and the entries have different network prefix lengths, then the packet is forwarded out of the interface associated with the route that has the longer network prefix length.

For example, a packet destined for 192.168.32.1 arrives on an interface of a ASA with the following routes in the routing table:

hostname# show route
         ....
         R   192.168.32.0/24 [120/4] via 10.1.1.2
         O   192.168.32.0/19 [110/229840] via 10.1.1.3
         ....
 
   

In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.2, because 192.168.32.1 falls within the 192.168.32.0/24 network. It also falls within the other route in the routing table, but the 192.168.32.0/24 has the longest prefix within the routing table (24 bits verses 19 bits). Longer prefixes are always preferred over shorter ones when forwarding a packet.

Dynamic Routing and Failover

Because static routing systems cannot react to network changes, they generally are considered unsuitable for today's large, constantly changing networks. Most of the dominant routing algorithms today are dynamic routing algorithms, which adjust to changing network circumstances by analyzing incoming routing update messages. If the message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly.

Dynamic routing algorithms can be supplemented with static routes where appropriate. A router of last resort (a router to which all unroutable packets are sent), for example, can be designated to act as a repository for all unroutable packets, ensuring that all messages are at least handled in some way.

Dynamic routes are not replicated to the standby unit or failover group in a failover configuration. Therefore, immediately after a failover occurs, some packets received by the ASA may be dropped because of a lack of routing information or routed to a default static route while the routing table is repopulated by the configured dynamic routing protocols.

For more information on static routs and how to configure them , see "Configuring Static and Default Routes".

Information About IPv6 Support

Many, but not all, features on the ASA supports IPv6 traffic. This section describes the commands and features that support IPv6, and includes the following topics:

Features that Support IPv6

IPv6-Enabled Commands

Entering IPv6 Addresses in Commands

Features that Support IPv6

The following features support IPv6.


Note For features that use the Modular Policy Framework, be sure to use the match any command to match IPv6 traffic; other match commands do not support IPv6.


The following application inspections support IPv6 traffic:

FTP

HTTP

ICMP

SIP

SMTP

IPSec-pass-thru

IPS

NetFlow Secure Event Logging filtering

Connection limits, timeouts, and TCP randomization

TCP Normalization

TCP state bypass

Access group, using an IPv6 access list

Static Routes

VPN (all types)

IPv6-Enabled Commands

The following ASA commands can accept and display IPv6 addresses:

capture

configure

copy

http

name

object-group

ping

show conn

show local-host

show tcpstat

ssh

telnet

tftp-server

who

write

The following commands were modified to work for IPv6:

debug

fragment

ip verify

mtu

icmp (entered as ipv6 icmp)

IPv6 Command Guidelines in Transparent Firewall Mode

The ipv6 address and ipv6 enable commands are available in global configuration mode instead of interface configuration mode. The ipv6 address command does not support the eui keyword. (The ipv6 address link-local command is still available in interface configuration mode.

The following IPv6 commands are not supported in transparent firewall mode, because they require router capabilities:

ipv6 address autoconfig

ipv6 nd prefix

ipv6 nd ra-interval

ipv6 nd ra-lifetime

ipv6 nd suppress-ra

The following VPN command is not supported, because transparent mode does not support VPN:

ipv6 local pool

Entering IPv6 Addresses in Commands

When entering IPv6 addresses in commands that support them, simply enter the IPv6 address using standard IPv6 notation, for example: ping fe80::2e0:b6ff:fe01:3b7a. The ASA correctly recognizes and processes the IPv6 address. However, you must enclose the IPv6 address in square brackets ([ ]) in the following situations:

You need to specify a port number with the address, for example: [fe80::2e0:b6ff:fe01:3b7a]:8080.

The command uses a colon as a separator, such as the write net and config net commands, for example: configure net [fe80::2e0:b6ff:fe01:3b7a]:/tftp/config/pixconfig.