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> Chapter 20 - Networking > Dynamic Routing Overview > Dynamic routing terminology

Dynamic routing terminology

Dynamic routing is a complex subject. There are many routers on different networks and all can be configured differently. It become even more complicated when you add to this each routing protocol having slightly different names for similar features, and many configurable features for each protocol.

To better understand dynamic routing, here are some explanations of common dynamic routing terms.

For more details on a term as it applies to a dynamic routing protocol, see one of Border Gateway Protocol (BGP), Routing Information Protocol (RIP), or Open Shortest Path First (OSPF).

Aggregated routes and addresses

Just as an aggregate interface combines multiple interfaces into one virtual interface, an aggregate route combines multiple routes into one. This reduces the amount of space those routes require in the routing tables of the routers along that route. The trade-off is a small amount of processing to aggregate and de-aggregate the routes at either end.

The benefit of this method is that you can combine many addresses into one, potentially reducing the routing table size immensely. The weakness of this method is if there are holes in the address range you are aggregating you need to decide if its better to break it into multiple ranges, or accept the possibility of failed routes to the missing addresses.

For information on aggregated routes in BGP, see Border Gateway Protocol (BGP), and Border Gateway Protocol (BGP).

To manually aggregate the range of IP addresses from to
  1. Convert the addresses to binary = 11000000 10101000 00000001 01100100 = 11000000 10101000 00000001 01100101 = 11000000 10101000 00000001 01100110 = 11000000 10101000 00000001 01100111


  1. Determine the maximum number of matching bits common to the addresses.

There are 30-bits in common, with only the last 2-bits being different.

  1. Record the common part of the address.

11000000 10101000 00000001 0110010X =


  1. For the netmask, assume all the bits in the netmask are 1 except those that are different which are 0.

11111111 11111111 11111111 11111100 =


  1. Combine the common address bits and the netmask.

Alternately the IP mask may be written as a single number:


  1. As required, set variables and attributes to declare the routes have been aggregated, and what router did the aggregating.

Autonomous system (AS)

An Autonomous System (AS) is one or more connected networks that use the same routing protocol, and appear to be a single unit to any externally connected networks. For example an ISP may have a number of customer networks connected to it, but to any networks connected externally to the ISP it appears as one system or AS. An AS may also be referred to as a routing domain.

It should be noted that while OSPF routing takes place within one AS, the only part of OSPF that deals with the AS is the AS border router (ASBR).

There are multiple types of AS defined by how they are connected to other ASes. A multihomed AS is connected to at least two other ASes and has the benefit of redundancy — if one of those ASes goes down, your AS can still reach the Internet through its other connection. A stub AS only has one connection, and can be useful in specific configurations where limited access is desirable.

Each AS has a number assigned to it, known as an ASN. In an internal network, you can assign any ASN you like (a private AS number), but for networks connected to the Internet (public AS) you need to have an officially registered ASN from Internet Assigned Numbers Authority (IANA). ASNs from 1 to 64,511 are designated for public use.

note icon NAs of January 2010, AS numbers are 4 bytes long instead of the former 2 bytes. RFC 4893 introduced 32-bit ASNs, which FortiGate units support for BGP and OSPF.
Do you need your own AS?

The main factors in deciding if you need your own AS or if you should be part of someone else’s are:

  • exchanging external routing information
  • many prefixes should exist in one AS as long as they use the same routing policy
  • when you use a different routing protocol than your border gateway peers (for example your ISP uses BGP, and you use OSPF)
  • connected to multiple other AS (multi-homed)

You should not create an AS for each prefix on your network. Neither should you be forced into an AS just so someone else can make AS-based policy decisions on your traffic.

There can be only one AS for any prefix on the Internet. This is to prevent routing issues.

What AS number to use?

In addition to overseeing IP address allocation and Domain Name Systems (DNS), the Internet Assigned Numbers Authority (IANA) assigns public AS numbers. The public AS numbers are from 1 to 64,511. The ASNs 0, 54272–64511, and 65535 are reserved by the IANA. These ASNs should not be used.

ASNs are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs) who then assign ASNs to companies within that RIRs geographic area. Usually these companies are ISPs, and to receive an ASN you must complete the application process of the local RIR and be approved before being assigned an ASN. The RIRs names and regions are:

AFRINIC Serves the African continent
APNIC Asia-Pacific including China, India, and Japan
ARIN American registry including Canada and United States
LACNIC Latin America, including Mexico, Caribbean, Central and South America
RIPE NCC Europe, the Middle East, former USSR, and parts of Central Asia

AS numbers from 64512 to 65534 are reserved for private use. Private AS numbers can be used for any internal networks with no outside connections to the Internet such as test networks, classroom labs, or other internal-only networks that do not access the outside world. You can also configure border routers to filter out any private ASNs before routing traffic to the outside world. If you must use private ASNs with public networks, this is the only way to configure them. However, it is risky because many other private networks could be using the same ASNs and conflicts will happen. It would be very much like your local network being made public — the resulting problems would be widespread.

In 1996, when RFC 1930 was written only 5,100 ASes had been allocated and a little under 600 ASes were actively routed in the global Internet. Since that time many more public ASNs have been assigned, leaving only a small number. For this reason 32-bit ASNs (four-octet ASNs) were defined to provide more public ASNs. RFC 4893 defines 32-bit ASNs, and FortiGate units support these larger ASNs.

Area border router (ABR)

Routers within an AS advertise updates internally and only to each other. However, routers on the edge of the AS must communicate both with routers inside their AS and with routers external to their AS, often running a different routing protocol. These routers are called Area Border Routers (ABRs) or edge routers. Often ABRs run multiple routing protocols to be able to redistribute traffic between different ASes that are running different protocols, such as the edge between an ISP’s IS-IS routing network and a large company’s OSPF network.

OSPF defines ABRs differently from other routers. In OSPF, an ABR is an OSPF router that connects another AS to the backbone AS, and is a member of all the areas it connects to. An OSPF ABR maintains a LSA database for each area that it is connected to. The concept of the edge router is present, but its the edge of the backbone instead of the edge of the OSPF supported ASes.

Neighbor routers

Routing involves routers communicating with each other. To do this, routers need to know information about each other. These routers are called neighbor routers, and are configured in each routing protocol. Each neighbor has custom settings since some routers may have functionality others routers lack. Neighbor routers are sometimes called peers.

Generally neighbor routers must be configured, and discovered by the rest of the network before they can be integrated to the routing calculations. This is a combination of the network administrator configuring the new router with its neighbor router addresses, and the routing network discovering the new router, such as the hello packets in OSPF. That discovery initiates communication between the new router and the rest of the network.

Route maps

Route maps are a way for the FortiGate unit to evaluate optimum routes for forwarding packets or suppressing the routing of packets to particular destinations. Compared to access lists, route maps support enhanced packet-matching criteria. In addition, route maps can be configured to permit or deny the addition of routes to the FortiGate unit routing table and make changes to routing information dynamically as defined through route-map rules.

Route maps can be used for limiting both received route updates, and sent route updates. This can include the redistribution of routes learned from other types of routing. For example if you don’t want to advertise local static routes to external networks, you could use a route map to accomplish this.

The FortiGate unit compares the rules in a route map to the attributes of a route. The rules are examined in ascending order until one or more of the rules in the route map are found to match one or more of the route attributes.

As an administrator, route maps allow you to group a set of addresses together and assign them a meaningful name. Then during your configuration, you can use these route-maps to speed up configuration. The meaningful names ensure fewer mistakes during configuration as well.

The default rule in the route map (which the FortiGate unit applies last) denies all routes. For a route map to take effect, it must be called by a FortiGate unit routing process.

The syntax for route maps are:

config router route-map

edit <route_map_name>

set comments

config rule

edit <route_map_rule_id>

set action

set match-*

set set-*



The match-* commands allow you to match various parts of a route. The set-* commands allow you to set routing information once a route is matched.

For an example of how route maps can be used to create receiving or sending “groups” in routing, see Border Gateway Protocol (BGP).

Access lists

Use this command to add, edit, or delete access lists. Access lists are filters used by FortiGate unit routing processes. For an access list to take effect, it must be called by a FortiGate unit routing process (for example, a process that supports RIP or OSPF). Use access-list6 for IPv6 routing.

Access lists can be used to filter which updates are passed between routers, or which routes are redistributed to different networks and routing protocols. You can create lists of rules that will match all routes for a specific router or group of routers.

Each rule in an access list consists of a prefix (IP address and netmask), the action to take for this prefix (permit or deny), and whether to match the prefix exactly or to match the prefix and any more specific prefix.

note icon If you are setting a prefix of, use the format The default route, can not be exactly matched with an access-list. A prefix-list must be used for this purpose.

The FortiGate unit attempts to match a packet against the rules in an access list starting at the top of the list. If it finds a match for the prefix, it takes the action specified for that prefix. If no match is found the default action is deny.

The syntax for access lists is:

config router access-list, access-list6

edit <access_list_name>

set comments

config rule

edit <access_list_id>

set action

set exact-match

set prefix

set prefix6

set wildcard

For an example of how access lists can be used to create receiving or sending “groups” in routing, see Border Gateway Protocol (BGP).

Bi-directional forwarding detection (BFD)

Bi-directional Forwarding Detection (BFD) is a protocol used to quickly locate hardware failures in the network. Routers running BFD send packets to each other at a negotiated rate. If packets from a BFD-protected router fail to arrive, then that router is declared down. BFD communicates this information to the routing protocol and the routing information is updated.

BFD neighbors establish if BFD is enabled in OSPF or BFP routers that establish as neighbors.

The CLI commands associated with BFD include:

config router bgp

config neighbor

set bfd


config router ospf

set bfd


Per-VDOM configuration:

config system settings

set bfd

set bfd-desired-min-tx

set bfd-required-min-rx

set bfd-detect-mult

set bfd-dont-enforce-src-port


Per-interface (override) configuration:

config system interface

edit <interface_name>

set bfd enable

set bfd-desired-min-tx

set bfd-detect-mult

set bfd-required-min-rx


For more information about BFD in BGP, see Border Gateway Protocol (BGP).