Multipath routing and determining the best route

Multipath routing and determining the best route

Multipath routing occurs when more than one entry to the same destination is present in the routing table. When multipath routing happens, the FortiGate unit may have several possible destinations for an incoming packet, forcing the FortiGate unit to decide which next-hop is the best one.

It should be noted that some IP addresses will be rejected by routing protocols. These are called Martian addresses. They are typically IP addresses that are invalid and not routable because they have been assigned an address by a misconfigured system, or are spoofed addresses.

Two methods to manually resolve multiple routes to the same destination are to lower the administrative distance of one route or to set the priority of both routes. For the FortiGate unit to select a primary (preferred) route, manually lower the administrative distance associated with one of the possible routes. Setting the priority on the routes is a FortiGate unit feature and may not be supported by non-Fortinet routers.

Administrative distance is based on the expected reliability of a given route. It is determined through a combination of the number of hops from the source and the protocol used. A hop is when traffic moves from one router to the next. More hops from the source means more possible points of failure. The administrative distance can be from 1 to 255, with lower numbers being preferred. A distance of 255 is seen as infinite and will not be installed in the routing table.

Here is an example to illustrate how administration distance works — if there are two possible routes traffic can take between two destinations with administration distances of 5 (always up) and 31 (sometimes not available), the traffic will use the route with an administrative distance of 5. If for some reasons the preferred route (admin distance of 5) is not available, the other route will be used as a backup.

Different routing protocols have different default administrative distances. These different administrative distances are based on a number of factors of each protocol such as reliability, speed, and so on. The default administrative distances for any of these routing protocols are configurable.

 

Default administrative distances for routing protocols and connections

 

Routing protocol Default administrative distance
 

Direct physical connection

 

1

 

Static

 

10

 

EBGP

 

20

 

OSPF

 

110

 

IS-IS

 

115

 

RIP

 

120

 

IBGP

 

200

Another method to determine the best route is to manually change the priority of both routes in question. If the next-hop administrative distances of two routes on the FortiGate unit are equal, it may not be clear which route the packet will take. Manually configuring the priority for each of those routes will make it clear which next-hop will be used in the case of a tie. The priority for a route be set in the CLI, or when editing a specific static route, as described in the next section. Lower priority routes are preferred. Priority is a Fortinet value that may or may not be present in other brands of routers.

All entries in the routing table are associated with an administrative distance. If the routing table contains several entries that point to the same destination (the entries may have different gateways or interface associations), the FortiGate unit compares the administrative distances of those entries first, selects the entries having the lowest distances, and installs them as routes in the FortiGate unit forwarding table. As a result, the FortiGate unit forwarding table contains only those routes having the lowest distances to every possible destination. While only static routing uses administrative distance as its routing metric, other routing protocols such as RIP can use metrics that are similar to administrative distance.

 


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