- Static Routing
- Static routing is the term used to refer to a manual method that is used to set up routing between networks. The network administrator configures static routes in a router by entering routes directly into the routing table of a router. Static routing has the advantage of being predictable and simple to set up. It is easy to manage in small networks but does not scale well. Compare this with dynamic routing.
- Static routes are simple and quick to configure.
- Static routing is supported on all routing devices and all routers.
- Static routes are easy to predict and understand in small networks.
- Static routes require extensive planning and have high management overhead. The more routers exist in a network, the more routes that need to be configured. If you have ‘N’ number of routers and a route between each router is needed, then you must configure N x N routes, so, for a network of nine routers, you need a minimum of 81 routes (9 x 9 = 81).
- Static routes do not dynamically adapt to network topology changes or equipment failures.
- Static routing does not scale well in large networks.
STATIC ROUTE CONFIGURATION (Cisco)
- Default Route
- Static Null Route
- Preferred Routes
- Backup Routes
- Static Load Balancing
A default route is often called the ‘route of last resort’. It is the last route tried by a router when all other routes fail because it has the fewest number of network bits matching and is therefore less specific. A default route is configured on a Cisco router with the following command:
CISCO ‘default ip route’ command:
ip route 0.0.0.0 0.0.0.0 <next hop or exit interface>
Static Null Route
A null route directs traffic to a non-existent interface called the null interface. The “Null 0″ interface is often referred to as a ‘bit bucket’. Network packets directed to the “Null 0″ interface are discarded as soon as they are received. A null route is useful for removing packets that cannot make it out of the network or to their destination, and/or to decrease congestion created when packets with no currently reachable destination float around the network, or the destination is under a denial of service attack. During a denial of service attack, a null route can temporarily be placed on the next to last hop closest to destination which will cause that device to drop all traffic generated by the attack.
CISCO ‘null route’ command:
! Null route (less specific) ip route 18.104.22.168 255.255.255.0 null0 ! Preferred routes (more specific) ip route 22.214.171.124 255.255.255.128 e1 ip route 126.96.36.199 255.255.255.128 e2
The route which has the greatest number of network bits in the network address matching the destination address stored in the packet is the preferred route to a destination. Routers are programmed to select a preferred route based on how closely the bits in the route and packet match. This is also referred to as ‘longest prefix match’.
CISCO ‘preferred route’ commands:
! Standard route ip route 188.8.131.52 255.255.255.0 e0 ! Preferred routes ip route 184.108.40.206 255.255.255.128 e1 ip route 220.127.116.11 255.255.255.128 e2
In cases where redundancy is needed and where more than one network connection between two network locations exists, a second route can be configured to be used when the primary network connection has failed. A backup route causes traffic to flow over a different physical path. When the first route fails, the second route over one or more less preferred path(s) will be used. By using a secondary routes the network administrator increases the redundancy and reliability of the network. Backup routes help compensate for network failures.
CISCO backup route configurations:
Scenario 1: Using more Specific Route(s)
SPECIFIC ROUTES (used unless down) ip route 18.104.22.168 255.255.255.128 e0 ip route 22.214.171.124 255.255.255.128 e1 BACKUP ROUTES (used when one of the specifics are down) ip route 126.96.36.199 255.255.255.0 e0 ip route 188.8.131.52 255.255.255.0 e1
Scenario 2: Using Cisco’s Weight mechanism
ip route 184.108.40.206 255.255.255.0 e0 ip route 220.127.116.11 255.255.255.0 e1 250
Static Load Balancing
Using static load balancing, you can configure traffic load balancing on a per-packet basis without using a dynamic routing protocol. Most routers will perform load balancing automatically if several equal cost paths to a destination exist on multiple interfaces. To configure this using static routing, you need only create multiple static routes for more than one interface. This creates more than one equal cost path which will balance the load.
Cisco routers perform load balancing on a per-destination, per-source-destination and per-packet basis. Load can be balanced across up to six equal cost paths.
CISCO router commands:
ip route 18.104.22.168 255.255.255.0 e0 ip route 22.214.171.124 255.255.255.0 e1 -or- ip route 126.96.36.199 255.255.255.0 188.8.131.52 ip route 184.108.40.206 255.255.255.0 220.127.116.11 ip route 18.104.22.168 255.255.255.0 22.214.171.124
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