Control plane
In
Depending on the specific router implementation, there may be a separate
In computing, the control plane is the part of the software that configures and shuts down the data plane.[2] By contrast, the data plane is the part of the software that processes the data requests.[3] The data plane is also sometimes referred to as the forwarding plane.
The distinction has proven useful in the networking field where it originated, as it separates the concerns: the data plane is optimized for speed of processing, and for simplicity and regularity. The control plane is optimized for customizability, handling policies, handling exceptional situations, and in general facilitating and simplifying the data plane processing.[4][5]
The conceptual separation of the data plane from the control plane has been done for years.[6] An early example is Unix, where the basic file operations are open, close for the control plane and read write for the data plane.[7]
Building the unicast routing table
A major function of the control plane is deciding which routes go into the main routing table. "Main" refers to the table that holds the unicast routes that are active. Multicast routing may require an additional routing table for multicast routes. Several routing protocols e.g. IS-IS, OSPF and BGP maintain internal databases of candidate routes which are promoted when a route fails or when a routing policy is changed.
Several different information sources may provide information about a route to a given destination, but the router must select the "best" route to install into the routing table. In some cases, there may be multiple routes of equal "quality", and the router may install all of them and load-share across them.
Sources of routing information
There are three general sources of routing information:
- Information on the status of directly connected hardware and software-defined interfaces
- Manually configured static routes
- Information from (dynamic) routing protocols
Local interface information
Routers forward traffic that enters on an input interface and leaves on an output interface, subject to filtering and other local rules. While routers usually forward from one physical (e.g.,
When an interface has an address configured in a
There also may be software-only interfaces on the router, which it treats as if they were locally connected. For example, most implementations have a "null" software-defined interface. Packets having this interface as a next hop will be discarded, which can be a very efficient way to filter traffic. Routers usually can route traffic faster than they can examine it and compare it to filters, so, if the criterion for discarding is the packet's destination address, "blackholing" the traffic will be more efficient than explicit filters.
Other software defined interfaces that are treated as directly connected, as long as they are active, are interfaces associated with
Static routes
Router configuration rules may contain static routes. A static route minimally has a destination address, a prefix length or subnet mask, and a definition where to send packets for the route. That definition can refer to a local interface on the router, or a next-hop address that could be on the far end of a subnet to which the router is connected. The next-hop address could also be on a subnet that is directly connected, and, before the router can determine if the static route is usable, it must do a recursive lookup of the next hop address in the local routing table. If the next-hop address is reachable, the static route is usable, but if the next-hop is unreachable, the route is ignored.
Static routes also may have preference factors used to select the best static route to the same destination. One application is called a floating static route, where the static route is less preferred than a route from any routing protocol. The static route, which might use a dialup link or other slow medium, activates only when the dynamic routing protocol(s) cannot provide a route to the destination.
Static routes that are more preferred than any dynamic route also can be very useful, especially when using traffic engineering principles to make certain traffic go over a specific path with an engineered quality of service.
Dynamic routing protocols
See
Installing unicast routes
Different implementations have different sets of preferences for routing information, and these are not standardized among IP routers. It is fair to say that subnets on directly connected active interfaces are always preferred. Beyond that, however, there will be differences.
Implementers generally have a numerical preference, which Cisco calls an "administrative distance", for route selection. The lower the preference, the more desirable the route. Cisco's IOS[8] implementation makes exterior BGP the most preferred source of dynamic routing information, while Nortel RS[9] makes intra-area OSPF most preferred.
The general order of selecting routes to install is:
- If the route is not in the routing table, install it.
- If the route is "more specific" than an existing route, install it in addition to the existing routes. "More specific" means that it has a longer prefix. A /28 route, with a subnet mask of 255.255.255.240, is more specific than a /24 route, with a subnet mask of 255.255.255.0.
- If the route is of equal specificity to a route already in the routing table, but comes from a more preferred source of routing information, replace the route in the table.
- If the route is of equal specificity to a route in the routing table, yet comes from a source of the same preference,
- Discard it if the route has a higher metric than the existing route
- Replace the existing route if the new route has a lower metric
- If the routes are of equal metric and the router supports load-sharing, add the new route and designate it as part of a load-sharing group. Typically, implementations will support a maximum number of routes that load-share to the same destination. If that maximum is already in the table, the new route is usually dropped.
Routing table vs. forwarding information base
See
Multicast routing tables
Multicast routing builds on unicast routing. Each multicast group to which the local router can route has a multicast routing table entry with a next hop for the group, rather than for a specific destination as in unicast routing.
There can be multicast static routes as well as learning dynamic multicast routes from a protocol such as Protocol Independent Multicast (PIM).
See also
References
- ^ Forwarding and Control Element Separation (ForCES) Framework, RFC 3746, Network Working Group, April 2004
- ISSN 2405-9595.
- ^ Conran, Matt (2019-02-25). "Named data networking: Stateful forwarding plane for datagram delivery". Network World. Retrieved 2019-10-14.
- S2CID 4269723.
- S2CID 2138863.
- ISSN 2405-9595.
- Bibcode:1986duos.book.....B.
- ^ Configuring IP Routing Protocol-Independent Features, Cisco Systems,July 2006
- ^ Nortel Ethernet Routing Switch 8600 Configuring IP Routing Operations, Nortel Networks, January 2007