Control plane
Encyclopedia
In routing
, the control plane is the part of the router architecture that is concerned with drawing the network map, or the information in a (possibly augmented) routing table
that defines what to do with incoming packets. Control plane functions, such as participating in routing protocols, run in the architectural control element. In most cases, the routing table contains a list of destination addresses and the outgoing interface(s) associated with them. Control plane logic also can define certain packets to be discarded, as well as preferential treatment of certain packets for which a high quality of service is defined by such mechanisms as differentiated services
.
Depending on the specific router implementation, there may be a separate forwarding information base
that is populated (i.e., loaded) by the Control Plane, but used by the forwarding plane
to look up packets, at very high speed, and decide how to handle them.
. "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. OSPF
and BGP
maintain internal data bases 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.
, serial
) to another physical interface, it is also possible to define multiple logical interfaces on a physical interface. A physical Ethernet
interface, for example, can have logical interfaces in several virtual LAN
s defined by IEEE 802.1Q VLAN headers.
When an interface has an address configured in a subnet, such as 192.0.2.1 in the 192.0.2.0/24 (i.e., subnet mask 255.255.255.0) subnet, and that interface is considered "up" by the router, the router thus has a directly connected route to 192.0.2.0/24. If a routing protocol offered another router's route to that same subnet, the routing table installation software will normally ignore the dynamic route and prefer the directly connected route.
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 tunneling protocols such as generic routing encapsulation
(GRE) or Multi-Protocol Label Switching (MPLS).
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.
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 implementation makes exterior BGP the most preferred source of dynamic routing information, while Nortel RS makes intra-area OSPF most preferred.
The general order of selecting routes to install is:
for more detail, but each implementation has its own means of updating the forwarding information base
with new routes installed in the routing table. If the FIB is in one-to-one correspondence with the RIB, the new route is installed in the FIB after it is in the RIB. If the FIB is smaller than the RIB, and the FIB uses a hash table or other data structure that does not easily update, the existing FIB might be invalidated and replaced with a new one computed from the updated RIB.
There can be multicast static routes as well as learning dynamic multicast routes from a protocol such as Protocol Independent Multicast
(PIM).
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
, the control plane is the part of the router architecture that is concerned with drawing the network map, or the information in a (possibly augmented) routing table
Routing table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...
that defines what to do with incoming packets. Control plane functions, such as participating in routing protocols, run in the architectural control element. In most cases, the routing table contains a list of destination addresses and the outgoing interface(s) associated with them. Control plane logic also can define certain packets to be discarded, as well as preferential treatment of certain packets for which a high quality of service is defined by such mechanisms as differentiated services
Differentiated services
Differentiated Services or DiffServ is a computer networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying and managing network traffic and providing Quality of Service on modern IP networks...
.
Depending on the specific router implementation, there may be a separate forwarding information base
Forwarding Information Base
A forwarding information base , also known as a forwarding table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should send a packet to be transmitted by the router.In contrast to routing information bases , also...
that is populated (i.e., loaded) by the Control Plane, but used by the forwarding plane
Forwarding plane
In routing, the forwarding plane, sometimes called the data plane, defines the part of the router architecture that decides what to do with packets arriving on an inbound interface...
to look up packets, at very high speed, and decide how to handle them.
Building the unicast routing table
A major function of the control plane is deciding which routes go into the main routing tableRouting table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...
. "Main" refers to the table that holds the unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
routes that are active. Multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
routing may require an additional routing table for multicast routes. Several routing protocols e.g. OSPF
Open Shortest Path First
Open Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
and BGP
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
maintain internal data bases 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., EthernetEthernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, serial
Serial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
) to another physical interface, it is also possible to define multiple logical interfaces on a physical interface. A physical Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
interface, for example, can have logical interfaces in several virtual LAN
Virtual LAN
A virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
s defined by IEEE 802.1Q VLAN headers.
When an interface has an address configured in a subnet, such as 192.0.2.1 in the 192.0.2.0/24 (i.e., subnet mask 255.255.255.0) subnet, and that interface is considered "up" by the router, the router thus has a directly connected route to 192.0.2.0/24. If a routing protocol offered another router's route to that same subnet, the routing table installation software will normally ignore the dynamic route and prefer the directly connected route.
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 tunneling protocols such as generic routing encapsulation
Generic Routing Encapsulation
Generic Routing Encapsulation is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol internetwork.-Overview:...
(GRE) or Multi-Protocol Label Switching (MPLS).
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 routing protocols. The routing table manager, according to implementation and configuration rules, may select a particular route or routes from those advertised by various routing protocols.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 implementation makes exterior BGP the most preferred source of dynamic routing information, while Nortel RS 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, 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.
Relationship between the routing table and forwarding information base
See forwarding planeForwarding plane
In routing, the forwarding plane, sometimes called the data plane, defines the part of the router architecture that decides what to do with packets arriving on an inbound interface...
for more detail, but each implementation has its own means of updating the forwarding information base
Forwarding Information Base
A forwarding information base , also known as a forwarding table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should send a packet to be transmitted by the router.In contrast to routing information bases , also...
with new routes installed in the routing table. If the FIB is in one-to-one correspondence with the RIB, the new route is installed in the FIB after it is in the RIB. If the FIB is smaller than the RIB, and the FIB uses a hash table or other data structure that does not easily update, the existing FIB might be invalidated and replaced with a new one computed from the updated RIB.
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
Protocol Independent Multicast
Protocol-Independent Multicast is a family of multicast routing protocols for Internet Protocol networks that provide one-to-many and many-to-many distribution of data over a LAN, WAN or the Internet...
(PIM).