TABLE OF CONTENTS There are several advantages to the single control card + multiple line card architecture, including a single point of control for the control plane and management protocols, distribution of information like routing table information, relatively easy software migration from a single-processor architecture (if we use messages), and so on.
However, the architecture also has some disadvantages:
The control card is a single point of failure if it goes down, so do all the control plane and management plane protocols.
The control card processor can become overloaded due to the large number of control plane tasks that need to be implemented. Some protocols such as OSPF and IS IS require complex SPF (Shortest Path First) calculations, potentially tying up the control processor for long periods.
The control card to line card control interface is a potential bottleneck, since the line cards need to forward messages destined to the router on this interface to the control card.
One way to address these issues is to add additional control cards. Some systems follow such a scheme for scalability for example, one control card could run all the interior routing protocols (OSPF, IS IS and RIP) while another could run the exterior routing protocol like BGP and also the multicast routing protocols like PIM (Protocol Independent Multicast), MOSPF (Multicast Open Shortest Path First), and DVMRP (Distance Vector Multicast Routing Protocol). The line cards are aware of this division and send the received packets to the appropriate control card.
