Routers

Routers act as network layer relays between networks.

While bridges are normally restricted to connecting LANs within an internetwork,

routers have the capability of connecting networks of different types

A router participates as a device on each attached network and exchanges information with devices on those networks. These are end node capabilities.

A router has the additional capability of exchanging information with other routers and with end nodes on remote networks as long as they support the same network layer protocol.

Therefore, routers are protocol-dependent, unlike bridges.

Packet

Packets (also known as frames) are defined as chunk data which has been packaged, addressed, and sent into the network towards its destination much as a letter is placed into an envelope, addressed, and dropped into a mailbox.

The data may represent an entire message or a segment of a message, depending on media or device limitations on the amount of information that may be enveloped.

Routers forward packets based on information held in the network layer headers of packets they receive on their attached networks. Network layer headers contain address information (typically consisting of a network identifier and a host identifier) and associated control to allow the packets to be routed to their correct destination. Packets that have no network layer header, or that have a network layer header for a protocol not supported by a particular router, are discarded.

There are a number of network layer protocols, each of which has its own network layer header format for addressing and control.

A router must be configured with the network layer address of each of its network connections. When it receives an incoming frame with a compatible network layer header, it determines whether the destination address is on the same network. If it is, then the frame is discarded. Otherwise, the router forwards the frame to the destination device (if on a network attached to the router), or to the next router in the path to the destination device.

In order to do this, a router must maintain routing tables containing information about the next router in the path to every reachable destination in the internetwork. The process for doing this includes two stages: 

• It must acquire route information.
  
• It must determine the best routes to insert into its routing table.
  

Route information can be acquired by manual configuration (these are called static routes), or can be learned automatically from other routers using routing table maintenance protocols (these are called dynamic routes)

Source-Route Transparent Bridging (SRT)

The IEEE 802.1 committee identified the need for source-route bridges to interoperate with transparent bridges in the same internetwork. A source-route transparent bridge (SRT) standard has been defined to achieve this goal. The principle behind SRT bridges is very simple. An SRT bridge inspects all received frames and looks for the presence of the routing information indicator (RII) and the routing information field (RIF). If these fields are present, the SRT bridge uses them and acts as a source-route bridge. If not, the SRT bridge operates in transparent bridge mode and forwards frames based on their MAC sublayer destination address and its associated entry in the filtering database. The source-route transparent bridge does not allow source-route bridge devices to communicate with transparent bridge devices. SRT bridge is the capability for its interfaces to understand both source-route bridging and transparent bridging devices. But an SRT bridge will never translate source-route bridge frames into transparent bridge frames, and vice versa.

Source-Route - Translational Bridge (SR-TB)

Source-Route - Translational Bridge (SR-TB) is not an ISO standard definition. However, more and more bridges are implementing the SR-TB because of the need to interconnect source-route bridge domain with transparent bridge domain. The goal of the source-route - translational bridge is to translate source-route bridge frame into a transparent bridge frame, and vice versa. The SR-TB bridges have to change the MAC layer protocol from (or to) Ethernet protocol to (or from) token-ring protocol. Actually, regarding the ISO bridge definition, this translation does not belong to a bridge. But it is implemented in a lot of bridges, in order to be able to interconnect source-route bridge domain and transparent bridge domain regardless of the protocol of the upper layer.

Tunnel Bridge

The tunnel bridge allows source-route bridge domains or transparent bridge domains to communicate across an IP network. The tunnel bridge receives bridged frames from its source-route bridge or transparent bridge domain. The frames are encapsulated into IP datagrams that are sent to the destination IP address. These IP datagrams are routed in the IP network as are other IP datagrams, with the IP rules. The destination IP address is actually another bridge implementing the tunnel bridge feature. This target bridge removes the IP envelope from these IP datagrams making them source-route bridge or transparent bridge frames. Then the target bridge sends these frames to its source-route bridge domain or transparent bridge domain in the same way that other bridged frames are sent.

With tunnel bridging, as far as the source-route bridge is concerned, the IP network is seen as a single LAN segment, regardless of the complexity of the IP network. Then it adds only one hop to cross this IP network. 

The number of hops from the source device to the source IP tunnel bridge, plus one hop to cross the IP network, plus the number of hops from the destination IP tunnel bridge to the destination device, must not exceed the 7-hop count limitation of the source-route bridge implementation.