Connecting Networks Exploring How Routing Works
Outline Overview Routers Routers and the IP Packet Delivery Process Path Determination Routing Tables Static, Dynamic, Directly Connected, and Default Routes Dynamic Routing Protocols Summary Lab Exercise 4-1: Creating a Default Gateway
Routers Linksys Cisco 2610 Router
router# show ip route D 192.168.1.0/24 [90/25789217] via 10.1.1.1 R 192.168.2.0/24 [120/4] via 10.1.1.2 O 192.168.3.0/24 [110/229840] via 10.1.1.3 1 2 Lets other routers know about changes Determines where to forward packets Router Functions
Routers and the IP Packet Delivery Process 1. Router receives frame. 2. Router de-encapsulates the frame and sees that it is a packet. 3. Router checks to see if the packet stays or goes to the next hop. If the packet needs to be forwarded, router consults the routing table. 4. If the packet stays, the router uses ARP. If the packet is to be forwarded, the router uses MAC of next hop. 5. Outgoing interface encapsulates the packet for correct media and sends it out.
Routing Table Entries Directly connected: Router attaches to this network Static routing: Entered manually by a system administrator Dynamic routing: Learned by exchange of routing information Default route: Statically or dynamically learned; used when no explicit route to network is known
Distance Vector Routing Protocols Passes periodic copies of routing table to neighbor routes and accumulates distance vectors
Link-State Routing Protocols After initial flood, passes small event-triggered link-state updates to all other routers
Summary Routers have certain components that are also found in computers and switches. These components include the CPU, motherboard, RAM, and ROM. Routers have these two primary functions in the IP packet delivery process: maintaining routing tables and determining the best path to used to forward packets.
Summary (Cont.) Routers determine the optimal path for forwarding IP packets between networks. Routers can use different types of routes to reach the destination networks, including static, dynamic, and default routes. Routing tables provide an ordered list of best paths to known networks, and include information such as destination, next-hop associations, and routing metrics. Routing algorithms process the received updates and populate the routing table with the best route. Commonly used routing metrics include bandwidth, delay, hop count, and cost.
Summary (Cont.) Distance vector routing protocols build and update routing tables automatically by sending all or some portion of their routing table to neighbors. The distance vector routing approach determines the direction (vector) and distance to any network in the internetwork. Link-state routing protocols build and update routing tables automatically, running the SPF algorithms against the link-state database to determine the best paths, and flood routing information about their own links to all the routers in the network. Cisco developed EIGRP, which combines the best features of the distance vector and link-state routing protocol.