Introduction to IP Routing

• A router decides where to send a packet using its route table and the destination network of the packet. It does not need the destination host, the originating network, or the return path to route the packet.
• Routers rebuild Ethernet frames at routed hops. Without NAT, source and destination IP addresses stay the same; TTL decreases and the IPv4 header checksum changes.
• A frame carries a packet to only two places: a host on the local network, or a router interface when the packet is going to a remote network.
• When the destination is on another network, the host sets the destination MAC address to the default gateway. The destination IP address never changes.

Ethernet frames are local and rebuilt at routed hops. Without NAT, the IP source and destination remain end to end, while routers update TTL and the IPv4 header checksum.

• MAC (hardware) addresses are always local. They are used only on the local LAN and never pass a router interface.
• Address Resolution Protocol (ARP) finds the MAC address of the default gateway, or of a local host, from an IP address.
• A host ARPs for the destination if it is on the same network. If not, it ARPs for the default gateway.
• At the far end, the router interface facing the destination network ARPs for the destination host.
• Reverse ARP (RARP) does the opposite, deriving an IP address from a MAC address, similar to DHCP. It does not update route tables.

The chapter text prints arp -g in Exercise 9.1 and in question 6, but the command on a host is arp -a (question 18). Cisco routers use show ip arp.

• The route table is the router's map of the internetwork. Routing decisions are made by destination network, not by host.
• A C in the table means the network is directly connected because it is configured on an interface. An S is a static route.
• The gateway of last resort is the default route. If it is not set and no route matches, the packet is dropped.
• To find the exit interface, match the destination IP address to the network that contains it. 172.16.20.94 falls inside 172.16.20.64/27, so the exit interface is FastEthernet 0/2 (question 14).

A router with these four routes has no match for 172.16.20.100. That address sits between the /27 blocks, so the packet is dropped.

The source MAC, destination MAC, and EtherType are included when the FCS is calculated, but only the FCS tells the receiver that the frame is intact.

• If the router has no route to the destination network, it drops the packet.
• The router also sends an ICMP destination unreachable message to the sender. The original packet is never returned.
• Logging happens only if it is configured. Routers do not ask for dynamic updates when they meet an unknown network.
• If a ping is lost along the way, the sender sees Request timed out. Destination unreachable appears when a router lacks the route.

• The main job of a routing protocol is to update routing tables.
• Convergence is the term for the result of all routes being updated in the routing table through a dynamic routing protocol. Faster convergence is better.

If a question asks for routing that is automated as new locations are added, the answer is dynamic routing, not static routing, DHCP, or RARP.

• An autonomous system (AS) is a collection of networks or subnets in the same administrative domain.
• An interior gateway protocol (IGP) is a dynamic routing protocol used inside an autonomous system. RIP, EIGRP, and OSPF are IGPs.
• An exterior gateway protocol (EGP) is a dynamic routing protocol used between autonomous systems. BGP is the EGP in use today.
• Administrative distance (AD) decides the most trusted route when two or more routes to the same network exist.
• Distance-vector and link-state describe how routing protocols share and calculate routes. Chapter 10 covers them in detail.

• Exam objective 2.1 lists three factors for route selection: prefix length, administrative distance, and metric.
• A router first prefers the route with the longest matching prefix, because it is the most specific.
• If prefix lengths tie, the router prefers the lowest administrative distance.
• If administrative distance also ties, the router prefers the lowest metric.

Host_A (172.16.10.2) connects to Lab_A through e0 (172.16.10.1). Host_B (172.16.20.2) connects through e1 (172.16.20.1). Host_C (172.16.30.2) and Host_D (172.16.30.3) connect through e2 (172.16.30.1).

• Open a command prompt, find the IP address of the default gateway, and ping a website such as www.wiley.com.
• Run arp -a and locate the entry for the default gateway. This gives you the MAC address of the router.
• Look at the other entries. The default gateway's MAC address is relevant only to local clients on the network.

The ipconfig command shows the default gateway on Windows. The chapter text writes ifconfig, which is the command on Mac and Linux.

This chapter has interactive visuals for the routing process, ARP, the route table, frame fields, static and dynamic routing, the written lab, and route selection. Open them at /network-plus/ch9.