Routing Concepts & Routing Tables Flashcards
(16 cards)
Describe what a router does when a packet arrives on its interface.
- Receives the Packet
- Checks the data link layer frame to verify that the frame is intended for the router.
- Error Checking (Optional)
- If the frame is invalid, it is discarded.
- Removes the Data Link Layer Header
- Once validated, the router strips off the data link layer header to access the encapsulated Layer 3 (IP) packet.
- Checks the Destination IP Address
- The router examines the destination IP address in the packet header and compares it against its routing table to determine the best next-hop address and outgoing interface.
- TTL Decrement and Check
- The router decreases the Time-To-Live (TTL) field in the IP header by 1. If the TTL reaches 0, the router discards the packet
- Routing Decision
- Based on the destination IP address, the router uses its routing table to:
- Find the most specific matching route (longest prefix match).
- Determine the next-hop IP address (if required).
- Identify the correct outgoing interface.
- Based on the destination IP address, the router uses its routing table to:
- Encapsulates the Packet for the Next Hop
- The router then re-encapsulates the packet in a new data link frame appropriate for the next network segment (with the next hop’s MAC address in an Ethernet frame).
- Forwards the Packet
- Finally, the router sends the newly encapsulated frame out through the selected interface toward the next hop.
What is meant by the term ‘dual-stack’?
A device or networking environment that supports the simultaneous use of both IPv4 and IPv6 addresses.
What is a loopback interface? What is their purpose?
The loopback interface is a logical interface that is internal to the router. It is not assigned to a physical port. It is considered a software interface that is automatically placed in an “up” state.
The loopback interface is useful for testing purposes. For example, it can be used for emulating networks behind the router.
What is a link-local address, and why is it needed in IPv6 but not IPv4?
A link-local address is a special IP address used to communicate with other devices on the same local link (no router needed).
- IPv6 requires link-local addresses for key functions like Neighbor Discovery Protocol (NDP), which replaces ARP. - IPv6 does not support broadcasts, so NDP uses multicast with link-local addresses to find other devices and routers. - Even on the same LAN, IPv6 devices must use link-local addresses to learn each other’s MAC addresses and communicate.
In IPv4, ARP uses broadcasts to find MAC addresses, so link-local addresses are not needed (except as a fallback if DHCP fails, e.g. 169.254.x.x).
TL;DR: IPv6 needs link-local because it has no broadcast, and all local communication (like ARP in IPv4) depends on it.
What are the primary functions of a router?
A router connects one network to another network, which means that it has multiple interfaces that each belong to a different IP network. It is responsible for the delivery of IP packets across different networks.
The primary functions of a router are:
- To determine the best path to forward packets based on the information in its routing table.
- To forward packets toward their destination.
How does a router build its routing table?
Directly Connected Networks: Added to the routing table when a local interface is configured with an IP address and subnet mask (prefix length) and is active (up and up).
Remote Networks: Networks that are not directly connected to the router. Routers learn about remote networks in two ways:
- Static Routes: Added to the routing table when a route is manually configured.
- Dynamic Routing Protocols: Added to the routing table when routing protocols dynamically learn about the remote network.
Default Route: A default route is similar to a default gateway on a host. It specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address.
- The default route can be entered manually as a static route, or learned automatically from a dynamic routing protocol.
- A default route has an IPv4 route entry of 0.0.0.0/0 or an IPv6 route entry of ::/0.
- The default route is referred to as a gateway of last resort.
What are the different routing table codes and what do they mean?
L - Identifies the address assigned to a router interface on that directly connected network.
- The purpose of the local route is for the router to know that the received packet is for its own interface.
- For IPv4 local routes the prefix length is /32, and for IPv6 local routes the prefix length is /128.
C - Identifies a directly connected network.
S - Identifies a static route manually created to reach a specific network.
O - Identifies a dynamically learned network from another router using the OSPF routing protocol.
- This route is a default route.
What are static routes?
They define an explicit path between two networking devices.
Static routes are manually configured and can be implemented for accessing remote networks.
They are not automatically updated and must be manually reconfigured if the network topology changes. It provides ease of routing table maintenance in smaller networks that are not expected to grow significantly.
What are Dynamic Routing Protocols?
Are used by routers to automatically share information about the reachability and status of remote networks.
They let routers automatically share info about their local networks with other routers, so each router can build and update its routing table with the best paths.
Dynamic routing protocols perform several activities, including network discovery and maintaining routing tables.
What is a default route?
The default route specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address.
A default route can be either a manually configured static route or learned automatically from a dynamic routing protocol. It routes to and from stub networks. A stub network is a network accessed by a single router, and the router has only one neighbor.
A default route has an IPv4 route entry of 0.0.0.0/0 or an IPv6 route entry of ::/0. This means that zero or no bits need to match between the destination IP address and the default route.
What is meant by ‘best path’ in a routing table?
The best path in the routing table is known as the longest match.
For there to be a match between the destination IP address of a packet and a route entry in the routing table, a minimum number of far-left bits must match between the destination IP address of the packet and the route entry in the routing table.
The minimum number of far-left bits that must match is determined by the prefix length of the route entry in the routing table.
Describe the steps in the route lookup process.
- When a packet arrives at a router, its header is examined for the destination IP address.
- Examine each parent route entry in the routing table for a match with the destination IPv4 address.
- If there is a match with a parent route, proceed to examine the child routes.
- If there is a best match (longest match) with a child route, that route is used to forward the packet.
- If there is no match with any entry, use the default route if there is one, else drop the packet.
- The route lookup process ends either with the determination of the exit interface, or the dropping of the packet.
When should static or dynamic routes be used?
Networks often use both static and dynamic routing together.
Static routes are used when:
- Sending all unknown traffic to a service provider (default route).
- Connecting simple networks with only one way in or out (stub networks).
- The admin wants to set a specific path for certain networks.
- Routing to networks that dynamic routing doesn’t know about.
Dynamic routing is used in bigger networks with many routers.
- It automatically finds the best paths when the network changes.
- Dynamic routing is good for growing networks because it learns new routes automatically.
What is Administrative Distance (AD)?
Cisco router uses what is known as the administrative distance (AD) to determine which route to install into its routing table. The AD represents the “trustworthiness” of the route. The lower the AD, the more trustworthy the route source when a router selects which route to use.
What happens if a router has two or paths to a destination with equal cost metrics?
The router forwards the packets using both paths equally. This is called equal cost load balancing
What is the difference between a local link and a broadcast domain?
Both refer to a group of devices that can talk to each other without a router, but “local link” is the IPv6 term, and “broadcast domain” is the Layer 2 / IPv4-era term.
