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15 Cards in this Set
- Front
- Back
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A protocol that defines routing and logical addressing is considered to be a network layer or layer 3 protocol.
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Network Layer (L3) protocols:
CLNS - Connectionless Network Services IP - Internet Protocol IPX - Novel Internetwork Packet Exchange DDP - Appletalk Datagram Delivery Protocol |
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IP is a connectionless Protocol
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Routing focuses on the end to end logic of forwarding data
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Between Hosts and router:
The sender sends a data-link frame across the medium to the nearby router; this frame includes the packet in the data portion of the frame. That frame uses data link layer (Layer 2) addressing in the data-link header to ensure that the nearby router receives the frame. |
routing table
A list of routes in a router, with each route listing the destination subnet and mask, the router interface out which to forward packets destined to that subnet, and, as needed, the next-hop router’s IP address. |
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When interacting with the data-link (L2) layer:
the data link layer adds the appropriate header and trailer to the packet, creating a frame, before sending the frames over each physical network. The routing process forwards the packet, and only the packet, end-to-end through the network, discarding data-link headers and trailers along the way. |
ARP is used to dynamically learn the data-link address of an IP host connected to a LAN.
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Routing as covered so far has two main concepts:
The process of routing forwards Layer 3 packets, also called Layer 3 protocol data units (L3 PDU), based on the destination Layer 3 address in the packet. The routing process uses the data link layer to encapsulate the Layer 3 packets into Layer 2 frames for transmission across each successive data link. |
Network layer protocols define the format and meaning of logical addresses.
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routing protocol
A set of messages and processes with which routers can exchange information about routes to reach subnets in a particular network. Examples of routing protocols include the Enhanced Interior Gateway Routing Protocol (EIGRP), the Open Shortest Path First (OSPF) protocol, and the Routing Information Protocol (RIP). |
routed protocol
A protocol which defines packets that can be routed by a router. Examples of routed protocols include AppleTalk, DECnet, and IP. |
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Routing protocols learn about all the locations of the network layer “groups” in a network and advertise the groups’ locations. As a result, each router can build a good routing table dynamically. Routing protocols define message formats and procedures, just like any other protocol. The end goal of each routing protocol is to fill the routing table with all known destination groups and with the best route to reach each group.
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Hosts actually use some simple routing logic when choosing where to send a packet. This two-step logic is as follows:
Step 1. If the destination IP address is in the same subnet as I am, send the packet directly to that destination host. Step 2. If the destination IP address is not in the same subnet as I am, send the packet to my default gateway (a router’s Ethernet interface on the subnet). |
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Routing Table:
A list of routes in a router with each route listing; the destination subnet, mask, router interface out which to forward packets detsined to that subnet and as needed the next hop routers ip address. |
When interacting with the data-link layer the data link layer adds the appropriate header and trailer to the packet creating a frame before sending the frames over the physical network. The routing process forwards the packet and only the packet end to end through the network discarding data link headers and trailers along the way.
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ARP - is used to dynamically learn the data-link address of an IP host connected to the LAN.
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Routing has 2 main concepts:
The process of routing forwards layer 3 packets, also called layer 3 protocol data units based on the destination layer 3 address in the packet. The routing process uses the data-link layer to encapsulate the layer 3 packets into layer 2 frames for transmission accross each sucessive data link. |
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Network layer protocols define the format and meaning of logical addresss.
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Routed Protocol - A protocol which defines packets that can be routed by a router.
Appletalk, DECNET, and IP are ROUTED protocols |
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Routing Protocols -
A set of messages and processes with which routers can exchange information about routes to reach subnets in a particular network. EIGRP, OSPF, RIP |
Hosts use simple routing logic.
If the destination IP address is in the same subnet as it is on, send the packet directly. If the destination IP is on a different subnet send the packet through the default gateway. |
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A router uses the following logic when recieving a data-link frame:
1. Use the datalink FCS field to ensure the frame has no errors, if errors occured discard the frame. 2. Assuming the frame was not discarded at step 1 discard the old data-link header and trailer leaving the IP packet. 3. Compare the IP packets destination IP address to the routing table and find the route that matches the destination address. This route identifies the outgoing interface of the router and possibly the next hop router. 4. Encapsulate the IP packet inside a new data-link header and trailer appropriate for the outgoing interface and forward the frame. |
IP Protocols
fill the routers IP routing tables with valid loop free routes |
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Goals of Routing Protocols:
to dynamicaly learn and fill the routing table with a route to all subnets in the network. If more then one route to a subnet is available to place the best route in the routing table. To add new routes - or replace lost routes with the best currently available route as quickly aws possible. The time between loosing a route and finding a working replacement route is called convergence time. To prevent routing loops. |
Network Layer Utilities:
ARP, DNS, DHCP, PING |
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An ARP broadcast is sent to a broadcast Ethernet address, everyone on the LAN receives it.
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If the host is on the same subnet, the sending host sends an ARP looking for the destination hosts MAC address.
If the sending host is on a different subnet then the destination host then the sending host routing logic results in the sending host needing to forward the packet to its default gateway. |
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DHCP messages to acquire an IP address:
1. DHCP discover message (LAN BROADCAST) 2. DHCP Offer message (LAN BROADCAST) 3. DHCP Request message directed to the server. 4. DHCP acknowledge directed to the client. |
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