Sem1 - Module 10 Routing Fundamentals and Subnets Review

Routable and routed protocols: A protocol is a set of rules that determines how computers communicate with each other across networks. A protocol describes the following: –The format that a message must conform to –The way in which computers must exchange a message within the context of a particular activity A routed protocol allows the router to forward data between nodes on different networks. Routed Protocols: –IP (Internet Protocol) –IPX/SPX –AppleTalk –DECnet –AppleTalk –Banyan VINES –Xerox Network Systems (XNS)

IP as a routed protocol: encapsulation

Routable and routed protocols: As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device. This is because Layer 2 data units, frames, are for local addressing. Layer 3 data units, packets, are for end-to-end addressing. encapsulation de-encapsulation

Routable and routed protocols: As a frame is received at a router interface, the destination MAC address is extracted. The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast. In either of these two cases, the frame is accepted. Otherwise, the frame is discarded since it is destined for another device on the collision domain. The accepted frame has the Cyclic Redundancy Check (CRC) information extracted from the frame trailer, and calculated to verify that the frame data is without error. If the check fails, the frame is discarded. If the check is valid, the frame header and trailer are removed and the packet is passed up to Layer 3. The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork. If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4.

Routable and routed protocols: Some protocols, such as IPX, require only a network number because these protocols use the host's MAC address for the host number. Other protocols, such as IP, require a complete address consisting of a network portion and a host portion. These protocols also require a network mask in order to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask. Consider following address & SNM: /27: IP: SNM: SubNet Addr: (Subnet for the IP /27)

Anatomy of an IP packet: The IP header consists of the following: Version – Indicates the version of IP currently used; four bits. If the version field is different than the IP version of the receiving device, that device will reject the packets. IP header length (HLEN) – Indicates the datagram header length in 32-bit words. This is the total length of all header information, accounting for the two variable-length header fields. Total length – Specifies the length of the entire packet in bytes, including data and header, 16 bits. To get the length of the data payload subtract the HLEN from the total length.

Anatomy of an IP packet: The IP header consists of the following: Flags – A three-bit field in which the two low-order bits control fragmentation. One bit specifies whether the packet can be fragmented, and the other specifies whether the packet is the last fragment in a series of fragmented packets. Time-to-live (TTL) – A field that specifies the number of hops a packet may travel. This number is decreased by one as the packet travels through a router. When the counter reaches zero the packet is discarded. This prevents packets from looping endlessly. Protocol – indicates which upper-layer protocol, such as TCP or UDP, receives incoming packets after IP processing has been completed, eight bits. Header checksum – helps ensure IP header integrity, 16 bits.

Anatomy of an IP packet: The IP header consists of the following: Source address – specifies the sending node IP address, 32 bits. Destination address – specifies the receiving node IP address, 32 bits. Padding – extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits. Data – contains upper-layer information, variable length up to 64 Kb.

Routing: Routing is an OSI Layer 3 function. The following are the two key functions of a router: –Routers must maintain routing tables and make sure other routers know of changes in the network topology. This function is performed using a routing protocol to communicate network information with other routers. –When packets arrive at an interface, the router must use the routing table to determine where to send them. The router switches the packets to the appropriate interface, adds the necessary framing information for the interface, and then transmits the frame. A router is a network layer device that uses one or more routing metrics to determine the optimal path along which network traffic should be forwarded.

Routing: Routing metrics are values used in determining the advantage of one route over another. Routing protocols use various combinations of metrics for determining the best path for data. Routed protocols transport data across a network. Routing protocols allow routers to choose the best path for data from source to destination. A routing protocol functions includes the following: Provides processes for sharing route information Allows routers to communicate with other routers to update and maintain the routing tables

Routing algorithms and metrics: Metrics can be based on a single characteristic of a path, or can be calculated based on several characteristics. The following are the metrics that are most commonly used by routing protocols: Hop count: The number of routers that a packet must travel through before reaching its destination. Each router the data must pass through is equal to one hop. A path that has a hop count of four indicates that data traveling along that path would have to pass through four routers before reaching its final destination. If multiple paths are available to a destination, the path with the least number of hops is preferred.

Routing algorithms and metrics: Bandwidth: The data capacity of a link. Normally, a 10-Mbps Ethernet link is preferable to a 64-kbps leased line. Delay: The length of time required to move a packet along each link from source to destination. Delay depends on the bandwidth of intermediate links, the amount of data that can be temporarily stored at each router, network congestion, and physical distance. Load: The amount of activity on a network resource such as a router or a link. Reliability: Usually a reference to the error rate of each network link.

Routing Protocols: Two families of routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). IGPs can be further categorized as either distance-vector or link-state protocols. Examples of distance-vector protocols include the following: Routing Information Protocol (RIP) – The most common IGP in the Internet, RIP uses hop count as its only routing metric. Interior Gateway Routing Protocol (IGRP) – This IGP was developed by Cisco to address issues associated with routing in large networks. Enhanced IGRP (EIGRP) – This Cisco-proprietary IGP includes many of the features of a link-state routing protocol. Because of this, it has been called a balanced- hybrid protocol, but it is really an advanced distance-vector routing protocol.

Link-state Routing Protocols: Link-state routing protocols were designed to overcome limitations of distance vector routing protocols. Link-state routing protocols respond quickly to network changes sending trigger updates only when a network change has occurred. Link-state routing protocols send periodic updates, known as link-state refreshes, at longer time intervals, such as every 30 minutes. Link-State Routing Protocols: –IS-IS (Intermediate System-to-Intermediate System ) –OSPF (Open Shortest Path First)

Routing versus switching: Routing and switching might seem to perform the same function to the inexperienced observer. The primary difference is that switching occurs at Layer 2, the data link layer, of the OSI model and routing occurs at Layer 3. This distinction means routing and switching use different information in the process of moving data from source to destination.

Router vs Switches: Another difference between switched and routed networks is switched networks do not block broadcasts. As a result, switches can be overwhelmed by broadcast storms. Routers block LAN broadcasts, so a broadcast storm only affects the broadcast domain from which it originated. Because routers block broadcasts, routers also provide a higher level of security and bandwidth control than switches.

Benefits of Subnetting: More efficient use of IPs Increased address flexibility Segments Broadcast domains (smaller) –Small amount of security

IPs & Subnetting: For each of the following IPs: – /26 – /25 – /28 Identify the following: –Class –Subnet Mask –# SN bits and # useable Subnets –# Host Bits and # useable IPs –Subnet address for the IP –Subnet Broadcast address –Useable IPs (range) –Major Network Address –Major Broadcast address

IPs & Subnetting /28 Identify the following: Class: Class C Subnet Mask (Host Bits = 0) : # SN bits and # useable Subnets: –4 Subnet bits –2 4 – 2 = 14 # Host Bits and # useable IPs: –4 Host Bits –2 4 – 2 = 14 Subnet address for the IP : Subnet Broadcast address (Host Bits = 1) : Useable IPs (range):  Major Network Address: Major Broadcast address:

IPs & Subnetting /26 Identify the following: Class: Class B Subnet Mask: # SN bits and # useable Subnets: –10 Subnet bits –2 10 – 2 = 1022 # Host Bits and # useable IPs: –6 Host Bits –2 6 – 2 = 62 Subnet address for the IP: Subnet Broadcast address: Useable IPs (range):  Major Network Address: Major Broadcast address:

IPs & Subnetting /25 Identify the following: Class: Class A Subnet Mask: # SN bits and # useable Subnets: –17 Subnet bits –2 17 – 2 = # Host Bits and # useable IPs: –7 Host Bits –2 7 – 2 = 126 Subnet address for the IP: Subnet Broadcast address: Useable IPs (range):  Major Network Address: Major Broadcast address:

IPs & Subnetting /28 Identify the following: Class: Class C Subnet Mask: # SN bits and # useable Subnets: –4 Subnet bits –2 4 – 2 = 14 # Host Bits and # useable IPs: –4 Host Bits –2 4 – 2 = 14 Subnet address for the IP: Subnet Broadcast address: Useable IPs (range):  Major Network Address: Major Broadcast address:

The number of lost IP addresses with a Class C network depends on the number of bits borrowed for subnetting. Host Subnet Schemes

Chapter #10 Test