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PowerPoint Presentation Routing Fundamentals ITNW 2312, Section II Router Purpose Router Purpose Overview: Two packet delivery types: Direct – packet’s destination address is on the same IP network as the sender’s one Indirect – packet’s destination address is on an IP network that is different from the sender’s one Indirect delivery requires routing, i.e. there must be at least one router between the source and destination Router Purpose TCP/IP Encapsulation: Each packet that the router receives has a destination IP address in its header that identifies final destination. Router Purpose Routing Decisions: A routing decision should result in the packet forwarded out proper interface, based on the destination IP address. Router Purpose Routing Decisions (continued): Upon receiving a frame, the router: Verifies integrity, checks the destination MAC address, and determines payload type (IP, IPv6, etc.) De-encapsulates the packet, verifies header checksum, and decrements & verifies TTL Determines if it should process the packet (if the destination IP address is the router’s) or forward it further Uses destination IP address to make routing decision (decides which interface to forward the packet out) Encapsulates the packet into new frame and forwards it out proper interface Router Purpose Routing Decisions (continued): Every TCP/IP host (including routers) stores known destination networks in its routing table A decision is made by matching the packet’s destination IP address against destinations in the routing table Two outcomes: Match found – router learns proper exit interface No match found – router drops the packet and elicits ICMP Destination Unreachable error message Router Purpose Routing Decisions (continued): By matching the destination IP address against its routing table, Boston learns exit interface to forward the packet out. Packet Forwarding Packet Forwarding Routing Table: Components: Logical Destinations – network IP addresses known to the router with corresponding subnet masks Next-Hop Addresses – IP addresses of immediate neighbor routers that are next to take over packet delivery Exit Interfaces – this router’s interfaces that connect to the neighbor routers or directly to the destination network A next-hop address must belong to the same logical network as this router’s connected interface Packet Forwarding Routing Table (continued): A routing table lists destination networks as well as the corresponding next-hop addresses and exit interfaces. Packet Forwarding Routing Table (continued): Every network assumes a range of host addresses that the packet’s destination address is actually matched against. Packet Forwarding Routing Table (continued): The tables are populated so that the most efficient routes are used in the first place An internetwork is converged when every router knows at least one path to each intended destination network Packet Forwarding Address Matches: The packet’s destination address can match one, several, or no routing table entries, so the router: Single match – uses proper exit interface to forward new frame out towards the neighbor router Multiple matches – chooses the table entry with the longest matching bit sequence No match – drops the packet, attempts to return an error to the sender Packet Forwarding Example I: Which interface should the packet be forwarded out? Packet Forwarding Example I (continued): Destination address 10.1.1.157 matches host address range of one network in its routing table… Packet Forwarding Example I (continued): … so the router learns exit interface & next-hop address … Packet Forwarding Example I (continued): … and forwards the packet out the exit interface to the neighbor router that has the next-hop address. Packet Forwarding Example II: Destination address 10.1.1.117 matches host ranges of several networks in its routing table… Packet Forwarding Example II (continued): … so the router chooses exit interface & next-hop address from the entry with the longest matching bits … Packet Forwarding Example II (continued): … and forwards the packet out the exit interface to the neighbor router that has the next-hop address. Failures: The workstation received ICMP errors from device at 172.16.1.1 which was unable to forward the Echoes. Packet Forwarding Failures (continued): The workstation gets no response from the network, possibly due to routing issues, and times out waiting. Packet Forwarding Packet Forwarding Failures (continued): Replies aren’t received from beyond certain point. Packet Forwarding Failures (continued): No reply is received from beyond the local router. Packet Forwarding Load Balancing: Routers may learn more than one path to a destination from the same specific information source Example – the administrator enters several next-hop addresses for the same destination network In such case, routers would load-balance data delivery between equal-cost routes to the destination Routing Tables Routing Tables Destination Networks: Two types: Directly connected – an IP address is configured on the router that belongs to the destination network Remote – no IP address is configured on the router that belongs to the destination network (not directly connected) Routing Tables Destination Networks (continued): For Router B … Routing Tables Destination Networks (continued): … there are two directly connected and two remote networks. Routing Tables Destination Networks (continued): Routers learn them: Directly connected – from IP configuration on connected interfaces (as soon as L1 through L3 become functional) Remote – via manual entry or from neighbor routers Routing Tables Connected Networks: For each network, the router stores the name of local interface that connects to this network Removed automatically if the connected interface’s: L1 or L2 go down, or IP address and subnet mask are removed Routing Tables Remote Networks: For each network, the router stores next-hop IP address as well as the exit interface Removed from routing table: Manually – if was learned via manual entry Automatically – if was learned from neighbor routers Routing Tables A route: Two uses: On a large network – L3 path between distantiated subnets, formed by multiple router-to-router connections On specific router – next-hop address that points at the next router along the path Routing Tables A route (continued): A L3 path within the internetwork. Routing Tables A route (continued): A path reference in specific routing table. Routing Tables Administrative Distance: If a router learns several routes to the same destination from different sources, routing data from which source is recorded into the routing table? Routing Tables Administrative Distance (continued): Defines trust level of a routing information source, with lower values defining more trustable sources Specific values are locally significant and don’t get exchanged between neighbor devices Values range from 0 (reliable most) to 255 (unknown) Routing Tables Administrative Distance (continued): Routing Tables Cisco: Routing Tables Other Notes: Stored and processed in RAM, so their content clears when the router is rebooted or powered down May consume substantial memory amounts, requiring optimizations such as summary routes Routing Tables Related Commands: Enable or disable forwarding of IPv4 packets (enabled by default): Router(config)# [no] ip routing View IPv4 routing table (user or enable mode): Router# show ip route Manual Routing Manual Routing Routing Function: Implements route learning, routing table construction and maintenance, and packet forwarding Two types: Manual Dynamic Manual Routing Overview: Requires human administrator to enter information on how to forward packets to each remote network Makes the administrator responsible for: Traffic estimates and route planning Routing information entry and corrections Traffic redirection Manual Routing Overview (continued): Devices: Use the information entered by administrator without evaluating or verifying it Place the commands entered into running configuration, requiring copy run start to save them IOS Command: To enter manual route: Router(config)#ip route {destination IP} {destination subnet mask} {next hop IP} [administrative distance] [permanent] Notice: Exit interface name – such as fastethernet0/0 – may be used instead of the next-hop IP address High administrative distance values are commonly used when entering secondary or backup routes Optional keyword permanent keeps the route in the routing table even after the exit interface goes down Manual Routing Example (continued): Manual Routing Example (continued): Manual Routing Manual Routing Limitations: Doesn’t account for changes automatically, so routes must be modified manually (possibly, on many routers) Manual work grows exponentially with the size of the network, nightmare if changes occur frequently Suffers from human errors, with each error causing havoc and, potentially, breaches in security Benefits: Minimizes CPU time and memory that is needed for carrying out routing function Gives security administrators complete control over packet forwarding outcomes Requires no inter-device communication, leaving all bandwidth to data transmissions Supported by all L3-aware devices (routers, firewalls, wireless APs) and platforms (Cisco, Linux, MS, etc.) Manual Routing Default Route Default Route Overview: Special-purpose manual route, used for forwarding packets that match no routing table entry Instructs the router to forward unknown packets to a device at specific next-hop address None exists by default, so packets that match no routing table entry are dropped Commonly used for outbound Web traffic Default Route Commands: To enter default route on a router that connects to ISP: (config)# ip route 0.0.0.0 0.0.0.0 {next hop IP} [AD] Keep in mind: On perimeter devices, next hop IP address is the address of ISP’s router High administrative distance values are used to create backup routes Quad-zeroes mean unknown destination Default Route Example: