1. 1 Network Layer Lecture 14 Imran Ahmed University of Management & Technology

2. 2 Agenda Introduction & Network layer functions Routing principles Hierarchical routing The Internet protocol (IP) Routing in the Internet

3. 3 IGRP Basics Cisco System’s proprietary dynamic routing protocol. (mid 1980’s) Its distance vector protocol. It broadcasts its router table out of all its interfaces at regular (adjustable) intervals. Class-full routing protocol because it cannot carry subnet mask information.

4. 4 Why IGRP? Not RIP Increased scalability – Improved fro running in larger networks compared to networks that use RIP. IGRP has a default maximum hop count of 100 hops, which can be configured to a maximum of 255 hops. Sophisticated metric – IGRP uses a composite metric that provides significant route selection flexibility. Multiple path support – IGRP can maintain up to six unequal cost paths between a network source and destination.

5. 5 IGRP Metrics IGRP supports five routing metrics:- –Bandwidth (K1) –Delay (K3) –Reliability (K2) –Load (K4) –MTU (K5)

6. 6 IGRP Metrics Bandwidth (Bw): It represents the speed of the link, where the speed can range from 1200bps to 10Gbps. Delay: This represents the delay on the medium. Its value can be from 1 to 2 * 10 23 microseconds. Reliability: Fraction of 255, where 255 means totally reliable. Administrative Distance: It can take a value between 1 to 255 and creates a Load or weighting on that particular link, the higher the number, the less attractive the link is. MTU: It is the maximum packet size that can be sent along the entire path without fragmentation (that is, it is the minimum of the MTU’s of all the networks involved in the path).

7. 7 IGRP Packet Format

8. 8 Version - Always 0x01 Opcode - 0x01 for a Request (a header with no entries) and 0x02 for an Update Edition - This number is incremented by the sender so that the receiving router does not use an old update Autonomous System number - The IGRP process ID Number of Interior routes - indicates how many of the routing entries in this update are subnets of a directly connected network. Number of System routes - indicates how many of the routing entries in this update are not from a directly connected network. Number of Exterior routes - indicates how many of the routing entries in this update are default networks.

9. 9 IGRP Packet Format Checksum - calculated on the header and the entries. With his field set to 0, the 16-bit one's complement sum is calculated and then inserted into this field. At the other end, the 16-bit one's complement is calculated again by the other router but this time including the already calculated value in the Checksum field. Destination - Destination network, just containing the last three octets for interior routes (e.g. 24.5.0 for the network 10.24.5.0) since the first octet will be known. For System and External routes, the routes would have been summarised so the last octet will always be zero (e.g. 10.24.5.0 will be entered as 10.24.5). Delay - The number of 10 microsecond chunks which is the sum of delays Bandwidth - IGRP bandwidth MTU - The smallest MTU encountered along the route to this particular destination network. Reliability - A number between 0x01 and 0xFF to indicate the error rates totally along the route. 0xFF is reliable. Load - A number between 0x01 and 0xFF expressing the total load along a route where 0xFF is totally loaded. Hop Count - A number between 0x00 (directly connected network) and 0xFF.

10. 10 IGRP Stability Features IGRP provides a number of features that are designed to enhance its stability, these includes:- –Hold-downs –Split horizons –Poison-reverse updates

11. 11 IGRP Stability Features Hold-downs are used to prevent regular updates messages from inappropriately reinstating a route that might have gone bad. Split horizons derive from the premise that it is never useful to send information about a route back in the direction from which it came. Poison-reverse updates are necessary to defeat larger routing loops. –To avoid routing loops, poison-reverse updates are sent to remove the route and place it in hold-down.

12. 12 IGRP Timers IGRP maintains a number of timers and variables containing time intervals, these includes an:- –Update timer –Invalid timer –Hold-time period –Flush timer

13. 13 IGRP Timers The update timer specifies how frequently routing update messages should be sent (default is 90 sec.). The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid (default is 270 sec.). The hold-time variable specifies the hold-down period (default is 280 sec.). The flush timer indicates how much time should pass before a route should be flushed from the routing table (default is 630 sec.).

14. 14 Unequal-Cost Load Balancing

15. 15 Unequal-Cost Load Balancing Unequal-cost load balancing: –R1 and R2 are linked by two links, a 64Kb/s link and a faster 2Mb/s link. –R1 needs to reach the network 172.16.0.0/16. –By default, IGRP will choose the faster link according to this formula Metric = Bw + Delay. By default, if there are multiple equal-cost paths to a destination, the router will load share across up to four paths (can be changed up to 6 paths). To share the load over unequal paths, IGRP uses variance command.