ROUTING BASICS

Why are Routers Necessary? One of the key components of the technical infrastructure of the network One of the key components of the technical infrastructure of the network Connect networks Connect networks Provide the best path from the source to destination Provide the best path from the source to destination A R1 B R2 R3 C R4R5

Sending Packets through the Network Using redirects Using redirects Sending packets on the same subnet Default router Discovering the local router A R1 B R2 Internet

The Internal Elements of a Router Interfaces Routing Engine Routing table Destination Next hop Interface

Schematic View of a Router Incoming packets Processing Outgoing packets

The Routing Table The crucial element of the router The crucial element of the router –defines the topology of the network –must be consistent with other router’s tables Static and dynamic routing tables Static and dynamic routing tables –static - when constructed by network administrator –dynamic - when constructed by routing protocols

Static Routes Advantages Advantages –predictability –no overhead –simplicity Disadvantages Disadvantages –lack of scalability –can not adapt to a failure in a network

Example: R1 R / / / / /24 R hostname Router1 interface e0 IP interface e1 IP IP route IP route IP route

Dynamic Routes Advantages –adapt to a failure in a network –work in large networks Disadvantages –increase in complexity –overhead on the lines and routers

Hybrid Routing Schemes Some parts use static and some parts dynamic routing Some parts use static and some parts dynamic routing –static routing on the access network –dynamic routing on the core and distribution network R1 R2R3 R6R5 R4 Core Distribution Access

Classification of the Routing Protocols Where the protocol is used Where the protocol is used –Interior protocols (IGP) –Exterior protocols (EGP) Kind of information that is carried and the way the routing table are calculated Kind of information that is carried and the way the routing table are calculated –Distance-vector protocols –Link-state protocols

IGP Vs EGP Interior Gateway Protocols Interior Gateway Protocols –within a single autonomous system single network administration single network administration unique routing policy unique routing policy make best use of network resources make best use of network resources Exterior Gateway Protocols Exterior Gateway Protocols –among different autonomous systems independent administrative entities independent administrative entities communication between independent network infrastructures communication between independent network infrastructures

Distance-Vector Vs Link-State Distance-vector protocols Distance-vector protocols –Each router periodically sends to his neighbors how far is the destination how far is the destination the next hop to get there the next hop to get there –Install routes directly in tables Link-state protocols –Each router sends information about links to which it is attached state of these links –it is flooded throughout the network –every router calculates its routing table

The Role of IGPs Maintain a coherent picture of the network topology and address domain in the router Maintain a coherent picture of the network topology and address domain in the router Distribute this information to the other routers Distribute this information to the other routers Maintain consistent routing tables, such that the path to every destination is “optimal” Maintain consistent routing tables, such that the path to every destination is “optimal” Converge quickly when there are changes in the network Converge quickly when there are changes in the network

Example: Choosing an Optimal Path R5 5 A R1 R2 R4 R3 R6 R7 R8 B

The Link Metric Possible metrics Possible metrics –hop count –inverse of the link bandwidth –delay –dynamically calculated –administratively assigned –combination Traffic should be monitored and metrics adjusted Traffic should be monitored and metrics adjusted

Example for Bad Metrics 2048K 1024K B A 256K 2048K 768K Bandw. 768K Metric 17 Bandw. 256K Metric 14

RIP - Routing Information Protocol IGP, distance-vector protocol IGP, distance-vector protocol First used in XNS (Xerox Network Systems) First used in XNS (Xerox Network Systems) Designed as a component of the networking code for the BSD release of UNIX Designed as a component of the networking code for the BSD release of UNIX incorporated in program “routed” (rote management daemon) incorporated in program “routed” (rote management daemon) First documented in rfc 1058 First documented in rfc 1058

RIP - Characteristics Packets are sent every 30 seconds or faster when necessary Packets are sent every 30 seconds or faster when necessary Route is considered down if not refreshed within 180 sec. (distance set to infinity) Route is considered down if not refreshed within 180 sec. (distance set to infinity) Two kinds of messages Two kinds of messages request request response response

RIP - Characteristics The metric is a hop-count The metric is a hop-count The value of 1 to 15 is used (16 denotes infinity) The value of 1 to 15 is used (16 denotes infinity) Bellman-Ford algorithm is used to find the shortest paths Bellman-Ford algorithm is used to find the shortest paths Doesn't support classless routing Doesn't support classless routing Used only in IP networks Used only in IP networks at first the intention was to be used in variety of networks at first the intention was to be used in variety of networks

Example: E C Dest. Link Hop B local 0 A 1 1 C 4 1 E 3 1 B Dest. Link Hop C local 0 B 4 1 D 5 1 F 6 1 A Dest. Link Hop A local 0 B 1 1 E 2 1 Dest. Link Hop D local 0 C 5 1 G 7 1 D Dest. Link Hop G local 0 D 7 1 F 8 1 Dest. Link Hop E local 0 A 2 1 B 3 1 E Dest. Link Hop F local 0 C 6 1 G 8 1 F G

Routing table for node A A C B D E F 1 Dest. Link Hop A local 0 B 1 1 C 1 2 D 1 3 E 2 1 F 1 3 G 1 4 G Dest. Link Hop A local 0 B 1 1 C 1 2 E 2 1 After two iterations After four iterations Dest. Link Hop A local 0 B 1 1 C 1 2 D 1 3 E 2 1 F 1 3 After three iterations

In Case of a Link Failure Routing table of node A Dest. Link Hop A local 0 B - - C - - D - - E 2 1 F - - G - - after the failure of link 1 G Dest. Link Hop E local 0 A 2 1 B 3 1 C 3 2 D 3 3 F 3 3 G 3 4 C B D E F 1 A Dest. Link Hop A local 0 B 2 2 C 2 3 D 2 4 E 2 1 F 2 4 G 2 5 before the failure of link 3

Split-Horizon and Poison Reverse Split-horizon Split-horizon –the information about destination routed on the link is omitted Poison reverse Poison reverse –the corresponding distance is set to infinity if the destination is routed on the link

Triggered Updates A timer is associated with each entry in the routing table A timer is associated with each entry in the routing table –much longer than the period of transmission of information Triggered updates Triggered updates –request nodes to send messages as soon as they notice a change in the routing table

Advantages and Disadvantages Advantages Advantages –Simple to implement –Low requirement in processing and memory at the nodes –Suitable for small networks Disadvantages Disadvantages –Slow convergence –Bouncing effect –Counting to infinity problem

RIP - Limitations Maximum hop count of 15 Maximum hop count of 15 –restricts the use of RIP in larger networks, but prevents the count to infinity problem (endless loops) Difference in links speed is not reflected in the hop-count metrics Difference in links speed is not reflected in the hop-count metrics –congested links can be still included in the best path

RIP II - Why Was Developed? Many superior IGP exists (RIP is often referred as Rest In Peace) Many superior IGP exists (RIP is often referred as Rest In Peace) There are still many implementations of RIP There are still many implementations of RIP Given that RIP will still be used, it deserves improvements Given that RIP will still be used, it deserves improvements RIP II is documented in RFC-1287, RFC-1388 and RFC-2453 RIP II is documented in RFC-1287, RFC-1388 and RFC-2453

RIP II - The Added Fields Routing domain Routing domain –used together with the next hop field to allow multiple autonomous systems to share a single wire Route tag Route tag –to flag external routes (for use by EGP and BGP) Subnet mask Subnet mask –to support subnets Metric Metric

RIP II - Improvements Authentication Authentication –uses a simple password procedure Routing per subnet Routing per subnet Support of multiple metrics Support of multiple metrics –hop count, throughput, measured as 10logC Routing domains Routing domains Multicasting Multicasting Compatible with RIP Compatible with RIP

RIP is not alone! IGRP and EIGRP Interior Gateway Protocol was developed in the mid1980s by Cisco Systems, Inc. Interior Gateway Protocol was developed in the mid1980s by Cisco Systems, Inc. Designed to overcome the limitations of RIP Designed to overcome the limitations of RIP Initially worked in IP environment, but latter ported to OSI CLNP networks Initially worked in IP environment, but latter ported to OSI CLNP networks

IGRP - Main Characteristics Distance vector protocol Distance vector protocol Uses a combination of metrics Uses a combination of metrics –internetwork, delay, bandwidth, reliability and load the weighting factors are set either by administrators or default values are used the weighting factors are set either by administrators or default values are used

IGRP - Additional flexibility Wide metric ranges Wide metric ranges –allow satisfactory metric setting in internetworks with widely varying performance characteristics Permits multipath routing Permits multipath routing –dual equal-bandwidth lines may run a single stream of traffic in round-robin fashion

EIGRP Enhanced version of IGRP Enhanced version of IGRP Improvements Improvements –convergence properties The Distributed Update Algorithm (DUAL) is used to obtain loop-freedom throughout a route computation The Distributed Update Algorithm (DUAL) is used to obtain loop-freedom throughout a route computation –operational efficiency Provides compatibility with IGRP Provides compatibility with IGRP