2013_Routing_protocols (1131233)
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Introduction to routing protocolsPrimary router responsibility:• determining the best path• forwarding packetsRouter components:1. CPU2. RAM• OS• running config• IP routing table• ARP cache• packet buffer3. ROM• bootstrap instructions• diagnostic soft• scaled-down OS4. Flash memory (OS storage)5. NVRAM (start-up config)Bootup process:1. Power-On Shelf Test (hardware testing)2. Bootstrap loading (program to locate OS and load it to RAM)3.
OS loading4. Config file loading (broadcast request if it doesn't exist)• interface addresses• routing info• passwords• etcInterface - physical connector on router. Samples:• LAN interface (RJ-45 UTP cable; ethernet encapsulation)• WAN (serial; PPP, frame relay, high-level data link control)Cases for establishing static routing:• Network consists of few routers.• Network is connected to internet through single ISP.1• Hub-and-spoke topology (central hub).Dynamic routing protocols perform:• network discovery• update and maintain routing tablesRouting table principles:• Router makes decision alone.• Different routers do not always have the same information in routing table.• Info about the path doesn't provide info about return path.Routes are added only after interfaces are configured.IP packet format:• version• IP header length• destination & source address• TTL• protocol (upper-layer)• etcEthernet frame fields:• preamble• source & destination address• start-of-frame• type/length• etcHop-count - number of routers that packet must travel.Bandwidth - data capacity of link.Equal cost load balancing - using multiple interfaces with same metric to split data.Unequal cost load balancing is used in EIGRP.By default 4 equal paths are allowed.ICMP unreachable message is sent is there is no appropriate record in routing table and no default route.Network address is determined by adding IP address and subnet mask.Cables can be straight-through or crossover.Recursive lookup - multiple lookups in routing table before forwarding (e.g.
to check connectivity andthen determine next-hop).If interface is down, all static routes to it are removed.In static routing when exit interface is ethernet network, both exit interface and next-hop IP should beconfigured.Route summarizing is used for merging different networks.2Cisco Discovery Protocol (CDP)Getting information about directly connected devices. Operates on Layer 2.Info messages are periodically sent to neighbors:• type of connected devices with interfaces• interfaces to make the connections• model numbersInformation provided about each neighbor:• device id• address list (for network layer)• port identifier• capabilities list• platformDynamic routing protocolsCompare to static routing:• independent of network size• advanced knowledge required• automatically adapts to topology• suitable for all topologies• less secure• uses more resources• route depends on current topologyRouting protocol purposes:• remote network discovery• maintaining up-to-date routing information• choosing best path• ability to find new best path is current is unavailableRouting domain - collection of routers under a common administration.Interior gateway protocols are used within routing domain or individual networks.Exterior gateway protocols are under control of different organisations.Types of IGPs:3• Distance vector routing - routes are advertised as vectors of distance (e.g.
hop-count metric) anddirection (next-hop router or exit interface). Complete routing tables are sent to all neighborsperiodically. Bellman-Ford or Ford-Fulkerson algorithm. Used when:– simple networks– not enough administrative knowledge– specific networks (e.g. hub-and-spokes)– worst-case convergence time are not a concern• Link-state routing - creating complete view of the topology by gathering information from otherrouters. After network has converged updates are sent when there is topology change. Used when:– hierarchical network design– good administrative knowledge– fast convergence is crucialClassful routing protocols don't send subnet mask info.
Examples: IGRP, RIPv1.Classless protocols include subnet mask in updates. Support discontiguous networks.Convergence -state when all routing tables are consistent.Metric is used to evaluate difference between available paths. Types:• hop count - number of routers• bandwidth• load• delay• reliability - probability of failure• cost - metric determined by OS or administratorAdministrative distance defines the preference of a routing source. The lower value is for more preferredroute source.Cold start - situation when routers know nothing about the connected devices.Loopback interface - software-only interface that emulates physical interface.
It is used by routing protocols, can be assigned an IP address.Null0 interface simulates exit interface. In is always up and discards traffic. Shouldn't be created orconfigured.Distance vector routingDistance is number of hops to destination, direction represents exit interface.RIP:• Hop-count metric (16 equal infinity).• Updates are broadcasted every 30 seconds.IGRP:• Bandwidth, delay, load and reliability as composed metric.4• Broadcasting updates every 90 seconds by default.EIGRP:• Unequal cost load balancing.• Diffusing Update Algorithm to calculate the shortest path.• No periodic updates.Entire routing table is broadcasted to 255.255.255.255 regularly.Routing protocols algorithm do the following:• send and receive routing information• calculate the best path• detect and reach topology changesDistance vector protocols features:• simple implementation• low resource requirements• slow convergence• limited scalability• routing loopsInitial exchange - updates including only information about directly connected networks.Speed of achieving convergence consists of:• speed of propagating a change to neighbors• speed of calculating best path using collected informationEIGRP uses bounded updates:• non-periodic• partial updates are sent only when topology is changed• only routers that need the information are updatedRouting loop - condition in which packet is continuously transmitting without reaching the destination.May be a result of:• Incorrectly configured static routes.• Incorrectly configured route redistribution.• Routing tables were not updated due to slow convergence.• Incorrectly installed discard routes.Count to infinity problem - situation when updates increase metric to infinity that is no longer reachable.Solution - setting maximum value of hops.Split horizon rule - not advertising a network through the interface from which the update came.
Inother words, router send records that don't contain current exit interface in routing table.Route poisoning is used to mark the route as unreachable and send updates to other routers.Split horizon with poison reverse - principle to ensure that router with unreachable network is not susceptible to incorrect updates about that network.Time to live (TTL) - 8-bit counter in IP header that limits number of hops the packet can traverse.5RIP version 1Features:• distance vector protocol• the only metric is hop count• routes with hop count greater than 15 are unreachable• broadcasting every 30 seconds• classful• no VLSM & CIDR support• administrative distance 120• by default updates are sent to all RIP-configured interfaces (even if there is no RIP-device) (can besolved using passive interface)• can have two exit interfaces with the same network address even if networks are discontiguous• if no version type is specified, receives v1 and v2 updatesTimers:• invalid (setting metric to 16 if update from current route hasn't been received for 180 seconds)• flush (removing route after 240 seconds silence)• holddown (unreachable route stays in this position for 180 seconds to let routers learn about thefailure)Holddown timer usage:1.
Router receives triggered update indicating that some network changed is no longer accessible.2. Router marks the network as possibly down and starts the holddown timer.3. If update with better metric is received, holddown timer is removed.4. Other updates are discarded.5. Packets are still forwarded.Triggered updates (immediately sent):• Interface changes state.• Changing state to/from unreachable.• Route is installed in routing table.Message format:1. data link frame header (MACs)2. IP header packet (IPs, protocol field-17 for UDP)3.
UDP segment header (src/dst port = 520)4. RIP message6••••command (request/responce)versionaddress family id (2 for IP, 0 for requesting full table)IP routes (25 maximum) + metricInformation exchange:1. Each RIP-configured interface sends request for full routing table.2. RIP-enabled neighbors send response.3. Routes are added or updated if needed.4. After updating triggered updates are sent to RIP-enabled interfaces.When there is a packet collision (in hubs), RIP _JITTER contains randomly chosen time to wait beforenext update.Boundary router has interfaces in more than one classful network.Update rules:• If the update and the received interface belong to the same major network, the subnet mask of theinterface is applied to tie network in routing table.• If networks differ, the classful mask of the network is applied.VLSM and CIDRNot supported by RIPv1 and IGRP.Designed for more effective use of IP addresses.Network classes:• A (1-126 in first octet; subnet mask 255.0.0.0)• B (128-191; 255.255.0.0)• C (192-223; 255.255.255.0)Significant bits refer to network, insignificant to host.Variable length subnet mask (VLSM) can be thought as sub-subnetting.• Conserves address space.• Ability to specify a different subnet mask for the same network number and different subnets.E.g.
10.0.0.0/8 can be divided in 256 networks 10.x.0.0/16.Classless inter-domain routing (CIDR) - form of route summarizing.• Reduces the number of entries in routing updates and routing tables.• Reduces bandwidth utilization.Algorithm (calculating a route summary):1. List networks in binary format.2. Count the number of left-most matching bits.3.
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