TELE 301Lecture 19: Interior Routing

David Eyers (dme@cs.otago.ac.nz)Telecommunications Programme

University of Otago

Today in Brief

Fundamental ConceptsRIP, its use and problemsBrief overview of OSPFVyattaVLANs

Static Routing

Advantagessimplicityless software that might fail

Disadvantageslimits on scalabilityless resilience and load-balancingeasy to miss return routes

Dynamic Routing

Routers calculate optimal routesfail-over and fail-backload balancingScalableessential complexity vs. accidental complexity

Autonomous Systems

“An AS is a connected group of one or more IP prefixes run by one or more network operators which has a single and clearly defined routing policy.” (RFC 1930)Identified by 16-bit ASNDetails depend on protocol(Shortages causing move to 32-bit ASNs)

Interior and Exterior

Interior Routing Protocols (IRP)Intra-ASeasy to set upauto-discovery of network topology

Exterior Routing Protocols (ERP)Inter-AScomplexity to match peering arrangementsexplicit choice of peers

Metric and Distance

Metric compares routese.g. hop count, speed, delay, reliabilityheterogeneous metrics incomparable

“Administrative Distance” (Cisco) compares protocols tooeg. direct=0, static=1, BGP=20, OSPF=110, RIP=120

low is better (more reliable)

✓

Formulated Metrics

Support heterogeneous networksOSPF108/bwidth e.g. 256kbps ⇒ 3,906,250

IGRP uses bandwidth, delaycan also include reliability, load and MTU

load flux can lead to “route flapping”

Multipath Routing

Boost capacityequal or proportionately

tests backup link(s) at same time

Process Switchingpacket-by-packet; never uses route cache

Fast SwitchingdestIP-by-destIP; first packet causes route table lookup, rest from route cache

RIP

Routing Information Proto.Early, easy, commonstill used today in SOHO gear

set and forget; few knobs

Distance-Vector algorithmrouting-by-rumour

metric is hop-countmetric of 16 indicates unreachability

alternative routes are not kept

RIP Metric

Network with Fast Ethernet links between A-B and B-C, and a 802.11g backup link A-CWhat is RIP’s next hop for A to C? A B

C

FE (100Mbps)

FE

(1

00

Mb

ps)

802.11g (<

54Mbps)

RIP LimitationsNo Variable Length Subnet Masks supporteg. /28, but is supported in RIPv2Slow convergence (~270s)Full advertisements broadcast every 30sLimited diameter of network (15 hops)Only metric is hop-countpoor support for heterogeneous networksAdvertisements not authenticated (v1)attacking the network made very easy

RIP is okay when…

Small networkor minimal IT support

Homogeneous networkwith respect to link speed

Lengthy convergence times OKLimited choice within hardwareSOHO grade may dictate RIP or RIPv2

RIPv2 and RIPng

RIPv2VLSM and authentication

RIPngRIP for IPv6. VLSM. No authentication

Count-to-Infinity

RtrA loses connection to NetA

Before RtrA sends update (of break), RtrB sends full update, advertising NetA at a cost of 1

RtrA now thinks it can get to NetA via RtrB at a cost of 1+1=2 hops

RtrA advertises this to RtrB, which sees an increased cost. RtrB advertises 2+1 = 3, continues to 16 (infinity/unreachability)

NetA NetBRtrBRtrA

Optimisations

Split Horizon w/ Poison Reversedon’t send routing information about a network on the interface it was received on, unless it is to tell the source that you recognise a route as being unreachable.

Triggered Updatessend updates as soon as something changes about a route’s metric or state

Optimisations (2)

Hold-down timerupon failure, hold the route as down until the network has converged, to expire old information

Although it will accept routes that are better than the original

Route Summarisation

Reduce size of routing tablessupernetting (route aggregation)

reflects hierarchical nature of a network

192.168.1.32/28 + 0010 0000192.168.1.48/28 = 0011 0000192.168.1.32/27 001we own everything in 192.168.1.32/27

Further Enhancements

incremental updateshello packetskeep alternativesauthenticate advertisementsall-routers multicastbroadcast stops at routers

Link State versus DV

Distance Vector“routing by rumour”

unaware of topology

Link Statemulticast local link states to all routers

each router then assembles topology

higher memory and processor reqs.

OSPF

Open Shortest Path FirstLink State protocol

Break AS into smaller areassimplifying calculation requirements

routes summarised crossing areas

Areas attached to backbonearea 0 is backbone area

External RIPNetwork

Area 0

Area 2

Area 1

Area 5

10.5

.0.0

/16

10.4.0.0/16

10.1.0.0/16

Inve

rcar

gill

Wellington

10.2.0.0/16

Area 4Auckland

10.0.0.0/16

Dunedin

R1

R6

R4

R3R2R7

R8

R5

each router be situated?Question: In which city will

Question: Classify each router.

OSPF Summary

Very large, complex, well-designed networks with skilled staffHigh memory and CPU requirementsFast convergence (but not fastest)There are others… EIGRP, IS-IS

Debian GNU/Linux-based routing platform called Vyattaruns on x86 COTS hardwareFeels like a real router!CLI similar to JUNOS interfaceRouting engine is QuaggaSupports RIP, OSPF, BGPInstallable Live CD

Vyatta Lab primer

Virtual LANsUse of VyattaStatic routing, and RIP

VLAN Intro

If we have time in the lecture, we shall preview a bit of information about Virtual LANs.If not, don’t worry, you’ll read about it in the labbook.

Broadcast Domains

VLAN Motivations Port Assignments

References

IP Routing Ravi MalhotraPublished by O’ReillyISBN: 0-596-00275-0TCP/IP Network Administration Craig Hunt Published by O’ReillyISBN: 1-56592-322-7