static routing dynamic routing - university of otago · 2012-05-29 · static routing advantages...
TRANSCRIPT
TELE 301Lecture 19: Interior Routing
David Eyers ([email protected])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