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Chapter 10: Designing Stable Internets

I. Route Instabilities on the Internet1. Factors that affect route instabilities on the Internet:

1. Interior Gateway Protocol (IGP) instability2. Faulty hardware3. Software proble s!. Insufficient "P# power $. Insufficient e ory%. &etwor' up rades and routine aintenance

. *u an error +. ,in' con estion

2. IGP Instability1. -hen in ectin an IGP into /GP0 you should ta'e easures to pre ent

instability0 such as a re atin the routes so that only the a re ate is in ectedinto /GP pre entin any sin le routes fro causin route flap.

2. So e networ' desi ners are forced to rely on dyna ic routin for alid reasons:1. /GP i ple entations can handle only a fi ed nu ber of networ' entries to be

ad ertised statically. he nu ber of static routes per itted aries fro endorto endor. -hate er that li it is0 networ's that want to o beyond this li itre4uire that ad inistrators in ect the IGP into /GP

2. So e ad inistrators are not co fortable with the fact that the networ's they arestatically ad ertisin i ht beco e unreachable by the router ad ertisin the .

his is understandable0 especially in cases where routes are ad ertised frodifferent points in the 5S. 5d ertisin a route that is not reachable can create

blac' holes.3. Faulty Har !are

1. Faulty hardware can be a proble 0 so it6s i portant to pro ide syste and lin'redundancy.

". So#t!are Proble$s1. Should test software before i ple entin 0 bu s are a reality and it6s i possible

to stop the all.%. Insu##i&ient CP' Po!er

1. "P# is needed to process updates0 peer sessions0 ana e ent plane traffic.7a'e sure you ha e the power necessary for your situation.

(. Insu##i&ient )e$ory1. 7e ory is needed to run the operatin syste alon with storin routin tables0

cache tables databases and bits of software to per it operation. Insufficiente ory can cause the router to not be able to reallocate unused fra ents and

will force you to reboot the router. 7a'e sure you ha e enou h e ory.*. +et!or, 'pgra es an Routine )aintenan&e

1. o reduce the li'elihood of causin disruptions0 networ' chan es should first besi ulated in non production en iron ents if possible. In addition0 ultiple

a or chan es should not be deployed at the sa e ti e. For e a ple0 it isunwise for a pro ider to perfor a or router software up rades0 switchhardware0 and chan e cablin all at the sa e ti e. Good plannin and networ'

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si ulation are the 'eys to successful networ' up rades.-. Hu$an rror

1. *u an error happens0 but ostly it happens because an ad inistrator decides tocircu ent policy and a'es chan es without 'nowled e of possible effects.8ou ust always plan0 prepare and test before i ple entin so that all possibleoutco es are found ahead of ti e. he ore co ple a networ' beco es the

ore infor ation that you ha e to consider before i ple entin additionalconfi urations. 9ules could be deployed that conflict with other rules whichlead to serious perfor ance and connecti ity de radation.

/. in, Congestion1. ,in' con estion can result in a or perfor ance de radation. 5 possible

scenario is when a redundant lin' fails0 and all that traffic is now sent throu hanother lin' on top of what traffic that lin' was already sendin causin the lin'to be con ested. en if the lin' can support the throu hput0 the router i htnot be able to handle the additional load.

II. GP Stability Features1. Controlling Route an Ca&he In ali ation See CH12 Page "2"

1. he use of soft reconfi uration re edies this. Instead of ta'in down the sessionor hard resettin it0 ust use soft reconfi uration to reconfi ure attributes on thefly without 'illin and already established "P session or anually introducina route flap.

2. -hen usin soft reconfi uration thou h0 you need to confi ure the router tostore the routes ad ertised to you (respecti e 5d ;9I/;In0 which should be inlinewith the peer6s 5d ;9I/;<ut) fro the specified peer(s) to be stored in local

e ory.2. GP Route Re#resh See CH12 Page "2/

1. &ewer ersion of soft reconfi uration= /GP route refresh. his re4uests that the peer re;ad ertise all it6s routes (its 5d ;9I/;<# ). his allows all thead ertise eents to run throu h the newly confi ured policies without ha in toreset the "P session and of course re o es the o erhead of e ory and "P#consu ption re4uired when usin soft reconfi uration.

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1. Stability Insi e the 4S1. If /GP is in ected (or so e portion) into the IGP0 route da penin can be useful

to pre ent an IGP eltdown fro route instabilities.

2. Instabilities 5utsi e the 4S1. 9oute >a penin can help the pro ider by not allowin the unstable routes fro

affectin the ISPs 5S0 as well as the rest of the internet. If the pro ider isa re atin the custo ers routes than this is not an issue0 e en if ost of thecusto ers routes are unstable. *owe er if the custo ers prefi is not of the ISPor if the custo er is ultiho in 0 route da penin beco es a consideration.1. Fro the custo ers perspecti e0 if the ISP is da penin the custo ers

routes e en after the custo ers routes beco e stable0 the custo er can bewithout internet connecti ity until their routes beco e unsuppressed.

2. Instabilities can also co e fro the pro ider itself0 and the custo ers boarder routers can6t handle this= thus route da penin is a ust.

3.