Routing Basics Recap
Umberto Poschi
u mberto@ p oschi. it
Most of the content of this presentation is derived from Cisco
docu mentation
Umberto P os c h i
1
Router O per ations
•I l Routing e’ i l t r a s f er i m en t o d el l ’i n f o r m a z i o n e a t t r a v er s o u n a r et e
d a t i d a u n a s o r g en t e a d u n a
•L u n g o i l p er c o r s o a l m en o u n n o d o i n t er m ed i o v i en e i n c o n t r a t o
•I l R o u t i n g , a d u n a a n a l i s i s u p er f i c i a l e, p u o ’ es s er e c o n f u s o c o n
b r id g in g :
• L a d iffe r e n z a p r im a r ia tr a i d u e e ’c h e r o u tin g a v v ie n e a liv e llo 3 d e lla
p ila O S I ( N e to w r k la y e r ) m e n tr e il b r id g in g a liv e llo 2 ( lin k la y e r )
• Q u e s ta d is tin z
b r id g in g u tiliz z
a lla d e s tin a z io
fu n z io n i in m o
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io n e d is
a n o p e r
n e .In q u
d o d e l tu
tin g u
il tr a
e s to
tto d
e a n c h e il c o n te n u to in fo r m a tiv o c h e r o u tin g e
s f e r i m e n t o d e l l ’i n f o r m a z i o n e d a l l a s o r g e n t e
m o d o r tn g e b r d n g a s s o lv o n o a lle lo r o
iffe r e n te
2
L 2
V er ses L 3
F or w ar d ing
• L 2 f orw a rd in g
–D etermine whic h p ort to
f orward the p ac k et f rom to
reac h that d es tination
S w itc h b a s e d o n L 2a d d r e s s
X
–T he layer 2 ad d res s is lef t
intac t ( in mos t c as es )
–T he layer 3 ad d res s is
always ig nored
10.1.1.1
MAC: Z
–Examine the layer two
ad d res s ( X )
X
10.1.1.1
10.1.1.1
M AC: X
10.1.1.2
M AC: Y
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3
L 2
V er ses L 3
F or w ar d ing
• L 3 f orw a rd in g
–D etermine whic h p ort to
f orward the p ac k et f rom to
reac h that d es tination
10.1.1.1
MAC: Z
–Examine the layer three
ad d res s ( 1 0 . 1 . 1 . 1 )
Z
S w itc h b a s e d o n L 3 a d d r e s s
X
–R ewrite the L 2 head er with
the c orrec t L 2 ad d res s f or
the next hop
10.1.1.1
10.1.1.1
M AC: X
10.1.1.2
M AC: Y
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Routing T ab l e
Un ica T a bel l a d i I n stra d a men to costru ita secon d o u n a d el l e seg u en ti
mod a l ita ’:
•S ta tica – l e rotte v en g on o d ef in ite ma n u a l men te
•D in a mica – l e rotte v en g on o a p p rese tra mite u n p rotocol l o d i rou tin g
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S tatic Routes C onf igur ation
172.16.14.33
172.16.14.34
Router(config)#ip route network [m a s k]
{a d d res s | i nterf a c e} [d i s ta nc e] [perm a nent]
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S tatic Route E x am pl e
Router(config) # ip route 173.15.14.32 255.255.252 172.16.14.34
Router(config) # ip route 19 2.168 .15.0 255.255.0
172.16.14.34
172.16.14.33
172.16.14.34
173.15 .14.33
173.15 .14.34
19 2.168 .15 .0 / 24
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D ef aul t Routes
Router(config) # ip route 173.15.14.32 255.255.252 172.16.14.34
Router(config) # ip route 19 2.168 .15.0 255.255.0
172.16.14.34
172.16.14.33
172.16.14.34
173.15 .14.33
173.15 .14.34
Router(config) # ip route 0 .0 .0 .0 0 .0 .0 .0 173.15.14.33
Umberto P os c h i
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V er if y ing th e S tatic
Route C onf igur ation
router#show ip route
C od es: C - c on n ec ted , S
D - E I G R P , E X E 1 - O S P F ex tern
i - I S -I S , L 1 U - per-user sta
G a tewa y
C
S *
of
- sta tic , I - I
E I G R P ex tern a l ,
a l ty pe 1 , E 2 I S -I S l ev el -1 , L
tic route
l a st resort is 0 . 0 . 0 . 0
G R
O
O S
2
P , R - R I
- O S P F , I
P F ex tern
- I S -I S l
to n etwork
P , M - m ob il e,
A - O S P F in ter
a l ty pe 2 , E ev el -2 , * - c a n
B
- B G P
a rea
E G P
d id a te d ef a ul t
0 . 0 . 0 . 0
1 0 . 0 . 0 . 0 / 8 is sub n etted , 1 sub n ets
1 0 . 1 . 1 . 0 is d irec tl y c on n ec ted , S eria l 0
0 . 0 . 0 . 0 / 0 is d irec tl y c on n ec ted , S eria l 0
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T h eL 3
F or w ar d ing T ab l e
• H ow d o w e bu il d a n L 3
f orw a rd in g ta bl e?
–R ou ters c ommu nic ate
u s ing rou ting p rotoc ols
–R ou ting p rotoc ols
exc hang e reac hab ility and
top olog y inf ormation
• R ou tin g p rotocol s bu il d a
l oca l top ol og y ta bl e or
d a ta ba se con ta in in g
rea cha bil ity in f orma tion
Umberto P os c h i
…
R e a c h a b ility a n d
to p o lo g y
in fo r m a tio n
10.1.1.0/24 via x .x .x .x
10.1.2.0/24 via y .y .y .y
....
T o p o lo g y
ta b le /d a ta b a s e
10
T h eL 3
F or w ar d ing T ab l e
• R ou ting p rotoc ols then ins tall
the “b es t p ath” f or eac h
d es tination into the loc al
rou ting tab le ( R I B )
–T h e b e s t p a t h i s h i g h l y
p ro to c o l d e p e n d a n t
• T he R I B
c ontains
–T h e s e t o f r e a c h a b l e
d e s tin a tio n s
–T h e n e x t h o p L 3 a d d r e s s t o
s e n d tr a ffic to
–T h e i n t e r f a c e t h r o u g h w h i c h t o
re a c h th e n e x t h o p
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…
R e a c h a b ility a n d
to p o lo g y
in fo r m a tio n
10.1.1.0/24 via x .x .x .x
10.1.2.0/24 via y .y .y .y
....
T o p o lo g y
ta b le /d a ta b a s e
10.1.1.0/24 via x .x .x .x
10.1.2.0/24 via y .y .y .y
....
R o u tin g T a b le
( R I B)
11
T h eL 3
F or w ar d ing T ab l e
• T he F I B is b u ilt f rom the R I B ,
and c ontains , es s entially, the
s ame inf ormation as the R I B
–T h e R I B i s o p t i m i z e d f o r
in te r a c tio n w ith r o u tin g
p r o to c o ls a n d h u m a n s
–T h e F I B i s o p t i m i z e d f o r
fo r w a r d in g
• T he F I B
c ontains
–T h e r e a c h a b l e d e s t i n a t i o n
–T h e n e x t h o p L 2 a d d r e s s ( t h e
M A C h e a d e r r e w r ite s tr in g )
–T h e i n t e r f a c e t h r o u g h w h i c h t o
fo r w a r d th e tr a ffic
…
R e a c h a b ility a n d
to p o lo g y
in fo r m a tio n
10.1.1.0/24 via x .x .x .x
10.1.2.0/24 via y .y .y .y
....
T o p o lo g y
ta b le /d a ta b a s e
10.1.1.0/24 via x .x .x .x
10.1.2.0/24 via y .y .y .y
....
R o u tin g T a b le
( R I B)
10.1.1.0/24 via x .x .x .x
10.1.2.0/24 via y .y .y .y
....
F o r w a r d in g T a b le
( F I B)
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T h eL 3
C o d e s :
C
- c o n n e c t e
D
- E I G R P , E
router#show
N 1 - O S P F N S
. . E . 1. - O S P F e x
i - I S -I S , L
* - c an d id at
- p e y r ioof d icl
G a P tewa
C
S
d , S - s t at ic , R - R I P , M
X ip- route
E I G R P e x t e r n al , O - O
S A e x t e r n al t y p e 1, N 2 t e r n al t y p e 1, E 2 - O S P F
1 - I S -I S l e ve l -1, L 2 - I
e d e f au l t , U - p e r -u s e r s
o w n resort
l o ad e d s is
t at icn otr o set
u t e
a d st
2 0 8 . 0 . 1 2 . 0 / 2 4
1 0 . 0 . 0 . 0 / 2 4
1 0 . 7 . 7 . 0
C
1 0 . 1 . 1 2 . 0
S
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F or w ar d ing T ab l e
[ 1 / 0 ]
is v a ria b l y
S
1 7 2 . 1 6 . 2 . 0 / 2 3
1 9 2 . 1 6 8 . 0 . 0 / 2 4
1 9 2 . 1 6 8 . 0 . 0 / 1 6
[ 1 / 0 ]
[ 1 / 0 ]
b il e ,
I A N S S A
n al t
l e ve l
r o u t
2
c on n ec ted ,
v ia
v ia
is d irec tl y
is d irec tl y
sub n etted ,
B
O S P
e x t
y p e
-2,
e ,
- B G P
F in t e
e r n al
2
ia o - O D
r
ar e a
t y p e 2
I S -I S
R
in t e r
ar e a
S eria l 0 / 2
sub n ets
1 0 . 1 . 1 2 . 1
is d irec tl y
1 7 2 . 1 6 . 1 . 0 / 2 4
C
v ia
m o
,
F
e r
S
ic
c on n ec ted ,
is sub n etted ,
1 7 2 . 1 6 . 0 . 0 / 1 6
S
is d irec tl y
S P F
O S P
e x t
S -I
t at
F a stE thern et0 / 1
1 0 . 1 . 1 2 . 1
1 0 . 1 . 1 2 . 1
c on n ec ted ,
c on n ec ted ,
2
sub n ets,
2
m a sk s
S eria l 0 / 1
N ul l 0
13
T h eL 3
F or w ar d ing T ab l e
N etwork router#show
+ R ou te
. . . .
N etwork
G a tewa y
C
S
of
l a st resort is n ot set
R ou te
2 0 8 . 0 . 1 2 . 0 / 2 4
1 0 . 0 . 0 . 0 / 2 4
1 0 . 7 . 7 . 0
C
1 0 . 1 . 1 2 . 0
S
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ip route
[ 1 / 0 ]
1 9 2 . 1 6 8 . 0 . 0 / 2 4
1 9 2 . 1 6 8 . 0 . 0 / 1 6
[ 1 / 0 ]
[ 1 / 0 ]
2
c on n ec ted ,
v ia
v ia
is d irec tl y
is d irec tl y
S eria l 0 / 2
sub n ets
1 0 . 1 . 1 2 . 1
is v a ria b l y
1 7 2 . 1 6 . 2 . 0 / 2 3
S
v ia
is d irec tl y
1 7 2 . 1 6 . 1 . 0 / 2 4
C
c on n ec ted ,
is sub n etted ,
1 7 2 . 1 6 . 0 . 0 / 1 6
S
is d irec tl y
sub n etted ,
F a stE thern et0 / 1
1 0 . 1 . 1 2 . 1
1 0 . 1 . 1 2 . 1
c on n ec ted ,
c on n ec ted ,
2
sub n ets,
2
m a sk s
S eria l 0 / 1
N ul l 0
14
T h eL 3
F or w ar d ing T ab l e
M aj or network
with
router#show
ip s route
s u b nets s how u p u nd er a s ing le
network with mu ltip le rou tes
. . . .
k rou is
tes n ot
s how
G S a ing
tewaley nativ
of l a estmas
resort
set
C
S
1 9 2 . 1 6 8 . 1 2 . 0 / 2 4
1 0 . 0 . 0 . 0 / 2 4
1 0 . 7 . 7 . 0
C
S
1 0 . 1 . 1 2 . 0
is sub n etted ,
[ 1 / 0 ]
1 7 2 . 1 6 . 0 . 0 / 1 6
S
is d irec tl y
is v a ria b l y
S
1 7 2 . 1 6 . 2 . 0 / 2 3
1 9 2 . 1 6 8 . 0 . 0 / 2 4
1 9 2 . 1 6 8 . 0 . 0 / 1 6
[ 1 / 0 ]
[ 1 / 0 ]
c on n ec ted ,
c on n ec ted ,
v ia
v ia
is d irec tl y
is d irec tl y
S eria l 0 / 2
sub n ets
1 0 . 1 . 1 2 . 1
is d irec tl y
1 7 2 . 1 6 . 1 . 0 / 2 4
C
v ia
2
u p as a s ing le entry
sub n etted ,
F a stE thern et0 / 1
1 0 . 1 . 1 2 . 1
1 0 . 1 . 1 2 . 1
c on n ec ted ,
c on n ec ted ,
2
sub n ets,
2
m a sk s
S eria l 0 / 1
N ul l 0
N ativ e mas k rou tes and their s u p ernets s how u p as
d if f erent network s
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Route S el ection
• T he metric is the cost of the rou te, a s comp u ted a n d rep orted
by the rou tin g p rotocol w hen in sta l l in g the rou te in the R I B
• T he time show n is the a mou n t of time sin ce the rou te w a s
tou ched
–EI G R P rec alc u lation of any typ e, inc lu d ing los ing an alternate
p ath, res ets this timer ( s ometimes )
–O S P F S P F ru n res ets this timer ( s ometimes )
–I S -I S S P F ru n res ets this timer ( s ometimes )
L as t M od if ic ation T ime
M etric
D
E X
1 9 2 . 1 6 8 . 2 5 4 . 0 / 2 4
Umberto P os c h i
[ 1 7 0 / 3 0 7 2 2 5 6 ]
v ia
2 0 8 . 0 . 2 4 6 . 1 0 ,
0 0 : 5 8 : 4 5 ,
S eria l 3 / 0
16
L ongest M atch
• W
w
in
ta
F or w ar d ing
hich of these tw o rou tes
il l be chosen f or
sta l l a tion in the rou tin g
bl e?
10 .1.1.0 / 24
C o s t : 10 0
A D : 9 0
v s .
10 .1.1.0 / 25
C o s t : 30
A D : 115
?
R o u tin g
T a b le
Umberto P os c h i
17
L ongest M atch
• W
w
in
ta
F or w ar d ing
hich of these tw o rou tes
il l be chosen f or
sta l l a tion in the rou tin g
bl e?
• Both! The prefix length is
d ifferent ( / 2 4 v s. / 2 5 ) , so they
a re d ifferent d estina tions!
• W hich on e w ou l d be u sed to
f orw a rd a p a ck et to 1 0 . 1 . 1 . 1 ?
10 .1.1.0 / 24
C o s t : 10 0
A D : 9 0
10 .1.1.0 / 25
C o s t : 30
A D : 115
R o u tin g
T a b le
?
10 .1.1.1
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L ongest M atch
F or w ar d ing
• W hic h one wou ld b e u s ed to
f orward a p ac k et to 1 0 . 1 . 1 . 1 ?
• 1 0 .1 .1 .0 /2 5
–B e c a u s e 10 .1.1.1 f a l l s w i t h i n
10 .1.1.0 / 25
–T h
d e s
th e
a lw
p a c
e ro u te w
tin a tio n a
lo n g e s t p
a y s u s e d
k e t
ith
d d
re
to
in w
re s s
fix m
fo rw
h ic h
fa lls
a tc h
a rd t
th e
w ith
is
h e
–T h i s i s c a l l longest match
f or w ar d i ng, o r longest p r ef i x
r ou ti ng
10 .1.1.0 / 24
C o s t : 10 0
A D : 9 0
10 .1.1.0 / 25
C o s t : 30
A D : 115
R o u tin g
T a b le
?
10 .1.1.1
10 .1.1.1
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19
A ggr egation T h eor y
–W e d o n ’t a d v e r t i s e r e a c h a b i l i t y
to in d iv id u a l h o s ts , b u t to
n e tw o rk s
• A s s ig ning 1 9 2 . 1 6 8 . 1 . 0 / 2 9 means
any ad d res s f rom 1 9 2 . 1 6 8 . 1 . 0
throu g h 1 9 2 . 1 6 8 . 1 . 7 is
reac hab le on this interf ac e
.1
1 9 2 .1 6 8 .1 .0 /2 9
• I P ad d res s ing is b u ilt arou nd
the c onc ep t of s u mmariz ing
reac hab ility inf ormation.
.2
.3
.4
.5
.6
–E a c h a c t u a l a d d r e s s m a y n o t
b e r e a c h a b l e , b u t i f i t i s , i t ’s
r e a c h a b le th r o u g h th is in te r fa c e
–I f a s
s o m e
a d v e r
in fo r m
Umberto P os c h i
p e c ific is r e a c h a b le o n
o th e r in te r fa c e , w e w o u ld
tis e m o r e s p e c ific r o u tin g
a tio n
20
A ggr egation T h eor y
• I n the s ame way, s u mmariz ing
mu ltip le network s into one
ad v ertis ement j u s t inc reas es
the s c op e of reac hab le hos ts
ters and d ev ic
mmariz ation p
f rom 1 9 2 . 1 6 8 . 1
8 . 1 . 1 5 are reac
hA
es b eyond
oint, all the
. 0 throu g h
hab le
1 9 2 .1 6 8 .1 .0 /2 9
.1
A
Umberto P os c h i
1 9 2 .1 6 8 .1 .8 /2 9
• T o rou
the s u
hos ts
1 9 2 .1 6
throu g
8 . 1 . 0 / 2 9 and
8 . 1 . 8 / 2 9 c an b e
ated ( s u mmariz ed ) to
v ertis ement,
8 .1 .0 /2 8
1 9 2 .1 6 8 .1 .0 /2 8
• 1 9 2 .1 6
1 9 2 .1 6
ag g reg
one ad
1 9 2 .1 6
.1
.2
.3
.4
.5
.6
.2
.3
.4
.5
.6
21
• S een f rom the bin a ry
p ersp ectiv e, a s y ou ma k e
the p ref ix l en g th shorter,
y ou mov e the n etw ork / host
sep a ra tion l in e to the l ef t.
28 d e s t i n a t i o n s
( 1 9 2 . 1 6 8 ) . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
( 1 9 2 . 1 6 8 ) . 0 0 0 0 0 0 1 0 . 0 0 0 0 0 0 0 0
( 1 9 2 . 1 6 8 ) . 0 0 0 0 0 0 1 1 . 0 0 0 0 0 0 0 0
21 0 d e s t i n a t i o n s
( 1 9 2 . 1 6 8 ) . 0 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 0
2 2 b its
• A s y ou mov e the red l in e to
the l ef t, y ou en comp a ss
more rea cha bl e d estin a tion s
in the sa me a d v ertisemen t,
bu t y ou ha v e f ew er
a d v ertisemen ts.
2 4 b its
A ggr egation T h eor y
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22
A ggr egation T h eor y
• A ssu min g 1 9 2 . 1 6 8 . 0 . 0 / 2 4
d oesn ’t ex ist. . . .
• 1 9
1 9
1 9
a d
2 .1 6 8 .1 .0
2 .1 6 8 .2 .0
2 .1 6 8 .3 .0
v ertised
• R a ther tha n
ea ch w ith 2
( 2 5 4 hosts)
sin g l e n etw
a d d resses
19 2.16 8 .0.0/ 22
1 n e tw or k
1 0 24 a d d r e sse s
/2 4 ,
/2 4 , a n d
/ 2 4 ca n be
a s1 9 2 .1 6 8 .0 .0 /2 2
3 n e tw or k s
256 a d d r e sse s e a c h
three n etw ork s,
5 6 a d d resses
, A a d v ertises a
ork , w ith 1 0 2 4
19 2.16 8 .1.0/ 24
19 2.16 8 .2.0/ 24
19 2.16 8 .3 .0/ 24
254 hosts
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23
A ggr egation T h eor y
• A d d res s s u mmariz ation als o
hid es c hang es in the network .
• Ev en if
C f ails ,
1 9 2 .1 6 8
( as long
reac hab
the link
A c an s
.0 .0 /2 2
as 1 9 2
le v ia s
su m m a r y d oe sn ’t
c ha n g e !
b etween A and
till ad v ertis e the
ad d res s s p ac e
. 1 6 8 . 2 . 0 / 2 4 is n’t
ome other p ath) .
19 2.16 8 .0.0/ 22
A
• R ou ters b eyond A d on’t need to
k now ab ou t the reac hab ility or
top olog y c hang e.
B
C
D
19 2.16 8 .1.0/ 24
19 2.16 8 .2.0/ 24
19 2.16 8 .3 .0/ 24
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24
A ggr egation T h eor y
• W hen a su mma ry is crea ted
throu g h a n y mecha n ism in
C isco I O S , a d isc a rd rou te is
in sta l l ed in the l oca l R I B
A
–S ame p ref ix and leng th as
s u mmary
–A d minis trativ e D is tanc e of
5
B
–N ext hop of N U L L 0
• W hy ?
r o u t e r ( c o n f ig -in t ) # ip
r o u t e r # s h o w
....
D
Umberto P os c h i
ip
10.1.0.0/23
r o u t e
is
s u m m ar y -ad d r e s s
a s u m m ar y ,
e ig r p
00: 00: 08 ,
100 10.1.0.0 25 5 .25 5 .25 4.0
N u l l 0
25
A ggr egation T h eor y
• W ha t w ou l d ha p p en if the
d isca rd rou te w a sn ’t
crea ted ?
A
–B has a rou te to 0 . 0 . 0 . 0 / 0
with a next hop of A
r o u t e r # s h o w
ip
r o u t e
r o u t e r # s h o w
ip
r o u t e
D
10.1.0.0/23
via < B >
–B
has a s u mmary f or
1 0 . 1 . 0 . 0 / 2 3 c onf ig u red
–A u s es B as the next hop to
1 0 .1 .0 .0 /2 3
–10.1.0.0/24 does not exist in
th is netw or k !
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B
D
0.0.0.0/0 via < A >
26
A ggr egation T h eor y
• A rec eiv es a p ac k et d es tined f or
1 0 .1 .0 .1
A
–T h e b e s t r o u t e A h a s f o r t h i s
d e s t i n a t i o n i s a l o n g 10 .1.0 .0 / 23
v ia B
• B
r o u t e r # s h o w
ip
r o u t e
r o u t e r # s h o w
ip
r o u t e
D
–A f o r w a r d s t h e p a c k e t t o B
10.1.0.0/23
via < B >
rec eiv es this p ac k et d es tined
to 1 0 . 1 . 0 . 1
–T h e b e s t r o u t e B h a s f o r t h i s
d e s tin a tio n is a lo n g th e d e fa u lt
r o u te , v ia A
–B
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fo rw a rd s th e p a c k e t to A
B
D
0.0.0.0/0 via < A >
27
A ggr egation T h eor y
• I f B had a the d is c ard rou te I O S
au tomatic ally c reates
A
• A rec eiv es a p ac k et d es tined f or
1 0 .1 .0 .1
r o u t e r # s h o w
ip
r o u t e
r o u t e r # s h o w
ip
r o u t e
D
10.1.0.0/23
–T h e b e s t r o u t e A h a s f o r t h i s
d e s t i n a t i o n i s a l o n g 10 .1.0 .0 / 23
v ia B
• B
via < B >
–A f o r w a r d s t h e p a c k e t t o B
rec eiv es this p ac k et d es tined
to 1 0 . 1 . 0 . 1
–T h e b e s t r o u t e B h a s f o r
10 .1.0 .1 i s a l o n g t h e d i s c a r d
r o u t e t o 10 .1.0 .0 / 23
B
D
0.0.0.0/0 via < A >
fo rw a rd s th e p a c k e t to
N U L L 0 , d is c a r d in g th e p a c k e t
–B
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28
A ggr egation T h eor y
• Many protocols allow you to remove the discard
route
–E I G R P a l l ow s y ou to set the a d min istra tiv e d ista n ce of the
d isca rd rou te so it is ig n ored or simp l y n ot in sta l l ed
–O S PF a l l ow s y ou to n ot crea te the d isca rd rou te
• T his is usef ul in some situations, b ut you must b e
very caref ul with this capab ility
• R outing loops can easily occur in a situation where
the discard route has b een removed
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29
W
h at I s a Routing P r otocol ?
• Routing p r otoc ol s a r e
us e d b e tw e e n
r oute r s to d e te r m ine p a th s
a nd m a inta in
r outing ta b l e s .
• A f te r th e p a th is d e te r m ine d , a
r oute r c a n r oute a r oute d
p r otoc ol .
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30
Routing A l ogor ith im
s
• S ta tic v e r s u s d y n a m ic
• S i n g l e -p a t h v e r s u s m u l t i p a t h
• F la t v e r s u s h ie r a r c h ic a l
• H o s t -i n t e l l i g e n t v e r s u s r o u t e r -i n t e l l i g e n t
• In tr a d o m a in v e r s u s in te r d o m a in
• L i n k -s t a t e v e r s u s d i s t a n c e v e c t o r
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S ingl e P ath
v s M ul tiP ath
• S ome sophisticated routing protocols support
multiple paths to the same destination.
–mu l tip l ex in g ov er mu l tip l e l in es.
• C an provide sub stantially b etter throug hput and
reliab ility.
–T his is g en era l l y ca l l ed l oa d sha rin g .
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32
F l at v s h ier ar ch ical
• In
• In
to
• R o
d o
a f l a t r outing s
a h i er a r c h i c a l
a r o u tin g b a c k
u t i n g s y s t em s
m a in s , a u to n o
y s te m ,
r o u tin g
b o n e. .
o f t en d
m o u s s
t h e r o u t er s a r e p eer s o f a l l o t h er s .
s y s t em , s o m e r o u t er s f o r m w h a t a m o u n t s
es i g n a t e l o g i c a l g r o u p s o f n o d es , c a l l ed
y s t em s , o r a r ea s .
–I n hierarchical systems, s o m e r o u t e r s i n a d o m a i n c a n c o m m u n i c a t e
w it h r o u te r s in o th e r d o m a in s , w h ile o th e r s c a n c o m m u n ic a t e o n ly w ith
r o u te r s w ith in th e ir d o m a in .
–I n v e r y l a r g e n e t w o r k s , a d d i t i o n a l h i e r a r c h i c a l l e v e l s m a y e x i s t , w i t h
r o u te r s a t th e h ig h e s t h ie r a r c h ic a l le v e l fo r m in g th e r o u tin g b a c k b o n e .
• H i er a r c h i c a l r o u t i n g m i m i c s t h e o r g a n i z a t i o n o f m o s t c o m p a n i es
a n d t h er ef o r e s u p p o r t s t h ei r t r a f f i c p a t t er n s w el l .
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33
Host Intelligence vs Router Intelligence
• S o m er o u tin g a lg o r ith
d et er m i n e t h e en t i r e r
r outing. I n s o u r c e-r o u
a n d -f o r w a r d d ev i c es ,
s to p .
m s a s s u m
o u t e. T h i s
t i n g s y s t em
m i n d l es s l y
• O t h er a l g o r i t h m s a s s u m e t h a t h o
t h es e a l g o r i t h m s , r o u t er s d et er m
i n t er n et w o r k b a s ed o n t h ei r o w n
t h e h o s t s h a v e t h e r o u t i n g i n t el l i g
h a v e t h e r o u t i n g i n t el l i g en c e.
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eth a t th es
is u s u a lly r
s , r o u t er s
s en d i n g t h
o u r c e en d n o d e w i l l
ef er r ed t o a s s our c e
m er el y a c t a s s t o r ee p a c k et t o t h e n ex t
s t s k n o w n o t h i n g a b o u t r o u t es . I n
in eth ep a th th r o u g h th e
c a l c u l a t i o n s . I n t h e f i r s t s y s t em ,
en c e. I n t h e l a t t er s y s t em , r o u t er s
34
I ntr ad om
ain v s I nter D om ain
• S ome routing alg orithms work only within domains;
others work within and b etween domains. T he
nature of these two alg orithm types is dif f erent. I t
stands to reason, theref ore, that an optimal
intradomain-routing alg orithm would not
necessarily b e an optimal interdomain-routing
alg orithm.
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35
A utonom ous S y stem s: I nter ior or
E x ter ior Routing P r otocol s
• An autonomous system is a collection of networks
und er a common ad ministrativ e d omain.
• I G P s op erate with in an autonomous system.
• E G P s connect d ifferent autonomous systems.
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36
D ef aul t A d m
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inistr ativ e D istances
37
C l assf ul Routing O v er v iew
– C l a ssf u l rou tin g p rotocol s d o n ot in cl u d e the su bn et ma sk w ith
the rou te a d v ertisemen t.
– W ithin the sa me n etw ork , con sisten cy of the su bn et ma sk s is
a ssu med .
– S u mma ry rou tes a re ex cha n g ed betw een f oreig n n etw ork s.
– T hese a re ex a mp l es of cl a ssf u l rou tin g p rotocol s:
• R I P v ers ion 1 ( R I P v 1 )
• IG R P
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38
C l assl ess Routing O v er v iew
–Classless routing protocols include the subnet mask with the
route adv ertisement.
–Classless routing protocols support v ariable-length subnet
mask ( V L S M ) .
–S ummary routes can be manually controlled within the network.
–T hese are ex amples of classless routing protocols:
•RIP version 2 (RIPv2)
•E IG RP
•O S PF
•IS -IS
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39
Routing P r otocol C om
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par ison C h ar t
40
U sing th e ip cl assl ess C om
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m
and
41