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Lecture 18
Internet Routing: The goal is to find any route that is loop free-Global optimization is a distant dream depending on economic and political drivers
Homework: 4.1-35, 37-45
Distance-vector routing: A’s routing table just after link A-E failure
D C NH
B B
C C
D C
E -
F 1 F
G F
Dest Cost NH
B B
C C
D C
E -
F 1 F
G F
Dest. Cost NHop
A A
B A
C A
D G
E 2 A
G G
A
F
Dest. Cost NHop
A A
B A
C A
D G
E -
G G
Dest. Cost NHop
A F
B F or C
C D
D D
E 3 F
F F
G
Dest. Cost NHop
A F
B F
C D
D D
E 3 F
F F
Dest. Cost NHop
A F
B F
C D
D D
E -
F F
What happens when G advertises its table to D? How does D get the message that A-E has failed?
Loops -A
• A advertises [E, ] to B and B advertises [E, 2] to A (they cross intransit)
• A advertises [E,3] and B advertises [E, ]
• etc.
• B resets [E, , -] (since A is next hop) A updates [E, 3, B] since 3<
• B updates [E, 4,A] and A resets [E, , -] since B is next hop
• etc.
Split horizon: B should not advertise a route
[E, 2] it got from A (in the first step)
Loops -B
• A advertises [E, ] to B & C
Advertisement to C is delayed
• B may advertise [E, ] to C
• C advertises [E, 2] to B
• A’s advertisement [E, ] arrives at C
• B advertises [E,3] to A
• C advertises [E, ] to B
• A advertises [E,4] to C
• B advertises [E, ] to A
• C advertises [E,5] to B
• B resets [E, , -] since A is next hop
• C will not update (why)
• B updates [E, , C] since 3<• C updates [E, , -] since A is next hop
• A updates [E,4, B] since 4<• B updates [E, , -] since C is next hop
• C updates [E, 5, A] since 5<
• A updates [E, , -] since B is next hop
• B updates [E, 6, A] since 6<
Split horizon will not solve this problem
Link State Routing
A B C D
A
B
C
D
You have a global view: routing table is spanning tree as seen from root node
The Internet circa 1990: A hierarchical collection of autonomous systems (AS)
Today’s multiple backbone
Stub AS
Multi-homed AS
Transit AS
Inefficiency of IP address classes
• If you have 257 end users, you need class B and then you have 16K addresses. We need finer distinctions.
• Two issues:– How do you give different network addresses to
physical networks within 1 class A, B or C network---subnetting
– How do you aggregate networks within an domain to simplifier routing outside the domain--supernetting
Forwarding Table of R1
Subnet Number Subnet Mask NextHop
128.96.34.0 255.255.255.128 Interface0
128.96.34.128 255.255.255.128 Interface 1
128.96.33.0 255.255.255.0 R2
More Subnetting
• You can break the same physical network into subnets—forcing hosts to speak through a router
• Each host has it’s own subnet in the new engineering network
• Subnet mask=IP address---non-contiguous 1’s and 0’s
Classless Interdomain Routing (CIDR)
• We can get better utilization if we hand out Class C addresses
• This would increase the size of forwarding tables
• We aggregate contiguous class C blocks
Aggregation of 16 Class C network addresses into a single 20 bit CIDER address
Class C network numbers
192.4.00010000.XYZ
192.4.00010001.XYZ
192.4.00010010.XYZ CIDR 20 bit network address
192.4.00010011.XYZ 192.4.0001 =194.4.16/20
192.4.00010100.XYZ
192.4.00010101.XYZ
192.4.00010110.XYZ
192.4.00010111.XYZ
192.4.00011000.XYZ
192.4.00011001.XYZ
192.4.00011010.XYZ
192.4.00011011.XYZ
192.4.00011100.XYZ
192.4.00011101.XYZ
192.4.00011110.XYZ
192.4.00011111.XYZ
Intradomain Routing
Border Gateway Routers—Default router for outbound traffic
Intradomain routing issues
• Scale—100,000 network addresses
• Autonomy
• Trust
• Flexibility—hot potato routing
BGP example
1 BGP speaker/AS: Advertise routes-prevents looping
Withdrawn broken routes
Routing Areas in OSPF