Transcript - Suraj @ LUMS

CS 672
Summer 2003
CS672: MPLS Architecture,
Applications and Fault-Tolerance
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CS 672
Summer 2003
Lecture 1
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CS 672
Summer 2003
Autonomous System (AS)
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AS is commonly defined as a set of interconnected routers that:
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Examples of AS include:
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are managed by single technical administration
use one or more interior gateway protocols (IGP) to route packets within the AS
(intra-AS)
use exterior gateway protocols (EGPs) to route packets between AS’s (inter-AS)
represent a single coherent picture of its reachable destinations.
ISP, larger organization (e.g., college campus, enterprise), …
To forward packets within (intra) and between (inter) ASs two different
types of protocols namely IGPs and EGP.
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IGPs include OSPF, IS-IS, …
EGPs include Border Gateway Protocol version 4 (BGP-4)
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CS 672
Summer 2003
Intra- and Inter-domain Routing
AS 1
Intra-domain routing
IGP (OSPF, IS-IS, etc.)
AS 2
Intra-domain routing
IGP (OSPF, IS-IS, etc.)
AS 3
BGP-4
Inter-domain routing
Intra-domain routing
IGP (OSPF, IS-IS, etc.)
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CS 672
Summer 2003
Stub, Multihomed and Transit AS’s
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If either source or destination of IP traffic resides inside an AS, that traffic
is called local traffic for the AS. Otherwise, it is known as transit traffic.
Based on local and transit traffic , AS can be classified into:
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Stub AS
– has a single connection to the other AS
– carries local traffic only (i.e., either source or sink)
Multihomed AS
– has connections more than one AS’s
– carries local traffic only
Transit AS
– Has connections to more than one AS’s
– carries local as well as transit traffic
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CS 672
Summer 2003
Stub or single home AS
AS 1
(e.g.,service provider)
Stub AS
(e.g., enterprise)
Stub AS
(e.g., enterprise)
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Summer 2003
Multihomed Non-transit AS
AS 1
(e.g.,service provider A)
AS 2
(e.g.,service provider B)
Stub AS
(e.g., enterprise)
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CS 672
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Transit AS
Transit AS
(e.g., backbone provider)
AS 2
(e.g.,service provider B)
AS 2
(e.g.,service provider B)
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CS 672
Summer 2003
Static Routing
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A router learns about routes (i.e., destination prefixes) in two ways:
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configurations and routing protocols
Routes that are manually configured are known as static routes. Static
routes continue to remain in the routing table independent of status of
the network and destination reachability.
Advantages of Static Routing
Simpler because does not require exchanging routing information.
 Does not cause routing churn in the routers due change in network topology
events (e.g., link up/down events).
Disadvantages
 Does not reflect actual status of the network and may result in blackholing the
traffic
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CS 672
Summer 2003
Dynamic Routing
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Routes that are learned through routing protocols are known as dynamic
routes. Dynamic routes are added or deleted based on network topology.
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Advantages of dynamic routing
 reachability of dynamic routes depends upon the status of the network. For
instance, if certain destination is not reachable due to link/node failure that
route is removed from the routing table.
Disadvantages of dynamic routing
 requires routing protocols and exchange of routing information
 False network topology change may cause generation of routing flaps,
recompilations of routes through the routing domain.
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CS 672
Summer 2003
Default Routing
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Default routing is the term commonly used for a routing entry that a
router uses to forward traffics for destinations unknown destinations.
The gateway of last resort can either be configured or learned via routing
protocols
Simplest form of routing for a Sub AS which have a single exit point to
another AS.
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