ECT 465

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ECT 465

• Routing protocols 2

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IP Routing
Static - Routes dont change unless an error is
detected Dynamic - Starts same as static -
Route propagation software
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Static Routing
Advantages - straightforward - easy to
specify - no routing software - doesnt consume
bandwidth - No extra CPU cycles - Routes dont
change unless an error is detected Disadvantages
- Relatively inflexible - Cannot accommodate
network failures or changes in topology
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Static Routing
Where used - Hosts two routes network
and default output to router - Used on PCs
Network prefix, subnet mask, address of router
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Dynamic Routing
Internet Dynamic Routing Each of the two
routers belong to a separate ISP. The Network
labeled Net 2 belongs to a corporate customer of
ISP 1, and the network labeled Net 3 belongs to a
corporate customer of ISP 2. Both routers know
about the network labeled Net 1, however R1 does
not know about Net 3 and R2 doesnt know about
Net 2. Routing Software Each router runs routing
software that learns about destinations other
routers can reach, and informs other routers
about destinations that it can reach. The
routing software uses incoming information to
update the local routing table continuously.
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Autonomous System Concept
A contiguous set of networks and routers all
under control of one administrative authority.
Routers are divided into groups where each
group is known as an autonomous system. The
routers within this system exchange routing
information, which is then summarized before
being passed to another group.
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Gateway Protocols
Interior Gateway Protocols (IGP) Routers
within an autonomous system. Exterior Gateway
Protocol (EGP) Routers exchanging information to
another router in another autonomous system. An
EGP summarizes information before passing it to
another system. An EGP can implement policy
constraints that allow a system manager to
determine exactly what information is released
outside the organization.
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Dynamic Routing
Routing Metric a measure of the path that
routing software uses when choosing a route. Hop
Count The number of intermediate networks on the
path to the destination. Administrative Cost
Assigned manually to control which paths traffic
can use. IGPs use routing metrics, EGPs do not.
EGPs just choose a path, not the optimum path.
This is because it cannot compare multiple
metrics from multiple autonomous systems.
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Route Advertising
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Border Gateway Protocol
Most popular Exterior Gateway Protocol. (BGP-4)
Characteristics - Routing among autonomous
systems - Provision for policies - Facilities
for transit routing (transit vs. stub) -
Reliable transport (TCP) All major ISPs use BGP
to exchange routing information Central database
of routing information is maintained to ensure
that routing remains consistent. Routing
Arbiter System Keeps a database of all possible
destinations in the Internet with information
about the ISP that owns each destination.
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Routing Information Protocol
Interior Gateway Protocol Characteristics -
Routing within an autonomous system - Hop count
metric - Unreliable transport - Broadcast or
multicast delivery - Support for default route
propagation - Distance vector algorithm -
Passive version for hosts
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Routing Information Protocol
Each outgoing message contains an advertisement
that lists the networks the sender can reach.
When it receives an advertisement, RIP software
on the receiving router uses the list of
destinations to update the routing table. Each
entry in the advertisement consists of a pair
(destination network, distance). - simple -
little configuration - default route is usually
connected to the ISP - each message is large
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Routing Information Protocol Packet Format
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Open Shortest Path First (OSPF)
OSPF is an interior gateway protocol that uses a
link-status algorithm to propagate routing
information. Each router participating in OSPF
must periodically probe adjacent routers and then
broadcast a link-status message. Characteristics
- Routing within an autonomous system - Full
subnet support - Authenticated message
exchange - Imported routes - Link state
algorithm - Support for multi-access networks
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Open Shortest Path First (OSPF)
Hierarchical Routing Autonomous system is
partitioned for routing purposes.
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Multicast Routing
IP Multicast abstraction allows an application
running on an arbitrary computer to - Join or
leave a multicast group at any time and begin
receiving a copy of all packets sent to the
group. Local Multicast Protocols - Flood and
Prune - Configuration and Tunneling -
Cored-Based Discovery Internet Multicast
Protocols - Distance Vector Multicast Routing
Protocol - Core Based Trees - Protocol
Independent Multicast- Sparse or Dense Mode -
Multicast Open Shortest Path First