Transcript Lecture_5_Routing Protocols

Chapter 21
Routing Protocols
Routing
Intradomain Routing
Interdomain Routing
Autonomous System
Distance Vector Routing Protocol
Link State Routing Protocol
Path Vector Routing Protocol
Routing cont…
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Autonomous System (AS)
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Intradomain Routing Protocol
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A group of networks & routers under the
authority of common administration
Routing inside an Autonomous system
Interdomain Routing Protocol
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Routing b/w Autonomous systems
Routing cont…
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Distance Vector Routing Protocol
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The routing table is shared with the neighbor
router(s)
Can not be used in large networks
The size of the N/W, in each direction, can not
exceed 15 hops
Examples are RIP and IGRP
Routing cont…
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Link State Routing Protocol
information about adjacencies
(neighborship) sent to all routers only
when there is a change
Examples are OSPF and IS-IS
OSPF
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OSPF is designed for large, scalable
networks.
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allows dividing a large internetwork into
smaller areas.
OSPF Terminology
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Link - physical and electrical connection between
two network devices.
Link-state - status of a link between two routers.
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information about router's interface and its
relationship to neighboring routers.
Cost - value assigned to a link (based on speed
of the network connection).
Area - collection of networks and routers (has
same area identification).
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Each router in an area, called an internal router, has
the same link-state information.
OSPF Terminology cont…
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Designated Router (DR) - a router elected to
represent all the routers in that network.
Backup Designated Router (BDR) - becomes the
DR, if original DR fails.
Adjacencies database - listing of all neighbors to
which a router has established communication.
Link-state database (topological database) information about all other routers in the
network.
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shows the network topology.
All routers in an area have identical link-state
databases.
OSPF Packet Format
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5 types of ospf packets.
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1
2
3
4
5
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Hello.
DBD – database description packets.
Link-state requests.
Link-state updates (LSUs).
Link-state acknowledgements.
OSPF Operation
Establishing Adjacencies
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Done using the Hello protocol.
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establish virtual point-to-point links, adjacencies.
ensure bi-directional
communication
between neighbors
before exchanging
link-state
information.
Hello packets
are multi-cast
Hello Protocol exchange process
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1. Router A first comes up - OSPF in down state.
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Router A sends Hello packet with its information.
2. All enabled OSPF routers receive the Hello
packet and store info locally (Init state).
3. Routers send unicast reply Hello packet back
to Router A with their own information.
4. From replies Router A adds all neighbor
routers to its own database (two-way state).
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Now, all routers that have learned about each other
establish bi-directional communication.
Hello Protocol exchange process (2)
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5. Neighbor routers select the DR and BDR.
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(before routers can begin exchanging complete linkstate information).
6. Neighboring routers determine master/slave
relationship, exchange LSAs, and establish full
adjacency.
7. Periodically (10 seconds by default), Hello
packets ("keepalive" updates) are exchanged to
ensure communication is still working.
DR and BDR
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Help reduce routing update traffic
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Act as a central point of contact.
Each router establishes adjacency with DR/BDR.
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DR forwards LSAs to all other routers in network.
Manage link-state synchronization
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sends LSAs only to DR/BDR.
Ensure all routers have same link-state information
about the internetwork.
BDR becomes DR, only if the DR fails.
Electing DR and BDR
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DR - router with highest priority.
BDR - second highest
priority.
Priority 0 cannot become
DR or BDR.
If priorities are same, the
higher Router ID is
elected.
No new election if another
router
is added.
If DR fails,
BDR becomes DR, and new BDR is elected.
Route Discovery
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After DR/BDR election, routers enters
Exstart state.
OSPF Configuration
OSPF Commands
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show ip ospf interface
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show ip ospf
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# times SPF algorithm executed, link state update
interval.
show ip ospf neighbors
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Verifies interface is configured – Router ID, DR &
BDR, timer intervals, neighbors.
List of neighbors, their priorities and states.
show ip ospf database
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Displays contents of topological database.
Path Vector Routing Protocol
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Interdomain Routing Protocol
One node, in each autonomous system,
functions on behalf of the entire AS
The node is called speaker node (for
discussion)
The speaker node in an AS creates routing
table and advertises it to speaker node in
the neighboring ASs
Example is BGP (Border Gateway Protocol)
Thank You
Questions???