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Chapter Overview
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Routing Principles
Building Routing Tables
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Routing Protocols
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Key to the network connection info
Contract routing tables dynamically
Costs are CPU time and network traffics
 IGP and EGP (Interior/Exterior gateway
protocols)
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Routing Basics
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Administrative Distances (AD)
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It is use to rate the trustworthiness of routing info received
on one router from its neighboring routers
A number between 0 to 255
 0 is the most trusted
 255 == no traffic should go there
Entering to routing table if two routers both can reach an IP
 Rule #1, go with a lower AD
 Rule #2, if the two ADs are the same, do hop count, etc
cost/metrics calculations
 If cost/metrics calculations are all the same, do a “Load
Balance” test by sending messages to the two routers
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AD numbers
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Three classes of routing protocols
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Distance vector
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Link State
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Router has 3 tables: (1) directly attached neighbors, (2) table used
to determines the topology of the entire internetwork, (3) actual
routing table
It knows more about the network than Distance Vector
It use an algorithm to calculate the shortest path
Hybrid
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Count the number of routers (hops) between two points
Combines the two, Cisco proprietary protocol
No “best” approach
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Dynamic Routing
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Distance-Vector Routing
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Routing by rumor
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A router passes COMPLETE routing-table to neighboring
routers
The received routing table is combined with a router’s
own routing table without verification
Use hop count to determine routing, then AD, then
“load balancing”
Know the directly connected networks initially
Build the tables afterward
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Example, initial routing tables
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Example, populated
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RIP Characteristics
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RIP: the acronym for Routing Information Protocol
Most common interior gateway protocol (IGP) in
the TCP/IP suite
Originally designed for UNIX systems as a daemon
called routed
Eventually ported to other platforms
Standardized in Request for Comments (RFC) 1058
Updated to version 2, published as RFC 2453
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RIP Communications
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RIP routers initiate communications when starting
up by broadcasting a request message on all
network interfaces.
All RIP routers receiving the broadcast respond
with reply messages containing their entire
routing table.
The router receiving the replies updates its own
routing table with the information in the reply
messages.
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RIP Version 1 Message Format
Refresh every 30 seconds
3 minutes == out
Not support subnet mask
Max hop count 15 – used by small networks only
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RIP Version 2 Message Format
Message are sent using UDP (user Datagram Protocol)
Port 520
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RIP 2
From
http://www.tcpipguide.com/free/t_RIPVersion2RIP2MessageFormatandFeatures-3.htm
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RIP vs. RIP v2
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VLSM = Variable Length Subnet mask
Discontiguous == two connecting subnets come
from different classful network
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Such as connecting 172.16.16.1/24 with 10.3.1.1/24
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Discontiguous
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Interior Gateway Protocols (IGP) and
Exterior Gateway Protocols (EGP)
Border Gateway Protocol
(BGP).
IGP
EGP
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OSPF Protocol
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OSPF: the acronym for Open Shortest Path
First (EGP/IGP)
Standardized in RFC 2328
Uses link-state routing
Offers several advantages:
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Updates routing tables more quickly when
changes occur on the network
Balances the network load by splitting traffic
between routes with equal metrics
Supports authentication of routing protocol
messages
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OSPF and RIPs
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How powerful/useful OSPF is
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IPv6 Routing
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Cannot broadcast
RIPng <== RIPv2
EIGRPv6  EIGPR
OSPFv3  OSPF
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? --cast
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