Transcript CS335 Networking & Network Administration

CS335
Networking &
Network Administration
Tuesday, May 18, 2010
Internet routing


Propagation of routing tables
Routing table updates
Static vs. dynamic routing

Static routes do not change


Routes are loaded when the system starts
Dynamic routes – table information changes
over time



Routes are also loaded when the system starts
System also starts route propagation software or
routing software
Routing software on one computer interacts with
routing software on others to learn about optimal
routes. Tables are then updated.
Static routing



Does not require extra routing software
Does not consume bandwidth
No CPU cycles used to propagate routing
info
Default route
Static routes




Most hosts use static routing
Host’s routing table contains two entries
One for the network to which the host
attaches
A default entry that directs all other traffic to a
specific router
Dynamic routing

Each router runs routing software that learns about
destinations other routers can reach and informs other
routers about destinations that it can reach. Routing
tables are updated continuously.
Routing in the global internet




To limit routing traffic, the Internet uses a twolevel routing hierarchy.
Routers and networks are divided into groups
All routers in a group exchange routing
information.
At least one router in each group summarizes
information before passing it on to other
groups
Autonomous system concept





The concept of groups of routers
Designers left the concept flexible to
accommodate many possibilities of routing
groups
Each group is an autonomous system
Each group shares routing information
The group’s information is summarized
before being sent to other groups
Internet routing protocols


Interior gateway protocols (IGP’s)
 Routers within an autonomous system use this to exchange
routing information
Exterior gateway protocols (EGP’s)
 A router from one system uses these protocols to exchange
routing information with another autonomous system
Optimal routes





Optimal route is hard to define
Depends on the application
For an interactive login app a path with least
delay is optimal
For a browser downloading a large graphic
file a path with maximum throughput is
optimal
For an audio webcast path with least jitter is
best
Routing metrics

The measure of the path that routing software uses
when choosing a route






Bandwidth – data capacity of a link
Delay – length of time required to move a packet along
each link from source to destination
Load – amount of activity on a network resource such as a
router or a link
Reliability – a reference to the error rate of each network
link
Ticks – the delay on a data link using IBM PC clock ticks
(approximately 55 milliseconds or 1/18 second)
Cost – an arbitrary value, usually based on bandwidth,
monetary expense, or other measurement, that is assigned
by a network administrator
Routing metrics


IGPs use routing metrics to find optimal
routes
EGPs merely finds a path because it cannot
compare routing metrics from multiple
autonomous systems
Routes and data traffic


A response to a routing advertisement is data
Data traffic for a given destination flows in
exactly the opposite direction of routing traffic
Border gateway protocol
(BGP)





Routing among autonomous systems – BGP provides routing
information at the autonomous system level
Provision for policies – BGP allows senders and receivers to
enforce policies, a manager can restrict which routes BGP
advertises to outsiders
Facilities for transit routing – BGP classifies each autonomous
system as a transit system if it agrees to pass traffic to another
autonomous system or as a stub system if it does not
Reliable transport – BGP uses TCP for all communication for the
reliability of data transfer that TCP provides
ISPs use BGP to exchange routing information among
autonomous systems in the global Internet
BGP resources



http://www.cisco.com/univercd/cc/td/doc/cisint
wk/ito_doc/bgp.htm
http://www.academ.com/nanog/feb1997/BGP
Tutorial/sld001.htm
http://www.bgp4.as/
RIP
Routing information protocol




Routing within an autonomous system – RIP is
designed as an IGP used to pass information among
routers within an autonomous system
Hop count metric – RIP measures distance in
network hops where each network between source
and destination is a single hop. Origin-one counting
where a directly connected network is 1 hop away,
not zero
Unreliable transport – uses UDP
Broadcast of multicast delivery – intended for use
over LANs that support broadcast
RIP
Routing information protocol



Support for default route propagation – allows a
router to advertise a default route. An organization
can use RIP to install a default route in each router
such that the default routes all forward traffic to the
ISP
Uses distance vector algorithm http://www.freesoft.org/CIE/RFC/1058/6.htm
Passive version for hosts – although only a router
can propagate routing information, RIP allows a host
to listen passively and update its routing table
RIP





Chief advantage is simplicity
Needs little configuration
The routers broadcast messages to each
other
After a short time all routers in an
organization will have all routes to all
destinations
Also handles propagation of the default route
RIP packet

A RIP update message
RIP




RIP is an Interior Gateway protocol (IGP)
Uses distance vector algorithm to propagate
routing information
A router running RIP advertises the
destinations it can reach along with a
distance to each destination
Adjacent routers receive the information and
update their routing tables
RIP
Routing information protocol



http://www.cisco.com/univercd/cc/td/doc/cisint
wk/ito_doc/rip.htm
http://www.faqs.org/rfcs/rfc1058.html
http://www.networksorcery.com/enp/protocol/r
ip.htm
Open shortest path first
protocol (OSPF)
OSPF Graph
Open shortest path first
protocol (OSPF)







Designed to scale better for large organizations
OSPF is an IGP to route within an autonomous system
Full CIDR support – includes the full 32 bit address mask with
each address
Authenticated message exchange – OSPF can ensure that
messages are only accepted from trusted sources
Imported routes – OSPF allows a router to introduce routes
learned form other means (e.g., From BGP)
Uses link-state algorithm http://www.answers.com/topic/linkstate-protocol
Has support for metrics
OSPF areas






Can scale to handle a larger number of routers than
other IGPs
Does hierarchical routing
Allows a manger to divide the routers and networks
in an autonomous system into subsets called areas
OSPF allows communication between areas
http://www.cisco.com/warp/public/104/1.html
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito
_doc/ospf.htm
Multicast routing

IP multicast allows applications to:



Join multicast group at any time
Leave the group at any time
Membership in a multicast group is dynamic
IGMP
Internet Group Multicast Protocol



The protocol uses a host to inform a router
when it joins or leaves an Internet multicast
group
IGMP is only used on the local network
Routers must use another multicast routing
protocol to inform other routers of group
membership
Forwarding and multicast
techniques



Flood and prune
Configuration and tunneling
Core-based discovery
Multicast protocols






Distance vector multicast routing protocol (DVMRP)
Core based trees (CBT)
Protocol independent multicast – sparse mode (PIMSM)
Protocol independent multicast – dense mode (PIMDM)
Multicast extensions to the open shortest path first
protocol (MOSPF)
The dynamics characteristics of Internet multicast
make the problem of multicast route propagation
difficult. Internet does not have an Internet-wide
multicasting facility
Routing summary





Both hosts and routers contain an IP routing
table. Hosts use a static table
Internet is divided into autonomous systems
EGPs and IGPs exchange routing information
Multicast route propagation is difficult
Many routing protocols