Download Chap 11 Routing

Chap 11 Routing
Andres, Wen-Yuan Liao
Department of Computer Science and Engineering
De Lin Institute of Technology
[email protected]
http://www.cse.dlit.edu.tw/~andres
Overview
Internetworking
function
Routing and routed protocols
Track distance between locations
Distance-vector, link-state, and
hybrid routing approaches
Routing Basics
 Path
determination
 Route packets from source to
destination
 Network and host addressing
 Path selection and packet switching
 Routed versus routing protocol
 Network-layer protocol operations
 Multiprotocol routing
Path determination
Evaluate
the available paths to a
destination and to establish the
preferred handling of a packet
Network part of IP
Layer 3
Route packets
The
consistency of Layer 3 addr.
improves the use of bandwidth by
preventing unnecessary
broadcasts
Broadcasts invoke unnecessary
process overhead and waste
capacity
Network and host addressing
The
router uses the network
address to identify the destination
network of a packet
Assigning host addresses
Network
administrator
Be partially or completely dynamic
Path selection and packet
switching
The
router uses the network
portion of the address to make
path selections
The switching function: accept a
packet on one interface and
forward it through a second
interface
Routed versus routing protocol
Routed
protocols define the
field formats within a packet
(IP): Carry data
Routing protocols provide
mechanisms for sharing
routing information: Maintain
table
Routing protocol




RIP (Routing Information Protocol)
IGRP (Interior Gateway Routing
Protocol)
EIGRP (Enhanced Interior Gateway
Routing Protocol)
OSPF (Open Shortest Path First)
Multiprotocol routing
Routers
are capable of
supporting multiple
independent routing protocols
and maintaining routing tables
for several routed protocols
Why Routing Protocols are
Necessary
Static versus dynamic routes
Default route
Dynamic routing
Various metrics
Three classes of routing
protocols
Time to convergence
Static versus dynamic routes
Static
route
Be
administered manually by a
network administrator who enters it
into a router's configuration
Dynamic
The
route
route knowledge: updated by
a routing process
Static route
Dynamic
routing: reveal
everything known about an internetwork
Security reasons
Hide
A stub
parts of an internetwork
network:A network is
accessible by one path
Default route
A routing
table entry that
directs packets to the next hop
when that hop is not explicitly
listed in the routing table
Dynamic routing
Offers
more flexibility
Loadsharing
Direct
traffic from the same
session over different paths in a
network for better performance
Dynamic routing operations
1.
2.
Maintenance of a routing
table
Timely distribution of
knowledge, in the form of
routing updates, to other
routers
A routing protocol
How
to send updates
What knowledge is contained in
these updates
When to send this knowledge
How to locate recipients of the
updates
Three classes of routing
protocols
Distance-vector
routing
Determines
the direction (vector)
and distance to any link
Link-state
(shortest path first)
Re-creates
Balanced
the exact topology
hybrid approach
Time to convergence
Convergence
The
knowledge: an accurate,
consistent view of new topology
Converged
All
routers in an internetwork are
operating with the same knowledge
Fast
convergence
Distance-Vector Routing
 Distance-vector
routing basics
 Exchange routing tables
 Topology changes propagate
 Routing loops
 Counting to infinity
 Defining a maximum
 Split horizon
 Hold-down timers
Distance-vector routing basics
Pass
periodic copies of a
routing table from router to
router
Do not allow a router to know
the exact topology of an
internetwork
Exchange routing tables
Directly-connected
A distance
of 0
network
Topology changes propagate
Topology
change updates
proceed step-by-step from router
to router
Send its entire routing table to
each of its adjacent neighbor
Routing loops
A network's
slow
convergence on a new
configuration causes
inconsistent routing entries
Routing loops
Packets
never reach their
destination but instead
cycle repeatedly through
the same group of network
nodes
Counting to infinity
The
invalid updates will
continue to loop until some
other process stops the
looping
Sol: Defining a maximum
Routing
metric (hop count)
Distance-vector protocols define
infinity as a specific maximum no.
The distance-vector default
maximum of 15 hops
Sol: Split horizon
Split-horizon
attempts: if a
routing update about Network
1 arrives from Router A,
Router B or Router D cannot
send information about
Network 1 back to Router A
Hold-down timers
 When
a previously
accessible network is now
inaccessible, the router
marks the route as
inaccessible and starts a
hold-down timer
Hold-down timers
They
help prevent a router
from immediately using an
alternate route that includes
the failed route
Hold-down timers
If
at any time before the holddown timer expires an update
indicates that the network is
again accessible
Marks
the network as
accessible and removes the
hold-down timer
Hold-down timers

If an update arrives from a
different neighboring router
with a better metric, the
router marks the network as
accessible and removes the
hold-down timer
Hold-down timers
If
before the hold-down
timer expires an update is
received from a different
neighboring router with a
poorer metric, the update is
ignored
Hold-down timers
Allows
more time for the
knowledge of a
disruptive change to
propagate through the
entire network
Link-State Routing
Link-state
routing basics
Exchange routing tables
Topology changes propagate
Two link-state concerns
Unsynchronized LSAs
Link-state routing basics
SPF
(shortest path first): OSPF
Maintain a complex database of
topology information
Maintains full knowledge of
distant routers and how they
interconnect
Link-state routing




Link-state advertisements (LSAs)
A topological database
The SPF algorithm; the SPF tree
A routing table of paths and ports to
each network
Exchange routing tables


Routers exchange LSAs with
each other
Each router in parallel with
the others constructs a
topological database
Exchange routing tables


The SPF algorithm computes
network reachability. It then sorts
these paths shortest path first (SPF)
The router lists its best paths, and
the ports to these destination
networks

Topology elements and status details
Topology changes propagate
Whenever
a link-state
topology changes, the routers
send information to other
routers or to a designated
router
Each Router
Keeps track of its neighbors:
( name, up or down, cost of the
link )
 Constructs an LSA packet
 Sends out this LSA packet

Each Router
When it receives an LSA packet,
updates the most recently
generated LSA packet
 Completes a map of the
internetwork by using the SPF
algorithm

Two link-state concerns
Processing and memory
requirements
Hold
information from various
databases, the topology tree, and
the routing table
Dijkstra's algorithm
Bandwidth
Initial
requirements
link-state packet flooding
Unsynchronized LSAs
Networks become unreachable
as a result of a disagreement
among routers about a link
Scaling up with link-state
protocols on very large
internetworks can expand the
problem of faulty LSA packet
distribution
Different Routing Protocols
Distance-vector
vs. link-state
routing
Hybrid routing protocols
LAN-to-LAN routing
LAN-to-WAN routing
Path selection and switching of
multiple protocols and media
Hybrid routing protocols
Use
distance vectors with more
accurate metrics
Using topology changes to trigger
routing database updates
Fewer resources
OSI's IS-IS, and Cisco's EIGRP
LAN-to-LAN routing
Routers
must be capable of
seamlessly handling packets
encapsulated into various
lower-level frames without
changing the packets' Layer 3
addressing
Summary
 Network addressing and best
path selection for traffic.
 Routed and routing protocols
 Distance-vector routing
 Link-state routing
 Balanced hybrid routing