Routing

1
Routing

• Shawn Shuoh-Ren Tsai
• Cisco Academy Training Center
• David Ta-Wei Shih
• CGUIC

2
Path Determination

• Network Layer
• provides best-effort end-to-end packet delivery
  across interconnected networks
• Use IP routing table
• Prevents unnecessary broadcasts

3
Routed versus Routing protocols

• Routed protocol is any network protocol that
  provides enough information in its network layer
  address to allow a packet to be forwarded from
  one host to another host based on the addressing
  scheme.
• For example, IP

4
Routed versus Routing Protocols (cont.)

• A routing protocol allows the routers to
  communicate with other routers to update and
  maintain tables.
• TCP/IP examples of routing protocols are
• RIP (Routing Information Protocol)
• IGRP (Interior Gateway Routing Protocol)
• EIGRP (Enhanced Interior Gateway Routing
  Protocol)
• OSPF (Open Shortest Path First

5
Network Layer Operation

• The router's network layer process
• examines the incoming packet's header to
  determine the destination network,
• references the routing table which associates
  networks to outgoing interfaces. 

6
Multiprotocol Routing

• Routers
• support multiple independent routing protocols
• maintain routing tables for several routed
  protocols.

7
Static versus Dynamic Routes

• Static Route
• Uses a programmed route that a network
  administrator enters into the router
• Dynamic Route
• Uses a route that a network routing protocol
  adjusts dynamically for topology or traffic
  changes

8
Why use a static route

• Static route enable you to reveal some restricted
  network
• When there is only one path, a static route is
  sufficient

9
How a default route is used

• If no entry in the routing table, try default
  route
• Default route is part of static configuration

10
Why dynamic route is necessary?

• Achieve load balancing
• Adapt to topology change
• The success of dynamic routing depends on two
  basic router functions
• maintenance of a routing table
• timely distribution of knowledge, in the form of
  routing updates, to other routers 

11
Routing Metrics

• Bandwidth
• Delay
• Load
• Reliability
• Hop counts
• Ticks
• Cost

12
Three Classes of Routing Protocols

• Distance Vector
• Determines direction and Distance for each link
  in the network
• Link State
• Generates the whole topology of the network
• Hybrid Routing
• Combines aspects of link state and distance
  vector

13
Time to Convergence

• When all routers in an internetwork are operating
  with consistent view of the new topology, the
  internetwork is said to have converged.
• Fast convergence
• A desired feature to reduce the period of time in
  which routers make inaccurate decisions.

14
Distance Vector Routing Basics

• Distance-vector-based routing algorithms pass
  periodic copies of a routing table from router to
  router.
• Each router receives a routing table from its
  directly connected neighboring routers.

15
Routing loop

• Routing loops can occur if a network's slow
  convergence on a new configuration causes
  inconsistent routing entries.

16
Count to Infinity

• To avoid count to infinity, distance-vector
  protocols define infinity as a specific maximum
  number.
• distance-vector default maximum 15 hops

17
Split Horizon

• Another possibility for a routing loop occurs
  when incorrect information that has been sent
  back to a router contradicts the correct
  information that it sent.
• 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.

18
Hold Down Timers

• When a router receives an update from a neighbor
  indicating that a previously accessible network
  is now inaccessible, the router marks the route
  as inaccessible and starts a hold-down timer.
• If at any time before the hold-down timer expires
  an update is received from the same neighbor
  indicating that the network is again accessible,
  the router marks the network as accessible and
  removes the hold-down timer

19
Link State Routing Basics

• Link-state based routing algorithms, also known
  as SPF (shortest path first) algorithms, maintain
  a complex database of topology information.
• a link-state routing algorithm maintains full
  knowledge of distant routers and how they
  interconnect.

20
Link State Routing Basics (cont)

• Link-state routing uses
• link-state advertisements (LSAs)
• a topological database
• the SPF algorithm, and the resulting SPF tree
• a routing table of paths and ports to each
  network

21
Two Link-State Concerns

• Processing and memory requirements
• Bandwidth requirements

22
Distance Vector versus Link State Protocols
23
Hybrid Routing

• use distance vectors with more accurate metrics
• use topology changes to trigger routing database
  updates
• converges rapidly
• Examples of hybrid protocols are OSI's IS-IS
  (Intermediate System-to-Intermediate System), and
  Cisco's EIGRP (Enhanced Interior Gateway Routing
  Protocol).