Network Layer

1
Network Layer

• ROUTING ALGORITHM

2
OSPF (Open shortest Path First)

• OSPF is intended to fix some of the deficiencies
  in RIP.
• OSPF has a number of advance features
• OSPF uses a link-state algorithm and a distance
  Vector algorithm

3
OSPF (Open shortest Path First)

• Each OSPF router monitors the cost of the link
  to each of its neighbors and then floods the
  link-state information to other routers in the
  network.
• The router then locally run Dijkstras
  shortest-path (distance vector) algorithm to
  determine a shortest-path tree to all subnets.

4
OSPF advanced features (not in RIP)

• Security all OSPF messages are authenticated (to
  prevent malicious intrusion)
• Multiple same-cost paths are allowed (only one
  path in RIP)

5
OSPF advanced features (not in RIP)

• Integrated support for unicast and multicast
  routing
• Hierarchical OSPF in large domains.

6
Hierarchical OSPF

• Two-level hierarchy local area, backbone.
• Link-state advertisements occur only in an area
• Only one area is configured to be a backbone

7
Four types of OSPF routers

• Internal routers perform intra-AS routing
• Area border routers summarize distances to
  nets in its own area, advertise it to other Area
  Border routers.
• Backbone routers run OSPF routing limited to
  backbone.
• Boundary routers connect to other ASs-
  exchanging routing information

8
Hierarchical OSPF
9
Inter-Autonomous System Routing

• Also called Border Gateway Protocol Version 4
  (BGP4)
• BGP provides each AS a means to
• Obtain subnet reachability information from
  neighboring ASs
• Propagate the reachability information to all
  routers internals to the AS
• Determine good routes to subnets based
• Allows a subnet to advertise its existence to
  rest of the Internet I am here

10
BGP Basics

• Pairs of routers exchange routing information
  over semi-permanent TCP connections using port
  179, also called BGP TCP.
• There are semi-permanent TCP connections between
  gateway routers and internal routers.
• BGP connection BPG messages BGP session

11
BGP basics

• When AS2 advertises a prefix to AS1, AS2 is
  promising it will forward any datagrams destined
  to that prefix.

12
Distributing reachability info

• With eBGP session between 3a and 1c, AS3 sends
  prefix reachability info to AS1.

13
Distributing reachability info

• 1c can then use iBGP do distribute this new
  prefix reach info to all routers in AS1

14
Distributing reachability info

• 1b can then re-advertise the new reach info to
  AS2 over the 1b-to-2a eBGP session

15
Distributing reachability info

• When a router learns about a new prefix, it
  creates an entry for the prefix in its forwarding
  table.

16
Path attributes BGP routes

• When advertising a prefix, BGP includes
  attributes.
• prefix attributes route
• Two important attributes
• AS-PATH contains the ASs through which the
  advert for the prefix passed AS10 AS 11
• NEXT-HOP Indicates the specific internal-AS
  router to next-hop AS. (There may be multiple
  links from current AS to next-hop-AS.)

17
BGP route selection

• Router may learn about more than 1 route to some
  prefix. Router must select route.
• Elimination rules
• Local preference value attribute policy decision
• Shortest AS-PATH
• Closest NEXT-HOP router