information is learned from other routers,
and routing protocols adjust routes automatically.
network administrator configures information
about remote networks manually. They are used to reduce
overhead and for security.
Because of the extra administrative requirements, static routing
does not have the
of dynamic routing.
In most networks static routes are often used in conjunction with a
dynamic routing protocol.
In order to forward packets correctly, routers must learn the
direction to remote networks.
Two types of routing:
Routing and Routed Protocols
allow the routers to communicate with
other routers to update and maintain tables.
RIP, IGRP, EIGRP and OSPF
provide enough information in their network
layer address to allow packets to be forwarded from one
host to another host based on the addressing scheme.
IP, IPX, AppleTalk
Routing consists of two basic mechanisms:
Router uses the routing table to determine the best path
Accept a packet on one interface and forward it to a second interface
Routing protocols create and maintain routing tables:
Simplified routing table.
The Default router entry
sends packets for any
other destinations out S1.
This command sets a
on a router:
Static routes between networks are manually configured by an administrator.
Static routes are added with the following command:
ip route 192.168.2.0 255.255.255.0 E0
ip route 0.0.0.0 0.0.0.0 192.168.1.1
Static routes out
Static routes to
next hop addresses
You can specify a non
default administrative distance for a static route:
ip route 0.0.0.0 0.0.0.0 192.168.1.1 130
Terms to Remember
Routing table entry that is used to direct frames for which a next hop is not
explicitly listed in the routing table.
A route that is explicitly configured and entered into the routing table. Static
routes take precedence over routes chosen by dynamic routing protocols.
Term describing the passage of a data packet between two network nodes
(for example, between two routers). See also
Routing metric used to measure the distance between a source and a
destination. RIP uses hop count as its sole metric. See also
Distance Vector and Link
The success of dynamic routing depends on two basic router
Maintenance of a routing table
Timely distribution of knowledge, in the form of routing
updates, to other routers.
Dynamic routing relies on the routing protocol.
Routing Protocols can be
Hybrid protocols (like EIGRP) contain some elements of both.
Different routing protocols use different
the best route to a network.
are used to rate the trustworthiness
of the various routing protocols.
Distance Vector Routing Protocols
RIP is a distance vector routing protocol:
Uses hop count as its metric
Each router the packet goes through is 1 hop
vector routing algorithm passes
to neighbor routers.
The neighbor routers combine the received routing table with their
own routing tables.
Link State Routing Protocols
state routing algorithms (Shortest Path First
maintain a complex database of topology information.
state routing uses:
state routing requires
than distance vector, and
requirements are often
higher as well.
OSPF is the most commonly used Link
, and the resulting
of paths and ports to each network
Distance Vector or Link State?
Also known as Bellman
Flood routing information to all routers
Requests routing information from directly connected neighbors
Complete view of the internetwork topology
Decisions based upon information provided by neighbors
Use fewer system resources
When a network link changes state LSA are flooded through network
Less errors, but they use more system resources
Calculate the shortest path to all known sites on the network
Small update packets contain only changes
OSPF and IS
Do not scale well to larger systems.
Because they converge more quickly less prone to routing loops
triggered updates, so convergence is fast
Based on finding the number of hops and direction to a link
Passes copies of complete routing table on a periodic basis
Each router simply inform its neighbors of its routing table
RIP and IGRP
more reliable, easier to debug, and less bandwidth
Distance vector, hop count metric, maximum 15 hops,
broadcasts updates every 30 secs.
Cisco proprietary distance vector, bandwidth / load / delay /
reliability composite metric, broadcast updates every 90 secs.
Cisco proprietary, enhanced distance vector (hybrid), load
balancing, uses DUAL to calculate shortest path.
Routing updates are triggered by topology changes.
state, open standard, Uses SPF algorithm. Routing
updates are sent as topology changes occur.
exterior routing protocol
, used between
ISPs, used to route traffic between ASs
AS is a collection of networks under a
and sharing a common routing strategy.
ARIN, ISP, or an administrator assigns the
16 bit AS number
IGRP, EIGRP and BGP require assignment of a unique AS
Each AS has its own set
of rules and policies.
The AS number uniquely
distinguish it from other
ASs around the world.
ASs divide the global internetwork into smaller, more