Distance vector means that routes are advertised as vectors of distance and direction. Distance is
defined in terms of a metric such as hop count and direction is simply the next-hop router or exit
interface. Distance vector protocols typically use the Bellman-Ford algorithm for the best path route
determination.
Distance vector protocols work best in situations where:
The network is simple and flat and does not require a special hierarchical design.
The administrators do not have enough knowledge to configure and troubleshoot link-state protocols.
Specific types of networks, such as hub-and-spoke networks, are being implemented.
Worst-case convergence times in a network are not a concern.
a router configured with a link-state routing protocol can create a "complete view" or topology of the
network by gathering information from all of the other routers. To continue our analogy of sign posts,
using a link-state routing protocol is like having a complete map of the network topology. The sign posts
along the way from source to destination are not necessary, because all link-state routers are using an
identical "map" of the network. A link-state router uses the link-state information to create a topology
map and to select the best path to all destination networks in the topology.
Link-state protocols work best in situations where:
The network design is hierarchical, usually occurring in large networks.
The administrators have a good knowledge of the implemented link-state routing protocol.
Fast convergence of the network is crucial.
What is Convergence?
Convergence is when all routers' routing tables are at a state of consistency. The network has converged
when all routers have complete and accurate information about the network. Convergence time is the
time it takes routers to share information, calculate best paths, and update their routing tables. A
network is not completely operable until the network has converged; therefore, most networks require
short convergence times.
The speed of achieving convergence consists of:
How quickly the routers propagates a change in the topology in a routing update to its neighbors.
The speed of calculating best path routes using the new routing information collected.
RIP Timers
In addition to the update timer, the IOS implements three additional timers for RIP:
Invalid
Flush
Holddown
Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the
default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing
table until the flush timer expires.
Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the
invalid timer. When the flush timer expires, the route is removed from the routing table.
Holddown Timer. This timer stabilizes routing information and helps prevent routing loops during
periods when the topology is converging on new information. Once a route is marked as unreachable, it
must stay in holddown long enough for all routers in the topology to learn about the unreachable
network. By default, the holddown timer is set for 180 seconds.
A triggered update is a routing table update that is sent immediately in response to a routing change.
Triggered updates do not wait for update timers to expire. The detecting router immediately sends an
update message to adjacent routers. The receiving routers, in turn, generate triggered updates that
notify their neighbors of the change.
Triggered updates are sent when one of the following occurs:
An interface changes state (up or down)
A route has entered (or exited) the "unreachable" state
A route is installed in the routing table
What is a Routing Loop?
A routing loop is a condition in which a packet is continuously transmitted within a series of routers
without ever reaching its intended destination network.
The split horizon rule says that a router should not advertise a network through the interface from
which the update came.
Route poisoning is used to mark the route as unreachable in a routing update that is sent to other
routers.
Features of RIP:
Supports split horizon and split horizon with poison reverse to prevents loops.
Is capable of load balancing up to six equal cost paths . The default is four equal cost paths.
RIPv2 introduced the following improvements to RIPv1:
Includes the subnet mask in the routing updates, making it a classless routing protocol.
Has authentication mechanism to secure routing table updates.
Supports variable length subnet mask (VLSM).
Uses multicast addresses instead of broadcast.
Supports manual route summarization.
EIGRP features include:
Triggered updates (EIGRP has no periodic updates).
Use of a topology table to maintain all the routes received from neighbors (not only the best paths).
Establishment of adjacencies with neighboring routers using the EIGRP hello protocol.
Support for VLSM and manual route summarization. These allow EIGRP to create hierarchically
structured large networks.
A level 1 route is a route with a subnet mask equal to or less than the classful mask of the network
address.
A level 1 route can function as a:
Default route - A default route is a static route with the address 0.0.0.0/0.
Supernet route - A supernet route is a network address with a mask less than the classful mask.
Network route - A network route is a route that has a subnet mask equal to that of the classful mask. A
network route can also be a parent route.
Ultimate Route
The level 1 route 192.168.1.0/24 can be further defined as an ultimate route. An ultimate route is a
route that includes:
either a next-hop IP address (another path)
and/or an exit interface
A level 1 parent route is a network route that does not contain a next-hop IP address or exit interface for
any network.
A level 2 route is a route that is a subnet of a classful network address. A level 2 route is a route that is a
subnet of a classful network address.
The route with the most number of equivalent left-most bits, or the longest match, is always the
preferred route.
Autonomous System
An autonomous system (AS) is a collection of networks under the administrative control of a single
entity that presents a common routing policy to the Internet.
A successor is a neighboring router that is used for packet forwarding and is the least-cost route to the
destination network.
Feasible distance (FD) is the lowest calculated metric to reach the destination network. FD is the metric
listed in the routing table entry as the second number inside the brackets.
D 192.168.1.0/24 [90/3014400] via 192.168.10.10, 00:00:31, Serial0/0/1
A feasible successor (FS) is a neighbor who has a loop-free backup path to the same network as the
successor by satisfying the feasibility condition.
The feasibility condition (FC) is met when a neighbor's reported distance (RD) to a network is less than
the local router's feasible distance to the same destination network.
The reported distance or advertised distance is simply an EIGRP neighbor's feasible distance to the same
destination network. The reported distance is the metric that a router reports to a neighbor about its
own cost to that network.