IP Routing

Routing

• The process that a router uses to forward packets toward the destination network.

• A router makes this decision based on the destination IP address

• If dynamic routing is used, routers have to learn routes from other routers.

• If static routing is used, the administrator configures this information into the router manually– So, you might say that updates to a routing table

are made by the administrator.

Configuring Static Routes with Outgoing Interface

Administrative distance of 1 - default

Outgoing interface

Configuring Static Routes with Next-hop IP Address

Administrative distance of 1 - default

Next hop interface

Configuring Static Routes

• An administrator actually enters static routes into the routing table.

• That makes them static route entries – because the router is not “discovering” those routes.

• If for some reason that outgoing interface goes down or is not available for some reason, then at that time the route will be removed from the routing table.

show ip route shows the routing table.The route would still be in the configuration (because it was

entered globally), but that route could now no longer be used by the router because the interface it refers to is down for some reason.

Administrative Distance

• What is the default for a outgoing interface?

• What is the default for the next-hop address?

• Defaults can always be changed.• Just make it higher if you want it to be a

“backup” route (lower numbers are preferred)

ip route 192.168.2.0 255.255.255.0 192.188.4.1 120

Router A Router B Router C

S0 192.168.2.1/24

S1 192.168.2.2/24

S0 192.168.4.1/24

S1 192.168.4.2/24

192.168.1.0/24 192.168.3.0/24 192.168.5.0/24

What would you enter to configure a static route from Router C to the LAN on Router A using outgoing interface?

The LAN on Router B from Router A using next-hop?

Static Default Routes

• A router should be configured with a special type of static route – a default route.

• This default route routes packets with destinations that do not match any of the other routes in the routing table

• It is a “gateway of last resort” that allows the router to forward “destination unknown” packets out a particular interface

ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]

Default Route on non-directly connected networks

Verifying a Default Route

• To verify a default route, use the following commands:– show run to make sure you typed it in correctly – show ip route to make sure the router placed it

in the routing table.

Examine the show ip route Command

[Administrative distance / Hop count]

Troubleshooting static route configuration

• Ping and traceroute should be used to test basic connectivity.

• But before using ping and traceroute, it is a good idea just to see if the physical interface is “up”.

show interfacesshow interface s0show interface s1show ip interface brief

Routed VS. Routing• Routed protocols

are protocols that are routed over an Internetwork (IP, AppleTalk, IPX)

• Routing protocols use algorithms to route routed protocols through the Internetwork (RIP, IGRP, OSPF)

•Routed protocols are used BETWEEN routers to direct traffic

•Routing protocols allow routers to share information about known networks with other networks (their purpose)

Routing Protocol and Autonomous System

• An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy.

• The goal of a routing protocol is to build and maintain the routing table.

• A routers learns about routes to a network by:– Gathering information from its own configuration

regarding directly connected networks.– Other routers forward information about known

networks.– Manually entered routes by a network administrator.

Convergence

• When all routers in an Internetwork are operating with the same knowledge, the Internetwork is said to have converged.

• Fast convergence is desirable because it reduces the period of time in which routers would continue to make incorrect routing decisions.

• In routers that use dynamic routing protocols, it is important to have fast convergence because routers could make incorrect forwarding decisions until the network has fully converged.

• A network has converged when the routers in the network are operating with consistent routing knowledge.

Router A Router B Router C

S0

S1

S0

S1

192.168.1.0/24 192.168.3.0/24 192.168.5.0/24

The routing table on these routers would be changed if something happened to the links BETWEEN the routers (a serial interface goes down for some reason). Anything that happens to the Ethernet interfaces would not affect the routing tables. Routing takes place between routers.

E0E0 E0

Distance Vector Routing

Neighbor to neighbor ONLY

Distance Vector Routing

Topology Changes (distance vector)

Routing table updates take place when the topology changes.

Router to router (neighbor tells neighbor)

Distance vector – each router sends its entire routing table to neighbor table

Link-state Routing• Shortest Path First (SPF)• Flood routing information about it’s OWN links• Analyze incoming routing update messages• If the message indicates that a network change has

occurred, the routing software recalculates routes and sends out new routing update messages

• These messages permeate the network, stimulating routers to rerun their algorithms and change their routing tables accordingly.

• After the initial flood, it passes small event-triggered updates to all other routers

In Order to Converge, a Router in a Link-State network must:

• Remember its neighbor’s name, when it’s link is up or down (status), and the cost of the path to that router.

• Create an LSP (link-state packet) that lists its neighbor’s name and relative costs.

• Send the newly created LSP to all other routers participating in the link-state network.

• Receive LSPs from other routers and update its own database. (Particularly when there are changes in the network)

• Build a complete map of the Internetwork’s topology from all the LSPs received, then compute the best route to each network destination.

Link-State Concerns• Processor overhead • Memory requirements (use more router resources

initially when flooding info)• Bandwidth Consumption on initial flood

All of these are considerations when selecting a routing protocol to be used over an enterprise network.

Routing Decisions

• The router uses the network portion of the address to choose the best path

• Router “switches” the packet to the best port for forwarding using path determination