NET-330 Module 8: Interior Routing

A day in a life of a router

find path

forward packet, forward packet, forward packet, forward packet...

find alternate path

forward packet, forward packet, forward packet, forward packet…

repeat until powered off

Routing versus Forwarding

Routing = building maps and giving directions

Forwarding = moving packets between interfaces according to the “directions”

IP Routing – finding the path

Path derived from information received from a routing protocol

Several alternative paths may exist

• best path stored in forwarding table

Decisions are updated periodically or as topology changes (event driven)

Decisions are based on:

• topology, policies and metrics (hop count, filtering, delay, bandwidth, etc.)

IP route lookup

Based on destination IP address

“longest match” routing

• More specific prefix preferred over less specific prefix
• Example: packet with destination of 10.1.1.1/32 is sent to the router announcing 10.1/16 rather than the router announcing 10/8.

IP route lookup

Based on destination IP address

IP Forwarding

Router decides which interface a packet is sent to

Forwarding table populated by routing process

Forwarding decisions:

• destination address
• class of service (fair queuing, precedence, others)
• local requirements (packet filtering)

Forwarding is usually aided by special hardware

Routing Tables Feed the Forwarding Table

RIBs and FIBs

FIB is the Forwarding Table

• It contains destinations and the interfaces to get to those destinations
• Used by the router to figure out where to send the packet
• Careful! Some people still call this a route!

RIB is the Routing Table

• It contains a list of all the destinations and the various next hops used to get to those destinations – and lots of other information too!
• One destination can have lots of possible next-hops – only the best next-hop goes into the FIB

Routing Protocols

Routers use “routing protocols” to exchange routing information with each other

• IGP is used to refer to the process running on routers inside an organization’s network
• EGP is used to refer to the process running between routers bordering directly connected ISP networks

What Is an IGP?

Interior Gateway Protocol

Within an Organization

Carries information about internal infrastructure prefixes

Two widely used IGPs:

• OSPF
• ISIS

What Is an EGP?

Exterior Gateway Protocol

Used to convey routing information between Organizations

De-coupled from the IGP

Current widely-use EGP is BGP (Border Gateway Protocol)

Why Do We Need an EGP?

Scaling to large network Hierarchy Limit scope of failure

Define Administrative Boundary Policy

• Control reachability of prefixes
• Merge separate organisations
• Connect multiple IGPs

Routing Protocols

Types of routing protocols:

• Interior Gateway Protocols (IGP)
• Exterior Gateway Protocols (EGP)

Interior versus Exterior Routing Protocols

Interior

• automatic neighbor discovery
• generally trust your IGP routers
• prefixes go to all IGP routers
• binds routers in one Autonomous System (AS) together

Exterior

• specifically configured peers
• connecting with outside networks
• set administrative boundaries
• binds AS’s together

Dynamic IGP Types

Distance Vector

• Routers send out broadcasts/multicasts with their routing table

• Other routers listen, and update their tables accordingly

• Typically send tables every 30-60 seconds

• Routing preference is based solely on Hops (how many routers between networks)

• Noisy (lot’s of broadcasts), slow to converge, does not scale to large or complex networks

• RIP v2 is common example

Link State IGP

More popular method for IGP

Routers figure out who their neighbors

After initial convergence – only send Hello’s (keep-alives) and Link State updates with changes

Uses more advanced route-selection metrics (notably – bandwidth)

Open Shortest Path First (OSPF) is most popular IGP

OSPF

Does not use a transport protocol (UDP or TCP)

• It is its own layer 4 – IP Protocol 89

Uses “Area” to determine which routers listen/talk to each other

Can just be one, or many if you have a large network

Configuring OSPF

Relatively easy on Cisco

Create an OSPF instance: The process ID specific to that router

#(config) router ospf instance_number (usually just 1)

Do not confuse “instance” and “area”

• Instance: A router can have multiple OSPF processes running. Imagine a router connecting two totally different organizations and configuring OSPF for both
• Area: Must be the same among all routers that need to share info within a particular instance

Add all of the Directly Connected Networks on that router

Network address with wildcard mask and area #

#(config-router) network 192.168.3.0 0.0.0.255 area 0