Notes8:InteriorRoutingandOSPF - echadbourne/NET-330 GitHub Wiki

What do routers do?

• Find path
• Forward packet...
• Find alternate path
• Forward packet
• Repeat until powered off

Routing vs forwarding

• Routing = building maps and giving directions
• Forwarding = moving packets between interfaces according to the "directions"

IP Routing - Finding the Path

• Path delivered from information received from a routing protocol
• Several alternative paths may exist (best are stored in a forwarding table)
• Decisions are updated periodically or as a topology changes (event driven)

Decisions are based on:

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

IP route lookup

Based on desination 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 Forwarding

• Router decides which interface a packet is sent to
• Forwarding table populated by routing process
• Forwarding is usually aided by special hardware

Forwarding decisions

• Destination address
• Class of service (fair queuing, precedence, others)
• Local requirements (packet filtering)

FIB is the forwarding table

• It containts destination 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

• IGB 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

IGP

• Interior Gateway Protocol
• OSPF, ISIS

EGP

• External Gateway Protocol
• Used to convey routing information between organizations
• De-Coupled from IGP
• Current widely-use EGP is BGP (Border Gateway Protocol)

Why do we need an EGP?

Scaling to large network

• Heirarchy
• Limit scop of failure

Define Administrative Boundary

Policy

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

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 (lots of broadcasts), slow to converge, does not scale to large or complex networks

• RIPv2 is common

Link State

• More popular for IGP
• Routers figure out who they neighbor
• After initial convergence - Only send Hellos (keep-alives) and Link State updates with changes
• Uses more advances 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

1. Create an OSPF instance: the process ID specific to that router

   • #(config) router ospf [instance_number] (usually just one)
   • Do not confuse "instance" and "area"
     • Instance: A router can have multple 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

2. 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