Routing/Routed Protocols

Remember:

• A Routed Protocol – defines logical addressing. Most notable example on the test – IP

• A Routing Protocol – fills the routing table with routing information. Examples on the test – RIP, IGRP, EIGRP, OSPF, IS-IS

Main Goals of Routing Protocols

• To fill the routing table with current best, loop-free routes

• To notice when routes in the table are no longer valid and remove them from the routing table

• To add new routes or replace lost routes– The time for finding a working route is called

convergence.

Two Categories of Routing Protocols

• Exterior Routing Protocols – used for use between different organizations such as ISPs or ISPs and their customers.– Ex: Border Gateway Protocol (BGP)

• Interior Routing Protocols – used to distribute routing information inside a single organization. – Ex: RIP, IGRP, EIGRP, OSPF, IS-IS

Key Points of Interior Routing Protocols:

• Each interior routing protocol can be characterized based on the underlying logic used by the routing protocol.

• The underlying logic is referred to as the TYPE of routing protocol.

• The three types are:1)Distance vector2)Link-state3)Hybrid

Support for VLSM

• Variable-length subnet masking (VLSM) means that, in a single Class A, B, or C network, multiple subnet masks can be used.

• The advantage of VLSM is that it enables you to vary the size of each subnet, based on the needs of that subnet.

• Some routing protocols support VLSM, and some do not

Classless or Classful

• Classless routing protocols transmit the subnet mask along with each route in the routing updates sent by that protocol.

• Classful routing protocols do not transmit mask information.

• Only classless routing protocols support VLSM. To say that a routing protocol is classless is to say that it supports

No VLSM with RIPv1 or IGRP

• Neither RIPv1 nor IGRP routing protocols has a field for subnet information.

• If a router running RIPv1 has a subnet mask of a certain value, it assumes that all interfaces interfaces within the classful address space have the same have the same subnet mask.

• This is called classful routing, and RIP and IGRP are both considered classful routing protocols.

VLSM with RIPv2, EIGRP, & OSPF

• Classless routing protocols do support the advertisement of subnet information.

• Use VLSM with routing protocols such as RIPv1, EIGRP, or OSPF.

• The advantage with this type of network is that bunches of IP addresses are saved.

More Routing Protocol

Basics

Static vs. Dynamic Routing

• Static routing occurs when you manually add routes in each router’s routing table. It will be covered in subsequent routing command procedures since it is mainly demonstrated through routing commands.

• Dynamic routing is when protocols are used to find networks and update routing tables on routers.

Interior vs. Exterior Gateway Protocols

• IGPs are used to exchange routing information with routers in the same autonomous system (AS).

• An AS is a collection of networks under a common administrative domain, which basically means that all routers sharing the same routing table information are in the same AS.

• EGPs are used to communicate between Ass such as in WAN links.

Administrative Distances

• The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router.

• An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means NO traffic will be passed via this route.

Default Administrative Distances

Route Source Default AD

Connected Interface 0

Static Route 1

EIGRP 90

IGRP 100

OSPF 110

RIP 120

External EIGRP 170

Unknown 255 (no traffic)

The Three Classes of Routing Protocols

• Distance Vector – finds the best path to a remote network using hop count. (RIP, IGRP)

• Link State – (also called shortest-path-first protocols) – the routers each create three separate tables. 1) keeps track of directly attached neighbors, 2) topology of network, 3) the routing table. (OSPF, IS-IS)

• Hybrid – uses aspects of both distance vector and link state. (EIGRP)

Distance Vector Terms

• See board for routing loop example.• Maximum Hop Count – prevents counting

to infinity. – RIP maximum hop count = 15 (16 = infinity)– IGRP maximum hop count default = 100

• Split Horizon – reduces incorrect routing information and routing overhead by enforcing the rule that routing information cannot be sent back in the direction from which it was received.

Distance Vector Terms• Route Poisoning – occurs when an

“unreachable,” or “infinite,” message is distributed when a link is down.

• Holddown Timer – A holddown prevents regular update messages from reinstating a route that is going up and down (called “flapping”). Typically, this happens on a serial link that is losing connectivity and then coming back up. Network might never converge otherwise.

Holddown Timers

• There are 3 instances when triggered updates will reset the holddown timer:1) The holddown timer expires

2) Another update is received with a better metric

3) A flush time, which is the time a route would be held before being removed, removes the route from the routing table when the timer expires.

Routing Information Protocol (RIP)

• A true distance-vector routing protocol.• Sends the complete routing table to all

active interfaces every 30 seconds.• RIP only uses hop count to determine the

best way to a remote network – max hop count = 15, with 16 being “infinite,” or unreachable.

• Works well in small networks.• RIPv1 uses only classful routing.

Interior Gateway Routing Protocol (IGRP)

• Cisco-proprietary distance-vector routing protocol (must use only Cisco routers).

• Classful• Default max hop count = 100.• Can be used in large networks.• Uses a different metric than RIP – IGRP

uses bandwidth and delay of line by default. This is called a “composite metric.”– Reliability, load, and MTU can also be used,

although they are not by default.

Open Shortest Path First (OSPF) Basics

• An open standards routing protocol (non-Cisco proprietary

• Good for large networks (over 15 routers)

• Uses the Dijkstra algorithm

• Only supports IP routing

• Classless

OSPF Logic

1) Each router discovers its neighbors on each interface. The list of neighbors is kept in a neighbor table.

2) Each router uses a reliable protocol to exchange topology information with its neighbors.

3) Each router places the learned topology information in its topology database.

OSPF Logic (cont.)

4) Each router runs the SPF algorithm against its own topology database to calculate the best routes to each subnet in the database.

5) Each router places the best route to each subnet in the IP routing table.

OSPF

• A link-state protocol uses a neighbor table and a topology database in addition to adding routes to the routing table.

• OSPF converges more quickly than do distance-vector protocols.

• Uses “Hello” packets to keep in touch with neighbors.

• Defined in RFC 2328

Enhanced Interior Gateway Routing Protocol (EIGRP)

• Cisco proprietary• Classless• Uses autonomous system numbers

– A number assigned to a group of routers under mutual administration.

• Referred to as a “hybrid routing protocol” • Provides support for IP, IPX, and Appletalk• Best path selection using the Diffusing

Update Algorithm (DUAL)

EIGRP

• Before EIGRP routers exchange routes with each other, they must become neighbors.

• There are three conditions that must be met for neighborship establishment:

1) Hello or ACK received

2) AS numbers match

3) Identical metrics

EIGRP Feature Comparison

Link-state Features• Converges quickly• Discovers neighbors

via “Hello” packets• Builds topology table• After learning its

neighbor’s routes, only changes to the routing table are propagated.

Distance-vector Features• Uses autonomous

system number (like IGRP)

• Uses metric based on bandwidth & delay

• Advertises entire routing table to new neighbors.