Introduction to Routing: Routed and Routing Protocols

Presented byJack Crowder, CCIE

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Agenda• Terms• Purpose, Function, and Considerations of routing

protocols• Classfull and Classless• Flat vs. Hierarchical• Routed Protocol vs. Routing Protocol• Protocol Table vs. Routing Table• Concepts

– Distance Vector vs. Link State Protocols• Comparison Matrix• Building a Routing Table

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Terms• Advertise[ment]• Authoritative• Autonomous System• Cache Table• Classfull• Classless• Convergence• Dampening• Default Route• Forwarding Decision• Messaging

• Metrics• Neighbors• Protocol Table• Redistribute• Routed Protocols• Routing Domain• Routing Protocols• Routing Table• Summarization• Timers• Updates

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Routing ProtocolsPurpose: Layer 3 Guidance

• Provide Layer 3 guidance to RoutedProtocols’ (IP, TCP, UDP) datagrams(packets) to reach destinations not connectedto the sources’ directly attached (Layer 2)media, across a contiguous (public and/orprivate) IP networks (WANs and/or LANs).

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Layer 3 Guidance

#VALUE!

Ethernet Ethernet

192.168.5.119

00:1A:68:F4:29:8E 8F:42:E0:01:A6:98

LAYER - 2

LAYER - 3

172.15.99.207

Router

84:ed:9a:3f:4c:7784:da:9c:56:be:32

172.15.99.1192.168.5.1

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Routing ProtocolsFunction: Messaging

• Routing Protocols build Routing Tables byexchanging messaging information– Convergence

• Routing Tables are used for “Forwarding”decisions

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Layer 3 Messaging

#VALUE!

Ethernet Ethernet

192.168.5.119

LAYER - 2

LAYER - 3

172.15.99.207

Router

172.15.99.1192.168.5.1

Ethernet

1.1.1.1 1.1.1.2

Router

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Routing ProtocolsConsiderations: Messaging

• Servers (Layer 4-7) serve and Routers (Layer 3-4) route– No overlap of function

• Router to Router communication– Next-hop consideration

• Autonomous System– Extent of Routing Domain(s) within your control

• Routing Domain– Extent of a routing protocol(s) within your control

• Routing Domain to Routing Domain communication– Redistribution

• AS to AS communication– Peering

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Classfull• Classfull routing protocols do not include the

subnet mask with the route advertisement.• Within the same network, consistency of the

subnet masks is assumed.• Summary routes are exchanged between

foreign networks.• Classfull

– Examples: RIPv1, IGRP

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Classless• Classless routing protocols do include the

subnet mask with the route advertisement.• Classless routing protocols support

variable-length subnet mask (VLSM).• Summary routes can be manually controlled

within the routing domain.• Classless

– Examples: RIPv2, EIGRP, OSPF, BGP

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Flat vs. Hierarchical Design• Flat

– Protocols that don’t support areas• Hierarchical

– Protocols that do support areas– Route summarization based on areas

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• Routed Protocols– Can be routed using Routing Protocol(s)– Carry information used by Routing Protocol(s)

• RIF: Routing Information Field– Examples: TCP, UDP, IP

• Routing Protocols– Routes a routed protocol datagram (packet)– IGP (Interior Gateway Protocol) vs. EGP

(Exterior Gateway Protocol)– Examples: RIP, OSPF, BGP

Routed vs. Routing Protocols

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Routing Concepts:How to reach a Destination

• Next-Hop based– Send packet to [Layer 3 protocol address] next hop– Examples: RIPv1&2, Next Hop Routing Protocol

• Metric based (i.e. Distance Vector)– Send packet to next hop with best (e.g. lowest) metric– Types: Bandwidth, Delay, Reliability, Load, MTU– Examples: IGRP, EIGRP

• Best Path based (i.e. Link State)– Send packet along a certain (best) path– Shortest Path First– Examples: OSPF, IS-IS

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Distance Vector• Distance: Cost based on metrics• Vector: Path to follow• Bellman-Ford Algorithm

– Update on timers• Router sends its’ full Protocol Table to router’s

direct neighbors ONLY• Loop avoidance based on Hop count• Examples: RIP v1 & 2, IGRP, EIGRP

– EIGRP has added update on network change

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Link State• Link: Cost to reach Link based on cost of Links• State: Update on “state” change (e.g. “up to down”

or “down to up”)• Dijkstra Algorithm: higher CPU requirements• Each router sends its [link] state to all other routers*• Router sends its’ Link State database (i.e. Protocol

Table database) to every other router in network*• Every router knows topology of entire network*

– Shortest Path First• Examples: OSPF, IS-IS

* Within the “area”

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Protocol Table vs. Routing Table

CONNECTED

ROUTES

STATIC

ROUTES

ADMINISTRATIVE

DISTANCE

cache

table

routing

table

EIGRP

ROUTES

OSPF

ROUTES

SPF

BGP

ROUTES

METRICS

RIP

ROUTES

DUAL

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Protocol Table vs. Routing Table• Protocols calculate one or more paths to destination subnet

and place [best] subnet into Protocol Table based on:– Next-Hop– Metric– Best Path

• Best subnet is compared to identical (i.e. same subnet andmask) best subnet in other protocols (if such a path exists)

• Subnet with lowest Administrative Distance (based onprotocol type) is then injected into Routing Table - *IF* nexthop is active

• Path with longest subnet match is used for ForwardingDecision: 192.168.5.0 /24 is used instead of 192.168.0.0 /16

• Copy of subnet used for Forwarding Decision is injected intoCache Table

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Packet “Forwarding Decision”1. The router first checks to see if its’ Cache Table is

populated2. If the Cache Table is populated, the router first consults

the Cache Table3. If router doesn’t find a match in the Cache Table, then the

router consults the Routing Table4. If the router finds a match in the Routing Table, that route

is then injected into the Cache Table5. If router doesn’t find a match in the Routing Table, then

the Default Route is used6. If there is no Default Route configured, then the packet is

dropped

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RIP

Sta

tic

IS-I

S

OS

PF

EIG

RP

IGR

P

Link

State

Admin

Distance

Distance

Vector

IGP

EGP

Classless*

120

BG

P

Int: 200

Ext: 20

interface: 0

next hop: 1

floating: ?115110

summ: 5

Int: 90

Ext: 170100

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RIP

Sta

tic

IS-I

S

OS

PF

EIG

RP

IGR

P

Metric(s)

Cost

calculation

Update on?

Max hops

Router

Codes

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Topology

A D

CB

11

4

3

2

1

203

202

12

204

201

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Routing Table: router ARoute Known via Type1 direct connected2 rtr B, C, D protocol3 rtr B, C, D protocol4 rtr B, C, D protocol11 direct connected12 rtr B, C, D protocol201 direct connected202 direct connected203 rtr B, C, D protocol204 rtr B, C, D protocol

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Routing Table – exampleTLab-term-srv> sh ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.5.1 to network 0.0.0.0

O 192.168.90.0/24 [110/11] via 192.168.5.1, 7w0d, Ethernet0/0O 192.168.15.0/24 [110/11] via 192.168.5.1, 7w0d, Ethernet0/0 192.168.240.0/24 is variably subnetted, 7 subnets, 3 masksO IA 192.168.240.251/32 [110/173] via 192.168.5.1, 7w0d, Ethernet0/0C 192.168.5.0/24 is directly connected, Ethernet0/0O E2 192.168.232.0/24 [110/20] via 192.168.5.1, 7w0d, Ethernet0/0 192.168.249.0/32 is subnetted, 1 subnetsS 192.168.0.0/16 [1/0] via 192.168.5.1O*E2 0.0.0.0/0 [110/1] via 192.168.5.1, 7w0d, Ethernet0/0

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Route Entry – example

O IA 192.168.240.251/32 [110/173] via 192.168.5.1, 7w0d, Ethernet0/0

Code: Inter-Area OSPF

Subnet & Mask

Administrative Distancebased on Protocol Type

Route Cost: based on Protocol metrics

Next HopIP Address

Uptime

Address onInterface