Dynamic Routing Protocols

Question 1

Dynamic vs. Static Routes

Answer 1

Dynamic routing protocols are more scalable than admin-defined static routes
Using purely static routes is only feasible in very small networks
Using a combo of the two is very common

Question 2

Dynamic Routing Protocol Advantages

Answer 2

Routers automatically advertise available subnets to each other without the administrator having to manually enter every route on every router

If a subnet is added/removed, the routers will automatically discover that & update their routing tables

Question 3

2 types of Routing Protocols

Answer 3

IGP (Interior Gateway Protocol)

EGP (Exterior Gateway Protocol)

Question 4

The only EGP in use today

Answer 4

BGP (Border Gateway Protocol)

Question 5

2 Types of IGPs

Answer 5

Distance Vector

Link State

Question 6

Distance Vector Protocols

Answer 6

AKA: “Routing by rumor”

Each router sends its directly connected neighbors a list of all its known network along with its own distance to each of those networks

Do not advertise the entire network topology

Router only knows its directly connected neighbors & the lists of networks those neighbors have advertised

No detailed topology info beyond neighbors

Question 7

Link State Protocols

Answer 7

Each router describes itself & its interfaces to its directly connected neighbors

Info is passed unchanged from one router to another

Every router learns the full picture of the network including every router, its interfaces, & what they connect to

Question 8

RIP

Answer 8

Routing Information Protocol:

Distance Vector

Uses hop count as the metric

Max hop count is 15
More than 15 hops will be marked unreachable

Only used in small/test networks

Question 9

OSPF

Answer 9

Open Shortest Path First:

Link State Protocol

Uses “Cost” as the metric

Automatically derived from interface bandwidth by default

You can manually configure the cost of links if you want to manipulate the path

Question 10

IS-IS

Answer 10

Intermediate System to Intermediate System:

Link State Protocol

Uses “cost” as the metric
(Not automatically derived from interface bandwidth)
(All links have equal cost by default)

Can manually configure cost of links to manipulate the path

If you do not want to manually set link costs, the lowest hop count will be used

Question 11

EIGRP

Answer 11

Enhanced Interior Gateway Routing Protocol:

Advanced Distance Vector (Kind of a hybrid between distance vector & link state)

Uses bandwidth & delay of links to calculate metric
(Load/reliability can also be considered but ignored by default)

A fixed delay value is used based on the interface bandwidth
(Does not measure current delay dynamically)

You can manually configure delay on links if you want to manipulate the path

Question 12

Equal Cost Multi Path (ECMP)

Answer 12

If multiple paths to a destination have an equal metric, the router will enter all of the paths into the routing table

Equal cost multi path will load balance the outbound traffic to the destination over the different paths

All IGP routing protocols perform ECMP by default

EIGRP is the only routing protocol capable of UnEqual Cost Multi Path
(Requires manual configuration)

Question 13

Administrative Distance (AD)

Answer 13

Measure of how trusted the routing protocol is
Protocol with the lowest AD wins
AD is considered first before calculating the metric to narrow the scope

Question 14

Administrative Distance Table

Answer 14

Connected Interface = 0
Static Route = 1
External BGP = 20
EIGRP = 90
OSPF = 110
IS-IS = 115
RIP = 120

Question 15

Floating Static Routes

Answer 15

If the best path to destination is lost
It will be removed from the routing table & replaced

We might want to configure a static route as a backup

Static routes have AD of 1, so it will be always preferred by default
You can change the AD of a static route (floating static route)

Example:
Ip route 10.0.1.0 255.255.255.0 10.1.3.2 115
Changes AD to 115

Question 16

Loopback Interfaces

Answer 16

Logical interfaces that allow you to assign an IP address to a router/L3 switch (not tied to a physical interface)

Can never go down

Question 17

Loopback Interfaces: Uses

Answer 17

Best practice to assign a loopback interface to routers

Commonly used for traffic that terminates on the router itself
(This could be management traffic, VoIP, BGP peering)

Used to identify the router (Router ID) in OSPF

The same loopback interface is used for multiple tasks

Multiple loopbacks can be configured (not common, only done when required)

Question 18

Adjacency

Answer 18

When the routing protocol is enabled on an interface the router will look for other devices on the link which are also running the same protocol

Routers do this by sending out & listening for hello packets (via multicast)

When a matching peer is found, the routers form an adjacency with each other
They will then exchange routing info

Question 19

Passive Interfaces

Answer 19

Allow you to include an IP subnet in the routing protocol without sending updates out of the interface

Best practice to configure loopbacks as passive interfaces

Impossible to form an adjacency on a loopback (not physical)

Making loopback passive means it will be advertised by the routing protocol but it will not waste resources sending out & listening for hello packets