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COURSES NETWORK+ > SECTION 8 : ROUTING > Flashcards

SECTION 8 : ROUTING Flashcards

1
Q

What is a router ?

A

Forwards traffic between subnets, between an internal and external network. Each subnet or external network is going to be its own broadcast domain. Keep in mind that a multilayer switches also perform routing functions.

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2
Q

Explain the action of the router when receiving a data frame from a switch

A

It packages up the frame to make an IP packet that will be sent to the other router of another network.

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3
Q

What is a routing table ?

A

Helps determine which route entry is the best fit for the network. Juste like a switch has its MAC address table (ARP packet).
A route entry with the longest prefix is the most specific network. (The one with the most important number of octets being specified). For instance 10.1.1.0/24 is more specific than 10.0.0.0/8. Every link in a routing table does have a cost associated with it.

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4
Q

What is an ARP cache ?

A

To map an IP address to a given MAC address.

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5
Q

What is a directly connected Route ?

A

Learned by physical connection between routers. If three routers are physically connected (direct cabling) zigzag lines are frequently used.

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6
Q

What is static route ?

A

Manually configured by an administrator. There is always one default static route —> 0.0.0.0/0. It’s like a default gateway.,

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7
Q

What is dynamic routing ?

A

Learned by exchanging information between routers. When R1 and R2 becomes connected they share their routing table. It’s done automatically using dynamic routing protocol. When you connect to the internet the ISP is the default route.

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8
Q

What are the three different types of route ?

A
  • directly connected
  • static
  • dynamic
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9
Q

How does the router know which is best ?

A

The dynamic routing protocol negotiates using bdwth, number of hops and so on.

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10
Q

What are the techniques to prevent network loops ?

A
  • split horizon
  • poison reverse
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11
Q

What is split horizon ?

A

Prevents a route learned on one interface from being advertised back out of that same interface. Similar to STP of layer 2.

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12
Q

What is poison reverse ?

A

Causes a route received on one interface to be advertised back out of that same interface with a metric considered to be infinite. The cost is so high so that it’s never used. The number of hop will be very very high.

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13
Q

What are the two types of Dynamic Routing protocols ?

A
  • internal : interior gateway protocol operates with an autonomous system
  • external : operates between autonomous systems - exterior Gateway Protocol
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14
Q

What is the router advertisement method

A

It’s a feature of every routing protocol. Every protocol will look at routing differently using :
- distance vector (number of hops)
- link state (bdwth available)
- hybrid
We have to think about that when we select the protocol.

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15
Q

What is distance vector ?

A

Sends full copy of routing table to its directly-connected neighbors at regular intervals. Number of routers you connect. Slow convergence (time it takes for routers to update their routing tables in response to a topology change). When you add a router to a network, it takes time for the other routers to know the new router is here. Once everybody knows it’s called a converged network.

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16
Q

How to speed-up convergence time ?

A

We use a hold-down timer. It prevents updates for a specific period of time and speeds up convergence. When we deal with distance vector we use hop-count. Number of routers from the source router through which data must pass to reach the destination network.

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17
Q

What does link state require ?

A

It requires all routers to know about the paths that all other routers can reach in the network.

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18
Q

What are the different link state protocols ?

A

OSPF and IS-IS

19
Q

Which feature allows us to have the fastest convergence time ?

A

The link state has a faster convergence time and uses cost or other factors as metrics. The link speed is really important.

20
Q

What is RIP ?

A

The Routing Information Protocol.
A distance vector protocol that uses hop count (maximum hops of 15; 16 is infinite). Above 16 the connection is dead. Oldest.
It is an interior gateway protocol.
It provides updates every 30 secs so hard to maintain convergence.
Easy to configure and runs over UDP.

21
Q

What is OSPF ?

A

Open Shortest Path First.
A link state protocol that uses cost. Very popular. It’s an interior gateway protocol. A lot more efficient. Cost is based on link speed between routers.

22
Q

What is IS-IS ?

A

A link state protocol that also uses cost and functions like OSPF protocol.
Not as widely popular. Also interior gateway protocol.

23
Q

What is EIGRP ?

A

Enhanced Interior Routing Protocol.
Hybrid of distance vector and link state protocols that uses bandwidth and delay. It takes into account the hops and the speed. It’s proprietary Cisco protocol that is popular in Cisco-only networks. OSPF is vendor neutral.

24
Q

What is BGP ?

A

It’s an external gateway protocol —> border gateway protocol.
A path vector that uses the number of autonomous system hops instead of router hops. All the others we’ve talked about are interior. It’s concerned about the number of AS.
Backbone of the internet.
Widespread utilization.
Slow convergence. It can take an hour.

25
Q

What is route believability ?

A

If a route has a lower administrative distance the route is more believable. For instance à directly connected network has an AD of 0.
A static connected network has an AD of 1 (Manually configured by the administrator). The order after those two routing information source is : EIGRP, OSPF, EIP, External EIGRP

26
Q

What are the metrics ?

A

Hop count, believability, reliability, bdwth, delay, cost and other.
Keep in mind that lower metrics are preferred over higher metrics when we talk about routing.

27
Q

What was the objective of NAT ? And what is the basic principle of a NAT-enabled router ?

A

Used to conserve the limited supply of IPv4 addresses. It translates the private IP addresses to public for routing over public networks.

The router keeps in memory the IPs of its network and reassign an inside global address to internal devices. The router strips off the source IP, replace it with the inside global address and then sends to the destination.

28
Q

What are the three different types of NAT ?

A
  • DNAT
  • SNAT
  • PAT
29
Q

What is DNAT ?

A

Dynamic NAT. Automatically assigns an IP address from a pool and gives a one to one translation. In microsecond we share an IP and give it back.

30
Q

What is SNAT ?

A

Static assignment. Manually assigns an IP address and gives a one-to-one translation. One card for every person. What is the advantage ?
Used as a security feature. It’s a kind of smoke screen.

31
Q

What is PAT ?

A

Sharing of one public IP by multiple private IP adresses which gives a many-to-one translation. Small offices are one IP for instance.
When PC1 and PC2 connects to internet they can’t have the same IP so we use ports to keep track of the different segments. The router remembers the port of the inside local and use the same for inside global address.

32
Q

What is the inside local address ?

A

Private IP address referencing an inside device. Any time we see «local» we must think about private IP address.

33
Q

What is the inside global IP address ?

A

Public IP address referencing an inside device. Any time we see «global» we must think about public IP address. It’s the address that our router will use. It’s the IP of a network becoming global.

34
Q

What is the outside local ?

A

Private IP address referencing an outside device. Any time we see «local» we must think about private IP address. It’s the private IP address that references an outside device such as a router. It’s the inside «side» of the router.

35
Q

What is the outside global ?

A

Public IP address referencing an outside device. Any time we see «global» we must think about public IP address.

36
Q

What is multi cast routing ?

A

Multicast sender sends traffic to a class D IP address known as a multicast group.

37
Q

What is the two protocols allowing Multicast routing ?

A

IGMP : internet group management protocol
PIM : protocol indépendant multicast

38
Q

What is IGMP ?

A

Let’s routers know which interfaces have multicast receivers and allows clients to join a multicast group.
Three versions:
V1: client requests joining the group and it asked every 60 s if it wants to remain. A lot of unnecessary traffic
V2: client can send a leave message to exit
V3: client can request multicast only from specific server and allows source-specific Multicast (SSM) and multiple video streams to a single multicast stream

39
Q

How does IGMP work ?

A

If a server wants to send out traffic it will send to the router and the IP address of the multicast group. If PC2 joins the multicast traffic by sending the join message to its default gate away, the router remembers that PC2 is part of the group.

40
Q

What is PIM ?

A

Routes multicast traffic between routers and forms a multicast distribution tree. Works a lot between routers where IGMP was about Client and server.

41
Q

What are the two modes of PIM ?

A

Dense mode (PIM-DM)
Sparse mode (PIM-SM)

42
Q

What is PIM Dense mode ?

A

Uses periodic flood and prune behavior to form optimal distribution tree. Not used in moderne network.

All of the routers will get all the information. Then if a router receives multicast traffic in the initial flood and the traffic is not needed then the router sends a prune message asking to be removed femme the Source Distribution Tree. Flood and prune —> every three minutes to find the best route. A lot of traffic.

43
Q

What is PIM sparse mode ?

A

Uses a shared distribution tree and creates an optimal distribution tree through shortest path tree (SPT) switchover. When we start this shared tree is not optimal but with time it becomes better.

When the server sends out the first message the router will send it over anywhere it can to get from the first router then to the last hop.
RDV point and last hop
It’s not optimal but overtime the optimal path will be found and Shortest Path tree will be reached.

COURSES NETWORK+ (7 decks)

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