Questions tagged [routing]

For questions about routing, the process of selecting to which network path traffic is sent. Consider using a more specific tag if your question is about a routing protocol like BGP or OSPF.

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NAT Overlapping

I am trying to do some networking lab. Basically, I have a network like this: So, is it possible to configure NAT router in such way, that PC1 and PC2 can access remote HOST and each other. The ...
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23 votes
4 answers
73k views

Why must all areas connect to an Area 0 backbone in OSPF?

This question is a literal repost of the same question asked on The Cisco Support Community. Answers are unique to Stack Exchange. Why is area 0 the backbone area in OSPF? Why must all other areas ...
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2 votes
3 answers
318 views

Is an entirely decentralised peer-to-peer network feasible? How far could it be scaled?

Most of the reading I've done about P2P networks are either about small scale simple mesh networks, where every peer can talk to another, or about high level protocols such as bittorrent, where data ...
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19 votes
2 answers
8k views

How does the router know how to route the packets to my terminal?

Suppose I have a network of 2 computers, each having a private network address of 192.168.1.101 and 192.106.1.102. For my public ip address, it is 10.2.10.172. When I make a http request to google....
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24 votes
7 answers
9k views

Does CIDR really "do away" with IP address classes?

I am still struggling to understand to what extent CIDR really renders IP address classes obsolete. Here's what I understood so far: It's ridiculously inefficient (and impossible, too) to assign ...
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15 votes
3 answers
22k views

MPLS vs Encrypted VPNs - traffic security?

Why do people say a lot of times that they have two connections between two offices - the main one being over MPLS and the backup one over VPN. Why not run a VPN over MPLS too? Is MPLS secure? Can no ...
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9 votes
3 answers
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How are MAC/IP addresses used in routing

I have to teach my sixth form about MAC Addresses and IP Addresses in a few weeks time and there are some areas I am unclear on which I am hoping some of you will be able to clarify for me. I have ...
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6 votes
2 answers
681 views

VLAN-Subnets 1:1?

I often hear people saying that each VLAN is a subnet (or is associated with a subnet) but is that really true? Assume I have 10 big offices. I will have a subnet for each office but only 2 VLANs ...
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15 votes
5 answers
15k views

When is BGP needed?

I have a question about BGP. When exactly is BGP needed? I remember reading that BGP is needed for communication between autonomous systems, but is this always true? For example, in packet tracer I ...
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20 votes
4 answers
23k views

Risks of using non-private IP addresses internally?

My company has received a large industrial machine with many networked devices on it. Unfortunately the engineer in charge has used a public IP address range on the machine. I'm in Europe. The chosen ...
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17 votes
2 answers
12k views

In which OSI/TCP-IP model layers do BGP, RIP protocols belong?

We know that in the OSI or TCP-IP model, the higher layer utilizes the services offered by the lower layer. For example, the data link layer uses services offered by the physical layer, network layer ...
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2 votes
2 answers
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What is nodal delay?

I keep seeing the definition of nodal processing delay as "The time it takes to process a packet in a network node (router, switch, hub, etc.), which is dependent on the speed of the device and ...
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11 votes
1 answer
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How is BGP "AS_SET" used?

RFC 1771 defines a path attribute type of AS_PATH as follows: AS_PATH (Type Code 2): AS_PATH is a well-known mandatory attribute that is composed of a sequence of AS path segments. Each AS path ...
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7 votes
2 answers
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IPv6 and online privacy and security

I am studying up on IPv6 and part of the talk was about how the machines generate or can have multiple IPv6 addresses for internal use and external use. Does this mean that there is no longer a "...
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3 votes
2 answers
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minimum hop limit for a packet where there are n routers between source and destination

I am having some confusion while calculating he minimum hop limit ... consider following scenario: (source)-------(R1)--------(R2)--------(destination) Now when an IP datagram is supposed to travel ...
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10 votes
1 answer
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How does NAT share one public IPv4 address across multiple private IPv4 addresses [closed]

I think it's a very classical question, but, thought I've have some background, I don't have enough vocabulary anymore to search and understand correctly in the Web. Let's say I have a home network ...
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10 votes
3 answers
4k views

Inbound BGP load-balancing from same ISP router

I have two BGP routers for DIA with an issue with one provider in that the two links to this provider land on the same SP router. My ethernet handoffs via two separate MetroE links from another ...
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8 votes
1 answer
6k views

Static routes: numerical next-hop vs interface

I'm reviewing old Cisco modules, and in CCNA 2, Module 2.5.1.2, it states, Under certain conditions, the network administrator will not want to configure the static route with an exit interface but ...
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5 votes
3 answers
4k views

Does a router send frames or packets?

Perhaps it's a novice question, but does a router send/receive frames? When I read about this subject, it seems that routers decapsulate the frame on arrival, and encapsulate the packet in a frame in ...
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3 votes
3 answers
6k views

At which layer does router operate?

I have studied from various sources that router is a layer 3 device. It does routing based on Network layer (layer 3) header, but I have a doubt. Network address translation (NAT) is a feature of ...
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7 votes
2 answers
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Dell PowerConnect 5524 VLAN routing

I have a Dell PowerConnect 5524 that I'm trying to configure. What I want is simple routing between VLANs and the user guide is less than helpful. On page 220, it says (or implies) that all you need ...
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5 votes
1 answer
485 views

Who decided the Admin Distance Values for each protocol?

OSPF has an administrative distance of 110 EIGRP has an administrative distance of 90 RIP has an administrative distance of 120 But where did 110, 90, and 120 come from? Who or what entity decided ...
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3 votes
4 answers
282 views

Do all network devices need to make routing decisions, regardless of communication across networks or within a network?

From Hunt's TCP/IP Network Adminstration: Gateways route data between networks, but all network devices, hosts as well as gateways, must make routing decisions. Is it correct that routing ...
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3 votes
1 answer
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What is the point of a subnet mask on a local network?

I have a question regarding subnet masks and my understanding (or misunderstanding of them). So when a subnet mask is used, a bitwise AND is performed using the subnet mask and the IP address(es). ...
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2 votes
2 answers
450 views

CGN and its advantages

I have been reading about CGN. Though similar to NAT it is used by service providers to route traffic without the need for the a public IP address. But when the packet leaves the ISP to the public ...
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2 votes
2 answers
365 views

Static Routes and selection

For what I know, Static Routes don't have a Metric. So, what if a Router has more than one Static Route to a Network? Let's say that on the topology below we have Static Routes configured, and R1 can ...
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2 votes
1 answer
551 views

Cisco Prefix List Won't Appear in Route Table

I have the following prefix-list configured on our router: ip prefix-list adsvr seq 5 permit 172.30.5.241/32 ip prefix-list adsvr seq 10 permit 134.239.130.143/32 ip prefix-list adsvr seq 15 permit ...
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2 votes
2 answers
5k views

What is the election process of Router ID in OSPF?

I am a little confused on the process OSPF uses to select the Router ID. Is following correct for the selection of the router ID in OSPF? Manual configuration Highest IP address on a loopback ...
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1 vote
1 answer
82 views

Best path in BGP Routing

Assuming 'Network A' is having 2 ISPs which are directly connected to 'Network B', which path will be used to connect to 'Network B'? Network A --> IP Transit A / IP Transit B --> Network B
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1 vote
1 answer
639 views

BGP Best Path Selection Algorithm

According to this cisco document (https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html), shortest AS_PATH is the #4 algorithm in the sequence to evaluate, out of 13 ...
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1 vote
1 answer
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Why does OSPF use flooding instead of broadcasting?

In flooding, we send unicast/multicast frames to all the routers in the network, whereas in broadcasting, we have a broadcast address by which we send packets to all the routers. In OSPF, "Router ...
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46 votes
8 answers
4k views

Network Neutrality, how does it actually work, technically speaking?

In this NPR article there is mentioned 'network neutrality' with very little details on what it actually is or how it actually works. I tried researching it on my own but I get a lot of non-technical ...
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23 votes
5 answers
54k views

Why use iBGP inside an Autonomous System, if IGP protocols fulfill the need for internal communication

Can anyone explain why we need iBGP for the routes when we have the IGP protocols (OSPF, RIP) for internal communication within the AS? I have read a lot of articles and books, but I could not find ...
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39 votes
8 answers
31k views

How would a router with /32 WAN subnet mask communicate with the rest of the Internet?

I'm studying networking and one of the most basic things I've been told countless times is that computers can't communicate outside of their own subnet. On the other hand, by looking at some ADSL ...
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22 votes
7 answers
39k views

Difference between access list and prefix list?

Can someone explain with an example whats the difference between an access list and prefix list.
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18 votes
8 answers
12k views

Why are IP addresses given to each interface and not device? What would the implications of that be?

Why do we need to give IP addresses to each interface? Wouldn't giving one to each device be enough?
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16 votes
7 answers
110k views

What are the differences between VRRP and HSRP?

Are there any significant differences between VRRP and HSRP?
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11 votes
3 answers
30k views

Whats's the function of a loopback interface in routing protocols?

So I've got this task where I have to setup a virtual network with an OSPF routing protocol. I firstly ignored this loopback interface I had to configure on the routers since it didn't do any ...
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13 votes
5 answers
15k views

Why does OSPF need Type2 LSAs?

Learning more about OSPF for CCNP studies. I'm looking at how OSPF builds its links, and have just covered Type1 LSAs. Looking at Type1 LSAs, I'm wondering why these are even necessary? The book ...
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12 votes
1 answer
29k views

RIB vs FIB differences?

Can someone explain the differences between a RIB and FIB a little further? I didn't see any similar questions on the site so I thought this would be a good addition. The RIB routes and the FIB ...
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5 votes
3 answers
7k views

Can a network with a default gateway successfully ping a PC in another network without gateway?

Please see the picture first. Let someone ping from PC1 to PC2 (which has no default gateway). Would it be possible for the ping to successfully reach to PC2 and then come back in the fashion ...
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  • 175
5 votes
1 answer
347 views

Optimizing Network Topology

Recently I wish to split the network by VLANs. Mainly for two reasons Exclude management interfaces from the commonly accessible lan Separate WiFi traffic (security and broadcasts) The current idea ...
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  • 205
14 votes
2 answers
10k views

What is a non-portable IPv4 address block?

For example, a Whois query for 4.4.4.0 says: ADDRESSES WITHIN THIS BLOCK ARE NON-PORTABLE Does the owner simply declare the block or a subnet are not up for sale? Or does non-portable have a ...
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13 votes
3 answers
12k views

OSPF Stuck in Exstart Adjacency State

According to this article two routers will get stuck in the exstart adjacency state when their configured MTU sizes do not match. Does a VLAN tag affect this as well even if the routers involved are ...
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  • 311
9 votes
3 answers
4k views

Cisco NAT hairpinning

I have a Cisco router configured NAT (4 static NAT and dynamic NAT). My problem is that I can't access internal servers using public IP addresses from the internal network. I know what the problem ...
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8 votes
1 answer
2k views

Something is filling my force10 ARP table

I have 2 force10 s25 switches that are used for a dark fiber link between two school buildings. One of my switches is filling its ARP table to the brim and then failing. This can happen as often as ...
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7 votes
2 answers
16k views

Poison reverse example

Can you give me an example when poison reverse is actually necessary? Distance vector routing protocols employ split horizon with poison reverse to minimize the convergence time when a route is no ...
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  • 393
7 votes
3 answers
5k views

OSPF and its use of Multicast

When it comes to OSPF and its use of multicast, am I correct in believing that IGMP forwarding and an IGMP querier is still required within the local segment? My understanding would be without this, ...
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  • 269
7 votes
3 answers
2k views

Alternatives to multiple Service Providers

I have one SP right now that serves 100 Sites and we are currently planning to migrate all the sites to a new SP. However, this will be a phased migration, so we will have some sites in the new SP ...
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6 votes
4 answers
72k views

How can hosts on two different VLANs communicate?

It is clear that hosts on two different VLANs can't directly communicate on a single switch, but how do they communicate to each other at all? Do they use a router? How does it work?
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