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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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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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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37 votes
3 answers
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Does a traceroute/tracert show every hop, or does it skip/hide some details of the path?

I am currently in college for a bachelor's degree in Network Engineering, and one of my Professors explained in class that a traceroute that shows, for example, 15 hops is actually abstracting the ...
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29 votes
2 answers
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Is multicast on the public internet possible? And if yes: How?

Is it possible to use multicast on the public internet? If yes: How? Are special IP addresses required and where do you get them from?
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24 votes
7 answers
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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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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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23 votes
5 answers
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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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22 votes
7 answers
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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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21 votes
3 answers
4k views

CEF operations on L3 switch vs router

I understand that L3 switches use CEF for faster packet forwarding. However, CEF will not work for packets that need to undergo NAT. Does this mean CEF isn't used by edge routers and edge L3 switches? ...
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20 votes
4 answers
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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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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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18 votes
8 answers
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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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18 votes
3 answers
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How does multihoming work with ipv6?

What are the differences between multihoming in IPv4 versus IPv6? Can an enterprise request a provider-independent IPv6 address space from its RIR/LIR which can be advertised to multiple upstream ...
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17 votes
3 answers
3k views

Is it possible to specify a route for a packet to take?

So, AFAIK, packets "hop" between routers. Packets are forwarded via a router's default path until it gets to destination IP. So is it possible to specify a specific set of routers this packet "hops" ...
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  • 275
17 votes
2 answers
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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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  • 441
16 votes
7 answers
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What are the differences between VRRP and HSRP?

Are there any significant differences between VRRP and HSRP?
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16 votes
3 answers
12k views

mtr output with high packet loss on one hop

I'm investigating a complaint of poor performance from an end user accessing a site I help maintain. I have these two mtr outputs to the end user, the first from the site: ...
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16 votes
6 answers
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Use BGP to defend against a DDoS attack originating from remote AS

I have a question regarding BGP and how to achieve this configuration. My enterprise core router is connected to an ISP (single homed). This router has already exchanged the specific public ip ...
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16 votes
2 answers
11k views

BGP multipath with different ASNs feasible for production networks?

on Cisco (this command is hidden for some reason) #bgp bestpath as-path multipath-relax The default BGP behavior only installs only routes with exact the same AS_PATH into RIB. With multipath-relax, ...
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15 votes
3 answers
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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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  • 253
15 votes
2 answers
10k views

ASICs vs x86 general-purpose routing/switching

SysAdmins often try to convince me that x86 general-purpose OS's can perform just as well as routers with low MHz CPUs and dedicated silicon (i.e., ASICs) at 1Gbps line rates. This thinking is ...
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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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  • 307
14 votes
2 answers
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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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14 votes
2 answers
2k views

BGP regular expression .+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+

could anyone tell me what ip as-path access-list 100 deny .+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+ stands for?! I searched over the internet for the meaning of this regular expression, but I was unable to ...
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  • 275
14 votes
3 answers
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How broadcasting works on different networks?

I was reading DHCP ( RFC 2131 ), I have basic knowledge about Relay Agent and Broadcasting. But for Understanding DHCP in depth, I felt like I have to know Broadcasting and Relay Agent ( RFC 1542 ) in ...
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14 votes
6 answers
26k views

What happens when the ARP cache overflows?

In at least one implementation there is a hard limit on the capacity of the ARP table. What happens when the ARP cache is full and a packet is offered with a destination (or next-hop) that isn't ...
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  • 869
13 votes
1 answer
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What is the difference between a VRF and GRF? (cisco)

I've heard about VRF's and GRF's, but was is the difference between those 2? (cisco)
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13 votes
5 answers
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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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13 votes
3 answers
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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
12 votes
6 answers
4k views

Best practice for dual-homing site with two ISPs?

I am subscribed to two ISPs, a fast expensive one and a cheap but slower one. They use different technologies, cable and ADSL, so there isn't much of a single point of failure, and all my comms ...
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  • 241
12 votes
2 answers
11k views

Error: `EIGRP is restricted to stub configurations only on this platform`

We just bought some new Cat4507R+E distribution switches. When we bought the switches, we asked for an Advanced Services K9 image, but our reseller told us they only come with a Universal K9 image. ...
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  • 193
12 votes
3 answers
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How does a switch know where to route packet, when there is another switch in front of the destination?

I am just wondering how a switch does know how to route packets to destinations "hidden" behind another switch. In my understanding a switch has a table that maps each of its hardware ports to exactly ...
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12 votes
1 answer
3k views

After TCP is established which BGP peer will send open message first?

After a Tcp connection is established between two bgp peers. which peer will send the open message first ? the active peer(which is initiating outbound connection) or the passive peer?(peer which is ...
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  • 1,095
12 votes
1 answer
5k views

Is it possible to connect two private eBGP peers with the same AS number?

The Problem A third party has implemented a small network and private BGP setup within one of our branches. We need to setup a eBGP peer relationship with this network. However we are both using the ...
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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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  • 940
12 votes
2 answers
17k views

why we can't create multiple loopback interfaces in Junos?

comparing to Cisco IOS , we can create any number of loopbacks on router , but on Junos you can create only one loopback interface (even logical units from same loopback) in the routing instance , ...
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11 votes
3 answers
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Do today's routers prevent fake IP headers?

I know people can modify the IP headers and change source IP address, but it should be simple for network devices to detect those messages. If they don't, why is it so hard? Does it add too much ...
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  • 267
11 votes
2 answers
4k views

Why is RIP not scalable?

Most of the references say "RIP is not scalable" hence can be used only in smaller networks. But none say "WHY?" What is that in RIP that actually preventing it from scaling to larger networks? And ...
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  • 113
11 votes
2 answers
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RIP routing is broken between two routers

There are 2 routers connected something like this: Network1 ----- (172.16.1.1/24) Router 1 (172.16.3.1/30) ------ (172.16.3.2/30) Router 2 ------ (172.16.2.1/24) Network 2 I try to add addresses for ...
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  • 213
11 votes
3 answers
801 views

Circular BGP propagation

This one is a difficult to describe, it's hypothetical because I'm learning about routing and right now I'm focused on BGP. Let's say my ASN is 65000, and I announce 192.0.2.0/24. My peer, AS 65001 ...
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  • 515
11 votes
5 answers
78k views

What does the notation F0/0 and F0/1 mean in this diagram?

I've recently been taking over an OpenStack environment that makes use of neutron networking with the use of GRE tunneling. We had a problem with network performance and I determined it to be an issue ...
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  • 377
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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  • 163
11 votes
1 answer
12k views

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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11 votes
3 answers
1k views

Quagga routing and security

I have a quagga router with two transit neighbors and announcing my own IP space. I recently joined a public peering exchange (IXP) and so I'm part of their local network (/24), together with all ...
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11 votes
2 answers
3k views

Why don't iperf, scamper and path MTU discovery packet captures agree on the path's MTU?

Let's do some path MTU discovery between two Debian hosts separated by a Debian router that runs Shorewall-generated iptables rules. Each of the two hosts uses a single Ethernet link while the router ...
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11 votes
2 answers
3k views

Trying to fit centralised firewall into network topology

I'm in the process of over-hauling our network, the issue I keep coming back to is: trying to bring layer 3 to the core, while still having a centralised firewall. The main issue I have here is that ...
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  • 1,458
10 votes
5 answers
2k views

Is policy routing bad?

I heard that network engineers tend to avoid policy routing. I am thinking to source-address dependent routing for example. Is policy routing bad practice? If so, why?
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  • 219
10 votes
5 answers
10k views

Asymetric routing - causes and effects?

It happens that I run across customers who have what I define as an "asymetric routing" in their networks. Simply said, they have two gateways on the same IP subnet. The clients are configured to ...
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  • 151
10 votes
3 answers
1k views

Does anycast addressing add additional latency in any way?

Does anycast addressing, in itself, add any additional latency to network connections? Specifically, if you had two servers located in the same data center that had extremely similar routes/number of ...
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10 votes
3 answers
15k views

Cisco: prevent vlan's of communicating with eachother on cisco router (ACL alternative)

Setup: Cisco router with multiple VLAN's configured on it. How can you prevent 2 VLAN's communicating with eachother? Normally I would do this with ACL's like this: access-list 102 deny ip 1.1.1.0 0....
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