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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46
votes
8answers
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 ...
37
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8answers
28k 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 ...
35
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3answers
49k views

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 ...
22
votes
3answers
61k 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 ...
22
votes
4answers
46k 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 ...
21
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7answers
33k views

Difference between access list and prefix list?

Can someone explain with an example whats the difference between an access list and prefix list.
21
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3answers
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? ...
21
votes
2answers
9k views

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?
20
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4answers
21k 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 ...
19
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7answers
6k 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 ...
18
votes
3answers
1k views

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 ...
16
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7answers
99k views

What are the differences between VRRP and HSRP?

Are there any significant differences between VRRP and HSRP?
16
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3answers
10k 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: ...
16
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6answers
15k views

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 ...
15
votes
3answers
20k 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 ...
15
votes
2answers
8k 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 ...
15
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2answers
9k 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, ...
14
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2answers
9k 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 ...
14
votes
2answers
1k 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 ...
14
votes
3answers
31k views

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 ...
14
votes
6answers
22k 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 ...
13
votes
2answers
8k 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 ...
13
votes
5answers
14k 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 ...
12
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8answers
6k 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?
12
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2answers
2k views

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 ...
12
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6answers
3k 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 ...
12
votes
2answers
10k 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. ...
12
votes
1answer
2k views

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)
12
votes
3answers
11k 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 ...
12
votes
1answer
4k 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 ...
12
votes
5answers
9k 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 ...
11
votes
3answers
1k views

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 ...
11
votes
2answers
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 ...
11
votes
5answers
48k 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 ...
11
votes
3answers
7k views

interpreting ping results

I'm pinging yahoo.com and am I'm puzzled by the result. C:\Users\jon>ping -t yahoo.com Pinging yahoo.com [98.138.253.109] with 32 bytes of data: Reply from 98.138.253.109: bytes=32 time=195ms TTL=...
11
votes
2answers
3k views

How can ISPs on one continent connect to ISPs on another continent?

How can ISPs on one continent connect to ISPs on another continent? From a physical layer standpoint? Say one ISP is located in Asia and another one in Europe how would they connect their fiber ...
11
votes
1answer
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 ...
11
votes
1answer
12k 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 ...
11
votes
1answer
8k 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 ...
11
votes
2answers
13k 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 , ...
11
votes
3answers
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 ...
11
votes
2answers
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 ...
11
votes
2answers
2k 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 ...
10
votes
5answers
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?
10
votes
3answers
14k 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....
10
votes
3answers
690 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 ...
10
votes
2answers
4k views

Why does the MTU need to match on interior link state routing protocols like EIGRP and OSPF?

If one tries to configure the neighbor adjacencys without matching MTU, The routers fail to become neighbors. I assume this is to protect the routing protocol from it self but I don't understand ...
10
votes
3answers
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 ...
10
votes
2answers
2k views

Why can't I ping from one computer to another using L3 cisco 3550 switch?

I have configured my 3550 L3 switch to redirect HTTP traffic to a significant Machine: This is my scenario ( PCs instead of Routers): This is the switch configuration: CAT3550# show running-config ...
10
votes
3answers
4k views

3 subnets, 2 OSPF areas - Will This Work?

Will any issues occur if I have a topology involving 3 subnets and two OSPF areas where one subnet is in AREA 0 and the other two subnets are both in AREA 1? For example: [area 1, subnet 1]---[ABR #...