Questions tagged [router]

For questions about network routers, devices that forward data packets between computer networks, commonly at the network layer (layer 3) of the OSI model.

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Upgrading Cisco 819 to 809, can I connect devices to 2 routed ports and be on same subnet?

The company I work for has multiple remote "IoT" style sites connected back to the datacenter via a Cisco 819 cellular router. We are in the process of upgrading these to the newer Cisco IR809 routers ...
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9 votes
3 answers
6k views

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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10 votes
2 answers
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What do these three options in a Cisco Router NAT configuration mean?

In a standard Static PAT configuration, where one IP:Port combination is always mapped to another IP:Port combination, there are three possible combinations of inside/outside/source/destination that ...
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How does a router know to which PC in a network to send a response from a server?

When my PC sends a request to a server, the transmitted IP address is the one of my private network. When the server responds to the request, how does the router know to which PC in the network to ...
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46 votes
6 answers
104k views

Difference between control plane, data plane and management plane?

I think I understand these concepts, but I'm a little rusty. Can someone give a concise, easy-to-understand explanation of these concepts? The planes are logical concepts, aren't they? Is this a Cisco ...
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What is a "Layer 3 Switch"?

What is a "Layer 3 Switch"? I essentially understand the OSI model, well maybe I don't and maybe that is why I don't understand. Its just that I don't see how/when/if a "managed switch" becomes a "...
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12 votes
7 answers
16k views

DHCP relay problem on interfaces secondary network

A secondary network was configured on an interface of a Cisco 2621 router and the ip helper-address commands put in place but no clients of that secondary network could pull DHCP addresses. We swapped ...
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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
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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 answers
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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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1 answer
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HSRP issue both device active stats

Here is the Diagram. Ip address: ISP: [VIP]- 10.107.25.5 [R1] - 10.107.25.6 [R2] - 10.107.25.7 Me: [VIP] - 10.107.25.10 [SW1] - 10.107.25.11 [SW2] - 10.107.25.12 Configuration on port: SW1 ...
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17 votes
4 answers
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Bad Mask for IP Address Error

I'm not too clear on subnetting rules and experienced an error when I tried to assign 148.26.1.176/28 to an interface on my Cisco router. Can someone please explain why it was not allowing me to ...
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9 votes
3 answers
26k views

Assign an IP address based on switch port

Short: I want DHCP server to assign a preconfigured IP address based on a port (on a switch, ideally) a device is connected to. Long: My situation: I am building an automated indoor farm (for ...
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1 answer
946 views

Why are routers not answering ARP Broadcasts?

Host A wants to send something to Host B who is in a different network. Between them is Router 1. Host A sends an ARP broadcast but Router 1 is not answering it. Why is that so?
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4 votes
2 answers
434 views

How to understand the output below from BGP?

I don't understand some data of the show route: admin@MX-RE0# run show route 36.250.236.6 inet.0: 736608 destinations, 806660 routes (736608 active, 0 holddown, 7 hidden) + = Active Route, - = Last ...
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4 votes
3 answers
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Collision domain question

I'm studying for my CCENT exam, and not quite sure why the book (official Cisco) says that there are 5 collision domains in this example. Isn't it supposed to be 4? The hub on the left, the hub in the ...
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3 votes
1 answer
2k views

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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3 votes
2 answers
5k views

Should I increase the timeout value for UDP on our VPN router-device?

We have a SonicWALL NSA 2400. We have numerous (100+) personnel in different offices in different physical locations that establish VPN sessions successfully every day. Some of these client computers ...
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2 votes
6 answers
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Is a "LAN" a layer 2 or a layer 3 concept, in TCP/IP?

I think my question is purely theoretical and a matter of definitions; it derives from the fact that a L2-switch with VLAN capabilities is able to "create" different virtual LANs, and thus I am led to ...
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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 answer
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how do i connect two networks with two routers in packet tracer?

I am am new to networking and I have recently started using packet tracer. I`m trying to get familiar with sub-netting and I've made a simple network that is going terribly wrong. I have 4 departments:...
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1 vote
3 answers
109 views

Why does a queue of a network device like router starts to fill up if the arrival rate equals the transmission rate of the outbound link?

In the book "Computer Networks: A Top Down Approach by Kurose" it is written that when the traffic intensity approaches 1, the average queue length gets larger and larger. Traffic intensity ...
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1 vote
2 answers
349 views

Decrease the Router's buffer size

Through this topology, I want to decrease the buffer size of R2 in order to generate an ICMP Source Quench Message, but I have no idea how to do it. I'm open to other suggestions to generate a Source ...
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1 vote
1 answer
344 views

What's the relationship of a Router with its loopback interface in logic?

I have a Router1 with a loopback 0 connected to the Router2: the interface brief is bellow: Router1>show ip interface brief Interface IP-Address OK? Method Status ...
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1 vote
1 answer
646 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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  • 121
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2 answers
482 views

can LAN port have multiple IP addresses of same subnet mask or different subnet mask

Can the router's ethernet interface have multiple IP's of the same subnet or different subnet mask? If yes what's the use?
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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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  • 1,511
14 votes
2 answers
8k views

Network Gateways vs Interfaces

When looking at the contents of a routing table section on the Wikipedia article on Routing Tables, it contains a table as an example of a routing table with two headings which have an unclear ...
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  • 253
22 votes
6 answers
6k views

What stops someone from configuring their network with IP addresses they do not own?

Here's the scenario. I was picturing a university that bought a range of IP addresses. I think their network would still be connected to an ISP (right?), but they'd have freedom to configure stuff the ...
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15 votes
2 answers
69k views

Why is my Router's WAN IP different from public IP? [closed]

My Router's WAN IP is a public IP - 172.100.x.x And my public IP is - 182.x.x.x Why do I have 2 public IP's? Does this probably mean that my router is behind another router? But then what is the use ...
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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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10 votes
4 answers
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how to get scp via snmp to work with cisco routers?

I have a lab setup where I'm trying to use SCP via SNMP to a cisco router.I found some online documentation such as: http://ccie20728.wordpress.com/2008/05/20/get-the-cisco-configuration-over-snmp/ ...
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  • 385
7 votes
3 answers
16k views

When exactly is ARP protocol is used

I am a quite a beginner in network engineering so please be nice. I am not sure I understand how ARP protocol works in some situation. For example, if we have following problem: ARP cache is empty ...
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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
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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9 votes
4 answers
3k views

Advantage of layer 3 switch over Router

What is the advantage of Layer 3 switch over the Router ??
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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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  • 832
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
6 votes
2 answers
619 views

MPLS can reroute, but can it route?

I have 4 Cisco routers, connected one to another like the following: **R1**(s1/0)--(s1/0)**R2**(s1/1)--(s1/0)**R3**(s1/1)--(s1/0)**R4** All of them have an IP address assigned on their interfaces. ...
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5 votes
2 answers
3k views

How does router prevent Broadcast radiation

I just learned networking and according to a wiki article on broadcast radiation prevention, it says: Filtering broadcasts by Layer 3 equipment, typically routers (and even switches that employ ...
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  • 153
2 votes
4 answers
20k views

How does ARP work between two routers?

When ARP is used by a host in a LAN, then it is broadcast because the host does not know where is the destination host, but a router knows which interface it must use to get to the next router. Then ...
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13 votes
2 answers
7k views

Is the MTU of Ethernet still 1500 Bytes?

So basically do the modern routers in the internet still use 1500 as the MTU of Ethernet? or the 1500 Byte was for the old days? What I'm asking here is what is the MTU of Ethernet in modern Networks?...
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  • 501
13 votes
1 answer
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Can a router send ARP requests to hosts?

I play with libpcap in C recently to create an arp sniffer program that captures arp requests on the air. I came up with a strange effect. I captured arp requests from my router. I don't know if this ...
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10 votes
3 answers
21k views

OSPF or iBGP for interior routing protocol?

So the plan is, two core routers each with a eBGP session to one of two ISPs with full routes. Both routers publish their full tables to each other so they can control traffic flow more intelligently ...
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  • 343
10 votes
4 answers
21k views

Setting up DHCP server on the router or on the switch?

We're using the Cisco 1921/K9 with a SG300 L3 switch. Does one set up DHCP server on the router or on the SG300? If both is capable, what is the reason to choose one over the other? Note: I have 4 ...
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9 votes
2 answers
2k views

Differences and functions of type codes 0800 and 0806

What are the primary differences and functions (or applications) of type codes 0800 and 0806? Through my own research, I found the 0800 was related to the 0800 to the IP/Layer 3 of the OSI model. ...
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  • 303
6 votes
3 answers
3k views

How is NAT a hack?

Forgive me if this sounds a little amateurish, or if this is the wrong stackexchange, but...most of my entirely life I've had ipv4 behind a NAT router. I've been told that this was a hack; simply ...
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  • 321
4 votes
3 answers
26k views

Load balancing with multiple ISP, two routers and firewall

I have two ISP connections on two Cisco 2901 routers, in front of Cisco ASA-5515 firewall. I need to configure failover load balancing network. Two Cisco 2901 router with dual (different ISP) ...
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3 votes
2 answers
3k views

Can't ping the management IP of Switch

My topology is: I am logged in to R1. I can ping 10.0.0.2 on R2 and 198.51.100.1 on R2, but can't ping the management IP of sw3 (198.51.100.2). What am I doing wrong? R1#sh ip arp Protocol Address ...
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