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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46 votes
6 answers
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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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26 votes
1 answer
94k views

What are the benefits and cons of IGMP snooping?

Our new router is asking if we want IGMP snooping enabled. Not being familiar with this, I looked to the internet and got the following wiki description: IGMP snooping is the process of listening ...
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  • 363
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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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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21 votes
7 answers
7k views

Building new network and need 200+ wireless devices

I'm planning the network for my company's new office space. I'm not an expert so I'd like to present my plan and ask y'all nice folk to tell me if and where I'm making mistakes. Assumptions: Office ...
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20 votes
3 answers
29k views

Difference between router and gateway

What is the exact difference between router and gateway? In Network configuration we set the default gateway IP address, but in reality we call the device configured with that IP address as router?
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  • 335
19 votes
2 answers
31k views

Cause of high CPU load on Juniper peering router's routing engine

Recently the routing engine CPU utilization on two of our Juniper peering routers increased from ~10-20% average load to 80+%. I'm trying to figure out what's causing this (and how to get this high ...
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18 votes
7 answers
62k views

Cisco command to show which interfaces an ACL is applied to

For Cisco routers and switches, is there a show command, or something similar, that will display what physical and logical interfaces an ACL is implemented on and what direction it is applied in? I ...
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17 votes
4 answers
68k views

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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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
16 votes
3 answers
5k views

Why would a switch be connected to a router using three ethernet cables?

In our rented office, we have a router (BT smart hub) connected to a large switch (model unknown). The two are connected using three ethernet cables, thus: ----------------------- | 1 2 3 4 ...
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  • 273
16 votes
7 answers
20k views

Automate configuration backup (every minute) in Cisco IOS

I'd like to make my Cisco router automatically do a copy run TFTP (and populate the fields automatically) every 1 minute. I'd also like it to be run in the background. Anyone know if this is possible/...
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16 votes
3 answers
10k views

Backing up Cisco router configuration - including ssh keys

We have some Cisco 2821, 2921 and 1921 routers in our shop. I can back up and restore the configurations by copying out or in the startup-config file, but what about the keys for ssh? I don't want ...
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16 votes
2 answers
4k views

Output Drops on Serial interface: Better queueing or Output queue size?

On Internet edge routers speaking eBGP to multiple carriers and iBGP to one another, all interfaces on the LAN and WAN side are GE except for one Serial full-DS3 (~45Mbps) on each router. Although I ...
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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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  • 1,271
16 votes
3 answers
4k views

class-default matches control traffic?

I'm seeing an issue with BFD on a link that's being policed egress where it appears during times where the policer is maxed BFD packets are not getting to the other side. I'm wondering if BFD hellos ...
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  • 335
15 votes
8 answers
10k views

How to connect 5 different networks together?

I have a network consisting of three servers in a data center, all of which are connected to each other via a switch. In addition to this I need this network to access 4 additional networks, most of ...
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  • 153
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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14 votes
3 answers
5k views

Should a router use SLAAC for IPv6 address assignment?

I am a little confused by the following 2 RFCs relating to IPv6: RFC 4862 (IPv6 Stateless Address Autoconfiguration) RFC 7084 (IPv6 CE Router Requirements) RFC 4862 states: The autoconfiguration ...
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  • 335
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
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
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
13 votes
2 answers
93k views

What is SNR Margin and Line Attenuation?

I'm currently using an adsl connection and last week an engineer from my ISP came and told me that my SNR margin and line attenuation is not good, and they need to replace the main telephone cable to ...
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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
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)
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  • 2,363
13 votes
1 answer
2k views

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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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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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
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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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
4 answers
105k views

What are the reasons for seeing an incomplete ARP?

As was mentioned in this post: The reason for seeing an incomplete ARP is that "An ARP request was sent for that address, but the host with that address is not up and running on the LAN, so there is ...
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  • 473
11 votes
6 answers
6k views

When does switch talk to router

I have read a lot on how switches forward within LAN, but little about how switch talks to router. When does a switch send packet to router ? Is it when it does not find recipient on the same LAN ?
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11 votes
3 answers
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 ...
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  • 267
11 votes
2 answers
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 ...
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  • 213
11 votes
4 answers
23k views

Using IP addresses from IP pool given by ISP

My ISP says that it can provide an optic fiber connection with a pool of 4 static IP addresses (subnet 255.255.255.248 i think). My question is that if I get that connection, 1 ip will be used for ...
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11 votes
1 answer
6k views

Cisco IOS built-in SSH client default version

You issue the command ssh 192.168.97.97 from a IOS device and the destination IOS router/switch SSH server is accepting both SSH versions. Being that you did not specify a version to use on the SSH ...
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11 votes
2 answers
280 views

IPv6 Subnetting - Block RA on Switches?

I'm currently planning a v6 only small datacenter network. Imagine for now one deployment consisting of 6 racks with 12 servers per rack. Each server has 2 NICs for two physically separate networks (...
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  • 113
11 votes
1 answer
3k views

How much RAM you actually need to keep whole global BGP routing table?

It seems that different sources claim very different RAM consumption for having whole BGP routing table information in RAM and having the router to correctly route the traffic. According to ...
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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
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
3k views

Embedded routers vs. software routers in the datacentre

I've been looking for a new 'core' router to put in our new rack which is colo'd at a nearby datacentre. What I've come across is the usual Cisco/Juniper/Brocade lineup, with the usual unattractive ...
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  • 515
10 votes
3 answers
1k views

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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10 votes
4 answers
6k views

Redundancy between router and servers

In the past, I was using a NetGear GS724T between my router and my servers. Unfortunatly the switch suddenly died. Now I've changed it for a GS748T and also bought one more and I am wondering how I ...
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10 votes
1 answer
7k views

If two NICs are bonded and connected to different switches how does a router decide which switch to forward the packets to?

I am trying to understand how the architecture works in case of NIC Bonding. The scenario is: There are 2 NICs on the same system. Both are bonded to support failover and load balancing. Hence the IP ...
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10 votes
7 answers
8k views

How does the computer detect the router?

Is the router always broadcasting "I'm here" (say, every period of time) and the computer listens or the router listens and once the computer broadcasts "is there a router here?" It responds? It ...
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  • 203
10 votes
1 answer
4k views

Long login delay when cisco device is booted

Whenever we boot or reboot a Cisco router or switch we have to wait a number of minutes before we can get a username prompt for login. We get a few fail messages Press RETURN to get started. % ...
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10 votes
4 answers
4k views

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
10 votes
2 answers
48k views

Cannot use 5GHz band Wi-Fi from channels 100-140

My AP operates in the 5Ghz band. There are no problems using Channel 36 to 64; however, clients can't even detect the SSID if I use Channel 100 to 140. Below is a screenshot of Netspot showing all ...
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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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10 votes
2 answers
3k views

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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