Questions tagged [cisco]

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48 votes
24 answers
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How can I see which switchports are not in use?

On a large Cisco Catalyst switch stack, almost all switchports are patched. I need to identify the ports, which are not in use, for connecting further devices. Following switchports, cabling, patch ...
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  • 3,354
46 votes
6 answers
103k 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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35 votes
3 answers
21k views

What is "oversubscription" in Networking?

I have a question about oversubscription in networking. I read a lot of documentation but I still don't understand what it means. I read the following on the Cisco website, oversubscription of the ...
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  • 3,078
33 votes
1 answer
81k views

Using SNMP to retrieve the ARP and mac-address tables from a switch

I would like to get ARP tables from a switch to a syslog-ng server that has been set up on Ubuntu Server 12.04 LTS. I have read about SNMP and I know the server will act as a manager and the switch as ...
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  • 685
29 votes
2 answers
56k views

What is the meaning of the CRC counter on a cisco device?

If the CRC counter of an interface is high, normally it's a bad sign, but why? If the count is high, what does this mean technically? What can cause this counter to go up? On which layer in the OSI ...
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  • 2,363
26 votes
4 answers
111k views

How can I capture traffic on Cisco IOS switches?

For investigating a problem in client to server communication, I need to capture packets for analysis. However it's not allowed to install a packet analyzer, such as Wireshark or tcpdump, on client or ...
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  • 3,354
24 votes
10 answers
306k views

How to find the port a device is connected to, based on IP, on a Cisco Catalyst switch

On a Cisco Catalyst switch, I need to find what port an end device is connected to. I have the IP address/host name. How do I quickly find the port?
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  • 8,258
24 votes
5 answers
29k views

Accidentally Removed Allowed VLANs from Cisco Switch Dot1Q Trunk

I am adding a new VLAN to an existing trunk port between two Cisco Catalyst switches (3750's). In the process of adding the new VLAN, it appears that I've removed the existing allowed VLANs on the ...
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  • 8,258
23 votes
8 answers
62k views

How can I force VTP clients to update their VLAN database from the VTP server, on Cisco devices?

I was doing some cisco exercises on configuring a vtp server and client, something I can't figure out is : I setup a vtp server, added my VLAN's I then took another switch that was attached to this ...
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22 votes
3 answers
62k views

Protocols: EIGRP vs OSPF [closed]

EIGRP and OSPF are both IGP protocols, the former is a mostly Cisco protocol and the latter is a well established open standard. What are the benefits of one over the other? Put another way, when ...
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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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22 votes
1 answer
2k views

All WiFi clients losing connectivity at the same time

In order to provide WiFi coverage for a small office area, a Cisco autonomous single-band 1041 (AIR-AP1041N-E-K9) is used, running software version 12.4.25d-JA1. The access point is having a basic ...
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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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  • 1,005
21 votes
5 answers
28k views

Cisco ASA site-to-site VPN failover

We recently replaced international MPLS with new ASA 5510s and site-to-site VPNs. However, when we deployed this we ran into a problem where each remote location has 2 ISPs for redundancy, but when ...
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19 votes
5 answers
18k views

Should portfast be used on a port potentially connecting to an unmanaged switch?

I understand the basics of how spanning tree works and why you would want to use portfast on user access ports. When dealing with a topology with a large number of dumb switches under desks and ...
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19 votes
3 answers
8k views

What factors drive a Cisco IOS upgrade?

In order of preference/priority, what factors do you consider in driving an upgrade (or downgrade) with Cisco IOS? If no compelling factors exist, how long would you allow a particular version of IOS ...
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19 votes
3 answers
83k views

Is the "default VLAN" simply the default native (untagged) VLAN on all interfaces that have no configuration?

... or does the "default VLAN" have some wider meaning? Also, can it / should it be changed? For instance, if a switch is going into part of a network that is only one VLAN and it's not VLAN 1, is ...
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19 votes
1 answer
3k views

Finding transparent firewall packet loss

We use the Cisco ASA 5585 in a layer2 transparent mode. The configuration is just two 10GE links between our business partner dmz and our inside network. A simple map looks like this. 10.4.2.9/30 ...
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  • 285
18 votes
3 answers
27k views

How do you know if a Cisco switch is handling traffic well?

I have just read Jeremy's question about 2960-S and iSCSI traffic handling, which drove me to ask this question: We all have lots of switches deployed in distinct infrastructures, how can you know ...
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  • 795
18 votes
3 answers
6k views

Writing text to the log

Is it possible to write an arbitrary line of text to the log of a Cisco IOS 12.x device? I should like to be able to put in markers when fetching configs via TFTP and I would like to be able to have ...
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  • 623
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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18 votes
4 answers
25k views

Cisco APs ignore DHCP Option 43 and use CAPWAP UDP broadcasts

Background I am trying to redeploy Cisco 1242 LWAPs on new Cat4507R+E switches, which means the IP address of the WLC is changing for that LWAP. For some reason, I have to configure ip helper-...
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18 votes
7 answers
12k views

Best practice for the combination of HSRP and ECMP

The combination of ECMP (or other causes of asymmetric paths) and HSRP is broken by default in Cisco IOS; the default behaviour with this design floods unicast traffic excessively. What is the best-...
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17 votes
4 answers
71k views

Confusing A, B, C network classes

I'm studying IPv4 addresses and came across this whole thing about classful addressing. I get the idea behind it, bit there is something I find confusing: There are two "ABC" ranges: First one: A: ...
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17 votes
4 answers
67k 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
8 answers
18k views

When *not* to create a SVI for a L2 VLAN?

When creating VLANs for just L2 on a switch -- routing will be handled by a device within that VLAN such as a load-balancer -- it isn't necessary to create the vlan interface. As a matter of habit, I ...
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17 votes
4 answers
8k views

Are there any reasons to not use BFD?

In looking to implement Bidirectional Forwarding Detection (BFD) it seems to be very flexible in terms of timer tuning, light weight regarding any overhead and it's flexibility in terms of overall ...
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  • 2,067
16 votes
5 answers
216k views

How can I reset a VPN tunnel on a Cisco ASA?

On a site-to-site VPN using a ASA 5520 and 5540, respectively, I noticed that from time to time traffic doesn't pass any more, sometimes just there's even missing traffic just for one specific traffic ...
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  • 3,354
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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  • 1,005
16 votes
5 answers
93k views

About the switchport mode

I am new to network Engineering. I find it is hard to distinguish and understand different switchport settings. Why can I have a switchport set to access, auto, or dynamic? What are they used for? ...
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  • 369
16 votes
5 answers
81k views

What causes total output drops on a cisco switch interface?

I have an HP c7000 blade chassis which contains Cisco 3120X and Cisco 3120G switches running ios 12.2(58)SE1. The blades themselves are very lightly loaded yet many interfaces on different blade ...
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16 votes
2 answers
2k views

Router won't hold configuration

I have reset the password on a Cisco 1700 router, now when I copy run start or write mem then reload or reboot, it defaults to factory settings, i.e. it is not holding the configuration I have just ...
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  • 188
16 votes
5 answers
25k views

Do I need to set the enable secret on Cisco device?

I'm setting up a Cisco 2901 router. I have a login password on the console line, and the vty lines are configured to only accept ssh connections with public key authentication. The auxiliary line is ...
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  • 163
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
4 answers
5k views

What happens when 2 computers transmits at the same time to a 3rd one in a full duplex switch?

Consider the following scenario: I have 2 PC's (PC1 and PC2) that wants to transmit at the same time to PC3 in a full duplex ethernet switch. Let's consider that all ports are in the same VLAN, so ...
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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
4 answers
5k views

Why does this router fail to use multiple, equal-cost routes from OSPF?

A Brocade router running NetIron 5.2, and a Dynamips emulation running IOS 12.4 both fail to dependably use equal cost paths in what seems to be a very simple configuration. I say "dependably" because ...
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  • 869
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
65k views

What does logging synchronous command do?

Can somebody describe me what does logging synchronous command do ?
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  • 489
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
5 answers
27k views

How to factory reset IOS-XR?

I want to factory reset IOS-XR. The router used is ASR9000 I'm used to do that easily in IOS doing config-register 0x2102, write erase followed by a reload. I search on documentation, but found no ...
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  • 2,018
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
3 answers
7k views

Configure Cisco ASA in Transparent mode: Layer2 DMZ w/ Vlan translation

I am in the middle of a project to migrate some existing switched ethernet dot1q trunks behind an ASA firewall... these trunks have five vlans each (numbered 51 - 55). This is a simple drawing of the ...
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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
4 answers
42k views

How are two "ip helper-address" statements handled?

I've discovered a subnet with two "ip helper-address" statements. This is not our normal configuration. interface Vlan300 ip address 10.1.3.2 255.255.255.0 ip helper-address 10.2.1.10 ip helper-...
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14 votes
3 answers
65k views

What is the meaning of RSSI?

When using mobile 3G routers, you see in the output of the "show cellullar 0 all" command. What is the significance of this value? Is this interpretation the same when talking about 2G or 3G? Ex: ...
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  • 2,363
14 votes
5 answers
719 views

In a multi-tenant environment what should be done to make your switchports silent on Cisco and Juniper switches?

For example preventing it from sending arp, stp, etc and to reveal as little as possible about the rest of the network. Example use case would be connecting to a peering exchange.
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  • 1,675
14 votes
3 answers
48k views

How to Monitor Vlan Traffic?

We have migrated to a new ISP who provide us several services (Internet+VoIP+VoD) and have divided each service in one vlan 100,101,102 respectively. Now I'm looking for a solution to monitor each ...
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  • 2,018
14 votes
4 answers
4k views

Enabling MLS QoS on a production 7600

I am looking at a 7609-S with a RSP720-3CXL running 12.2(33)SRE3. I have been asked to help with rate limiting some servers attached to a couple of ports running as L2 ports (switchport mode access), ...
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  • 5,270
14 votes
3 answers
3k views

Why we get high fluctuation on bandwidth measurement Cacti graph?

We were on redundancy test of Etherchannel and Routing on our network. During this intervention we made some measurement. Our monitoring tool is Cacti for graph. The equipment monitored is a 4500-X ...
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  • 2,018

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