Questions tagged [cisco]

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NAT Overlapping

I am trying to do some networking lab. Basically, I have a network like this: So, is it possible to configure NAT router in such way, that PC1 and PC2 can access remote HOST and each other. The ...
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11 votes
2 answers
15k views

In IPsec VPN, how is the pre-shared key encrypted?

I was doing IPsec VPN on ASA 8.0, and I understand a little about that. The initiator starts by sending its ISAKMP policy to the responder, and the responder sends back the matched policy. After that, ...
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8 votes
3 answers
7k views

Implementing Cisco QoS model to end users

I have implemented the 4-Class QoS Model as described in this earlier post from 2013. This Core configuration is implemented on a Cisco VSS 6509-E setup running software version: Cisco IOS Software, ...
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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
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
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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3 votes
1 answer
255 views

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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48 votes
24 answers
99k views

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
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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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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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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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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10 votes
3 answers
8k views

How are Weighted Fair Queuing and Weighted Random Early Detection related?

I'm trying to understand how QoS systems work and I'm not sure how exactly would WFQ and WRED interact. At first, I thought that WFQ is a queuing mechanism and that WRED is a congestion avoidance ...
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  • 685
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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12 votes
1 answer
4k views

Mac / CAM entries from non-Cisco switches with Q-BRIDGE-MIB

How do I do community string Indexing for other switches like D link DES 3526 and Brocade Fastiron SX800? I am running a java program to poll a specific vlan on a Brocade sx 800 l3 switch. The ...
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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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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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  • 940
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
8 votes
1 answer
2k views

Staging Many Lightweight Cisco AP's for Deployment

My organization is soon going to be deploying approximately 600 new Cisco 2602i access points in a CAPWAP deployment with several 5508 controllers. This is an infrastructure refresh; currently we ...
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  • 8,258
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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10 votes
3 answers
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 ...
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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
2k views

Traceroute doesn't show one hop?

I have an interesting issue where my traceroute skips one hop on the way. I do not know why but it seems to be some kind of Juniper behaviour which I do not understand yet. Blue routers are Cisco ...
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  • 141
1 vote
1 answer
5k views

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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  • 4,237
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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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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13 votes
2 answers
29k views

Open a range of TCP ports in Cisco IOS NAT

ip nat inside source static 192.168.1.10 10.10.10.9 route-map RANGE ! route-map RANGE permit 100 match ip address 102 ! access-list 102 permit tcp host 192.168.1.10 range 3000 3389 any The ...
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  • 131
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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  • 193
7 votes
1 answer
2k views

BGP authentication-key-chain vs. authentication-key

I found a neat little behavior on a Juniper box I was working with today. We were migrating from a Cisco ASR to an MX and all of our internet exchange peers that had authentication configured didn't ...
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  • 6,440
6 votes
1 answer
3k views

Cisco 867 forward UDP port range

I need to forward an UDP port range to a specific private IP address in my LAN. In order to forward one port to an internal IP I do: access-list <acc_list_number> permit <port_type> any ...
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  • 206
4 votes
3 answers
1k views

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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4 votes
3 answers
11k views

Cisco 3725: Why default-gateway does not appear on `show ip route`

My network topology is as follows From R1, I was not able to ping R3. Packet capture on R3 shows that echo request received, but no reply from R3. It turns out that default gateway did not appear ...
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  • 301
2 votes
1 answer
551 views

Cisco Prefix List Won't Appear in Route Table

I have the following prefix-list configured on our router: ip prefix-list adsvr seq 5 permit 172.30.5.241/32 ip prefix-list adsvr seq 10 permit 134.239.130.143/32 ip prefix-list adsvr seq 15 permit ...
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1 vote
2 answers
348 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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  • 883
-1 votes
1 answer
268 views

CISCO PACKET TRACER "INVALID IP ADDRESS"

Hi I am still new and studying about networks have here 234.1.18.6/29 address and mask but when I try it out on Packet tracer it gets me an invalid message. I cannot understand why this address are ...
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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
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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14 votes
1 answer
124k views

What does TCP DUP ACK mean?

In Wireshark, I see TCP duplicate ACK packets sent from the receiver to the sender. What does it mean? Does it imply packet loss? Thank you
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  • 163
13 votes
1 answer
14k views

What is the 5 level QoS model (through config example)?

I have heard people stating that their network "operates a 5 level (or layer) QoS model" Sometimes they also mention a 7 level QoS model. Can anyone provide me with a Cisco IOS config example of ...
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  • 5,270
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
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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  • 3,300
11 votes
3 answers
7k views

What are the alternatives to OTV for stretching a layer 2 network across 2 datacenters?

We want to find a "safe" way to stretch layer 2 network across 2 data-centers, and since we don't have any equipment running NX-OS on either data-centers we can't use OTV. So what are the alternatives ...
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  • 505
9 votes
3 answers
7k views

Generate list of subnets on switch using SNMP?

Is anyone familiar with an approach to generate a list of subnets on a particular switch (let's pick a more popular one - Cisco) using SNMP? How about a list of VLANs? If so, can you point me to the ...
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9 votes
3 answers
2k views

VM on a client connected to a Cisco 1262 unable to DHCP (but IPv6 works fine)

I recently replaced wireless at a site from the internal AP on an 877-WM with a (standalone) 1262 running 15.2(2)JB. I've configured the AP pretty much the same as I always do for Cisco AP's for ...
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  • 1,743
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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13 votes
6 answers
326k views

Finding Non-Cisco End Device IP Addresses Connected to Cisco Switch

I have Cisco catalyst switch, with many VLANs and address ranges. How do I find IP addresses connected to the switch that are not Cisco devices?
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  • 812
12 votes
1 answer
7k views

Translating SNMPWALK output into human-readable format

I am asking this question which is related to a question I previously asked and was comprehensively answered by @MikePennington. When I asked that question, I intended to know how to poll arp table ...
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  • 685
10 votes
4 answers
15k views

Can anyone explain to me the difference between the PSH and URG flags in TCP segment?

According to me when PSH flag is set, it indicates that data will have to be sent immediately to the wire and on receiving side the data has to be pushed to the application immediately. When URG flag ...
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10 votes
4 answers
34k views

converting AP from CAPWAP to autonomous - BVI1 interface gets reverted back to DHCP

Has anyone ever gotten this message in the middle of converting a CAPWAP access point from CAPWAP to autonomous? *Mar 1 00:47:59.399: %CAPWAP-3-STATIC_TO_DHCP_IP: Could not discover WLC using static ...
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