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13
votes
4answers
16k views

MTU and Fragmentation

Consider the following: NAS with 10G interface connected to 10G interface on switch. Client computers connected to switch via gigabit Ethernet. Will there be any gain in performance if MTU size is ...
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
4answers
1k views

Possible (bit) patterns of a netmask

Given a prefix Y, it's easy to calculate the corresponding netmask: Put Y times a set bit, and then fill up "to the right" with zeros until one has 32 bits (IPv4) in total. Example: Prefix 24, thus ...
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
4answers
8k views

What is the use of wild card masks vs normal masks

I've always wondered what the use of wild card masks was. When I learned about them in college I wondered what the use was, we saw that you could separate subnets by for instance even and uneven IPs. ...
11
votes
2answers
1k views

How did I break (half of) my network?

I'm looking for some post-event advice so this event doesn't happen again. We have a network core of two Cisco 4500x switches, configured for VSS redundancy. From those, we have iSCSI devices, our ...
11
votes
11answers
4k views

Locate a host on Network

What is the best method of locating a particular workstation on a VLAN ? I sometimes need to do this, if a workstation IP address, shows up on an ACL Deny torrent use High bandwidth usage (Top ...
10
votes
1answer
6k views

Authenticate ssh key via Cisco ACS (TACACS+)

I can set a router to authenticate via a ssh public-key with: ip ssh pubkey-chain username admin key-string <ssh-pub-key> exit exit Is it possible to do something similar with Cisco ...
10
votes
4answers
140k views

What can cause “FastEthernet is up, line protocol is down” on a Cisco Switch?

I wonder what can cause the first line output of the command show interfaces to be: FastEthernet is up, line protocol is down. In other words, Is the cable that is connected to the local interface, ...
10
votes
3answers
34k views

Spanning tree Default Priority Values

This question is a repost of the same question in The Cisco Learning Network; however, the answers are unique to Stack Exchange. I have always accepted that the defualt priority value of STP (802.1d) ...
9
votes
2answers
17k views

IPv6 link-local vs unique local

When do you use link-local or unique local in IPv6 networks? (Cisco doesn't manage to explain it properly in their online material.)
8
votes
1answer
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 ...
8
votes
3answers
2k 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 ...
7
votes
4answers
501 views

Why is the IP layer aware of higher layers in the network stack?

In the IP packet header there is an 8-bit Protocol field (at offset 9) that holds the "next level protocol", with assigned numbers given in RFC 790. For example, TCP is 6. My question is, why is the ...
7
votes
5answers
21k views

Copy running config in one line

I want to make a script to copy the running config of a switch 4506 in one line. So far, I see two option : copy run tftp: sw#copy run tftp://10.0.0.1/cfg.cfg Address or name of remote host [10....
7
votes
3answers
25k views

ARP request outside of LAN; Target machine or router response?

If I sent an ARP request to the target machine, which is in over Internet and has 2 routers between, what will the target IP address on my request (0001) packet be? Will it be the Router IP address, ...
7
votes
1answer
43k views

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

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 ...
7
votes
4answers
737 views

What exactly is the difference between SDN & NFV?

What is the difference between SDN (Software Defined Networking) & NFV (Network Function Virtualization)? Wikipedia says NFV relies on standard client-server model. How is this model different ...
7
votes
3answers
2k 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, ...
7
votes
5answers
6k views

Moving interface names on a Cisco ASA while maintaining the rest of the configuration in place

My initial problem: I’ve run out of physical ports on my ASA 5525-X My initial solution: Create sub-interfaces on a port-channel and use a switch to aggregate my VLANs The subsequent issue: How do I ...
6
votes
2answers
14k views

Poison reverse example

Can you give me an example when poison reverse is actually necessary? Distance vector routing protocols employ split horizon with poison reverse to minimize the convergence time when a route is no ...
6
votes
2answers
296 views

What Cisco will discard TTL=1 packets for own interface?

There have been a number of questions about router behaviour with IPv4 packets with TTL=1 and apparently contradictory answers. This question is not about what should a host or router do, it is a ...
6
votes
2answers
554 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. ...
5
votes
1answer
295 views

Optimizing Network Topology

Recently I wish to split the network by VLANs. Mainly for two reasons Exclude management interfaces from the commonly accessible lan Separate WiFi traffic (security and broadcasts) The current idea ...
5
votes
2answers
4k 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 ...
5
votes
5answers
799 views

How we can assume which network protocol is working in which OSI Layer?

For a long time, I have this kind of silly question: How we can say some network protocols are working in X layer of the OSI model? For example, HTTP working in L7 of OSI, OSPF is L3, ICMP is L4, TCP ...
5
votes
1answer
555 views

Cannot get port-forward working on ASA 5505: “Drop-reason: (acl-drop) Flow is denied by configured rule”

Months ago I made this post here on NE, but I still for the life of me just cannot figure our port-forwarding on my ASA 5505. I have since started over and am now trying the DMZ port instead, but ...
5
votes
2answers
9k views

Why each and every single port on Layer 2 switches need to have it's own MAC address?

A switch doesn't alter the frame, and source/destination mac addresses remain how sending host specified them,so let me say it again Layer 2 Switching never changes the frames means no changes in MAC ...
5
votes
4answers
2k views

eBGP Dual ISP, Dual Edge Router

I am currently using a very simple eBGP and iBGP configuration in my network to route out through a single ISP. This diagram gives a rough idea of what I currently have in operation: I am running out ...
5
votes
4answers
14k views

what is the difference between a CSU/DSU and a modem?

I'm so confused, both do the same function, converting the analog to digital, but what's the difference?
5
votes
2answers
3k views

The best way to understand Vlans?

Every time i try reading about the configuration of a device and adding it to the network, i always face a problem. The problem is basically am not able to understand Vlan's, and how for example some ...
4
votes
3answers
741 views

Is VPN a layer 3 concept?

From Tanenbaum's Computer Network This demand soon led to the invention of VPNs (Virtual Private Networks), which are overlay networks on top of public networks but with most of the properties of ...
4
votes
3answers
3k views

Telnet - “Destination unreachable” response

I'm looking to implement, what I feel, is Telnet security when it comes to unwanted hosts initiating a Telnet connection to one of my devices. I can lock down a specific subnet or only allow a range ...
4
votes
1answer
186 views

Strange flooding during installing new switch in network

This is very strange issue i am noticing. In our datacenter we have every switch in rack has cisco vPC configured in short whole datacenter running vPC so there is no loop for spanning-tree anywhere ...
3
votes
1answer
568 views

Juniper EX4200 how to select s-vlan from c-vlan on dot1q-tunneling access port?

How do I tell Juniper EX4200 to push specific S-VLAN based on C-VLAN on q-in-q (dot1q-tunneling) port? For instance I want to instruct EX4200 to perform Q-in-Q using these rules: 1) If a single-tag ...
19
votes
2answers
2k views

How do you get an ASA to announce NAT'd 'outside' addresses in to an OSPF zone?

Device arrangement is as follows: BGP Peers + | | +------+-------+ | | | Juniper MX5 | | | +------+-------+ |.254 OSPF | 10.0.1.0/24 ...
16
votes
4answers
66k views

Best practice using Cisco's “ip-helper” for DHCP?

Our topology is such that we have two 4510's in our IDF closets. Each switch has a data VLAN and a voice VLAN. The switches are layer 2 trunked to the core, where the VLAN interfaces are, routing ...
14
votes
5answers
9k views

Duplicate IP Address - which one wins?

If there's a duplicate IP address, which one "wins"? First, last, flaps, neither?
14
votes
2answers
3k views

Why are there 3 ranges of private IPv4 addresses?

Why are there three separate ranges of private IPv4 addresses of different sizes? Why not just reserve the largest (10.0.0.0 - 10.255.255.255) range and let people create /16 or /24 (or whatever) ...
13
votes
3answers
31k views

Mac-flap due to roaming clients on wireless

We are running an Aerohove wireless network (controller-less).. I occasionally get mac-flaps within the VLAN tied to a SSID. I know this is coming from a roaming client.. The problem is that the ...
12
votes
4answers
10k views

POE is compatible with Gigabit Ethernet?

I'm loocking for a specifications of Gigabit Ethernet connexion with POE. As I saw somewhere 1000BASE-T(X) is using the 8 lines of thisted pair ? So this is my Question, how It could work with POE?
12
votes
3answers
21k views

POE, POE+, UPOE

I see that Cisco now has a 60 watt POE option. What was the limitation that caused POE to have to be developed in stages of 15, 30, and then 60 watts? Why could they not just do 60 watts from the ...
11
votes
1answer
9k 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
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 ...
10
votes
4answers
16k 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 ...
10
votes
2answers
833 views

Transaction based CLI on Ethernet Switches

I am familiar with CLI on managed Ethernet switches. However, recently I came across a term 'transaction based CLI' on switches. I am not exactly sure what is that and purpose of having it in switches....
10
votes
3answers
4k views

Why does my UDP Broadcast wireless communication is capped at 1MBs?

I am trying to implement Network Coding over a meshed network. Therefore I need to send packets to several destinations, this is way I am using Broadcast packets. But I can not understand why this is ...
9
votes
2answers
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. ...
9
votes
2answers
8k views

pfSense multi-wan Bridge, NAT, Load balancing and CARP

Context I currently have : 1 pfSense 2.0.2 router (on a Firebox X-Peak X5000) 2 WAN 1 LAN 3 Servers My interfaces WAN1 68.XX.XXX.98 to 69.XX.XXX.102 WAN2 65.XXX.XXX.58 to 66.XXX.XXX.62 LAN 192.168....
9
votes
2answers
2k views

Finding the IP of a MAC Address

Is there a way to derive the IP of a device from their MAC address on a Cisco device? Assume you are not on a local segment where ping sweeping / arp are possible, instead you are remote and there ...

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