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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11 votes
4 answers
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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
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
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
8 votes
3 answers
1k views

DHCP Relay by MAC Address of CPE

We are testing a new Provisioning Server in DOCSIS/CMTS environment and we need to setup a lab with 5 models of CPE. I'm wondering if we can configure on Cisco Routers the dhcp relay (ip helper) ...
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  • 2,018
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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  • 15.7k
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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7 votes
3 answers
314 views

Is there a way to recognize the location where a client connects to the network?

So yes, I'm pretty new to networking and I hope this isn't a stupid question. I couldn't find an answer on google nor could I find something here (but I'm also sure about the terminology), so I ...
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  • 173
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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6 votes
2 answers
2k views

forwarding / non forwarding Route Reflector?

I am considering running route reflectors in our current network topology of 20 od MX80's however all current devices are active traffic forwarders. from a scaling perspective RR makes logical sense ...
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  • 1,609
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
5 votes
1 answer
134 views

ip-fwd-entry allocation fail OSPF

Any idea what this error means? I can't find much on googling. Device is a Brocade SX. Output is from: show ip ospf error **** Warning! ip-fwd-entry allocation fail 3629337 times --- The ...
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8 votes
2 answers
521 views

Determining optimal bandwidth needs for the office

We're currently using the Cisco 1921/K9 router along with SG300 L3 switch and other L2 switches in the office environment. We're hooked up to a 10M fiber line for internet but not sure if we need that ...
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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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5 votes
3 answers
3k views

Is a router always necessary to link two subnets?

I am thinking especially of VLANs, which represent separate subnets that may need only one switch.
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9 votes
4 answers
829 views

How does one specify certain IPs or MAC addresses for NBAR policy enforcement?

In an office environment, if I wanted to block youtube using a Cisco ISR router, I would set up the following with NBAR: class-map match-all YOUTUBE match protocol http host "*youtube.com*" ! policy-...
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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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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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7 votes
3 answers
3k views

ISR routers - difference between routed and switched ports

On a Cisco ISR router - would you say there are any benefits from purchasing an HWIC or EHWIC to connect to the next set of switches within the branch network? For instance. Lets say Gig0/0 and Gig0/...
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  • 2,527
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
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
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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4 votes
2 answers
3k views

OSPF LSA Refresh Time

Is there a Cisco command that would allow one to modify the default value of 1800 seconds (30 min) for the OSPF LSA refresh rate?
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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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  • 683
4 votes
4 answers
2k views

Cisco 2600 series router logging a lot of "Memory allocation failed" errors

We are getting the below two error messages randomly on a 2600 series router. Syslog shows the router has been fine up until a few days ago. Would a reboot most likely cure the issue or is it most ...
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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
2 votes
1 answer
137 views

How can I log authentication errors in 3G routers?

What command (debug) can I use to see authentication errors on a cisco 3G router specifically for the 3G connection?
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  • 2,363
9 votes
3 answers
11k views

What is the use of a tunnel interface on a cisco router?

What is a tunnel interface used for on a cisco router? What is the difference between setting up a VPN connection and a tunnel interface; or does this serve the same purpose?
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  • 2,363
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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5 votes
3 answers
1k views

Is it necessary to use layer 2 switches with microwave point-to-point links?

For a university project we are designing a network across 3 buildings for a small business. I've decided to implement a microwave point-to-point to connect each building. A lot of microwave point-to-...
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2 votes
4 answers
2k views

Cisco 1921 security features in the default bundle

Bought a Cisco 1921 router without any additional modules. This is the first time I'm using a non-home router. What security features should I enable so that users are relatively safe when using the ...
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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
9 votes
2 answers
5k views

What is backpressure in the context of routers?

There are several references to backpressure as regards to the internal bandwidth and switching fabrics of routers and switches. What exactly does backpressure mean, and what are the real world ...
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