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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.

22
votes
4answers
44k 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 ...
21
votes
6answers
5k 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 they'd still be inside a ISP (right?), but they'd have freedom to configure stuff the way they wanted. ...
20
votes
2answers
25k 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 ...
20
votes
7answers
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 ...
17
votes
2answers
19k 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, correct? Is this a cisco only ...
17
votes
7answers
50k 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 ...
16
votes
3answers
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 ...
16
votes
6answers
19k 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/...
16
votes
4answers
40k 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 ...
16
votes
3answers
9k 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 ...
16
votes
3answers
3k 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 ...
15
votes
2answers
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 ...
15
votes
2answers
8k 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, ...
14
votes
7answers
8k 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 ...
14
votes
2answers
1k 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 ...
12
votes
2answers
9k 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. ...
12
votes
2answers
4k 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?...
12
votes
1answer
1k 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)
12
votes
1answer
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 ...
12
votes
5answers
14k 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 ...
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
6answers
21k 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 ...
11
votes
3answers
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 ...
11
votes
7answers
12k 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 ...
11
votes
2answers
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 ...
11
votes
3answers
2k 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 ...
11
votes
1answer
5k 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 ...
10
votes
6answers
4k 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 ?
10
votes
4answers
2k 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 ...
10
votes
4answers
5k 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 ...
10
votes
1answer
3k 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. % ...
10
votes
4answers
3k 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/ ...
10
votes
7answers
4k 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 ...
10
votes
2answers
227 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 (...
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
1answer
27k 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 ...
9
votes
4answers
2k views

Measuring per-link latency/jitter/packet-loss

I need to measure the latency, jitter and packet-loss per link in a network, as opposed to end-to-end, e.g. like this: s1-eth0 <--> s2-eth0 (~ 20µs avg. latency, standard derivation 10µs, 57 % ...
9
votes
4answers
735 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-...
9
votes
2answers
4k 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 ...
9
votes
3answers
3k 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?
9
votes
3answers
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 "...
9
votes
1answer
6k 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 ...
9
votes
3answers
21k 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 ...
9
votes
3answers
17k 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 ...
9
votes
3answers
8k views

Cisco Router config alternatives to Cisco SDM (i.e. no Java required)

We have an old Cisco 850 Series router hanging around here and happen to have a need for it temporarily until we get our new Palo-Alto into place. Now, after I reset it and try to pull up the ...
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
1answer
2k views

Summarized OSPF routes advertised?

Preparing for CCNP, doing some lab work. ZEUS is summarizing two loopbacks with area 2 range 172.16.2.0 255.255.254.0, and the summary route is arriving on router 'WODAN' but the subordinate routes (...
8
votes
2answers
4k views

How can hosts with public IP addresses have nodes with private IP addresses in their traceroute?

I'm doing some research on users connected to Internet via cable operators and I've seen some unusual route traces. A bit about setup: Basically user has a home router connected to a DOCSIS modem. ...
8
votes
2answers
835 views

routing between vlans not working

I need to put a Cisco router behind an ISP router. (The ISP doesn't allow us to use their modem only.) I've called the ISP technical service and they've set up the connection with me. Their router ...
8
votes
3answers
10k 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?