Hot answers tagged

44

These terms are abstract logical concepts, much like the OSI model. Data plane refers to all the functions and processes that forward packets/frames from one interface to another. Control plane refers to all the functions and processes that determine which path to use. Routing protocols (such as OSPF, ISIS, EIGRP, etc...), spanning tree, LDP, etc are ...


33

Most likely if they're a big university they are their own ISP, using BGP to connect their network to the internet via a number of upstream networks. Nothing stops them from using IP addresses they should not be using, and it would work in their local network. However, it won't work on the Internet. Their upstream networks providing them connectivity should ...


32

The switch is probably a managed switch on which multiple VLANs are configured. The three cables between the router and the switch are used to provide inter-VLAN routing. Another possibility is that the multiple cables are used for link aggregation (e.g. LACP) to increase throughput.


26

Can anyone explain me what is the need of IBGP communication for the routes, when we have the IGP protocols (OSPF, RIP) for internal communication? Scalability1: Imagine that you're receiving 500,000 EBGP routes in more than one location2, and you need to influence the per route exit point in your AS. BGP can handle many more routes than IGP protocols. ...


24

Forwarding Plane - Moves packets from input to output Control Plane - Determines how packets should be forwarded Management Plane - Methods of configuring the control plane (CLI, SNMP, etc.) Undestand the difference between Forwarding, Control and Management Plane


22

ARP is used by a host on a LAN to resolve a layer-3 address to a layer-2 address so that a frame can be built for the LAN. A router is just another host on a LAN, and it will need to resolve layer-3 addresses to layer-2 addresses, the same way a PC on a LAN does.


21

It's theoretically possible, but not really in a practical sense. The IP protocol includes two options: Loose Source and Record Route (LSRR) Strict Source and Record Route (SSRS) They're both described in RFC 791. The difference between them is that LSRR can specify a partial route, while SSRS specifies the complete, exact route. With LSRR, each router ...


20

Does anycast addressing, in itself, add any additional latency to network connections? No. one using unicast and the other using anycast Anycast is unicast. It is just that the same network is in two different places, and the routing protocol chooses the closest network to which it send traffic destined for that network. It seems like having to ...


19

I don't believe that there is anything simpler than show interfaces | <some regex> unfortunately. Edit: From the comments below, @Santino pointed out a more concise RegEx: show ip interface | include line protocol|access list My testing so far indicates that this gives the same results as my longer RegEx below. I usually use the following to find ...


19

You have several options to get this type of functionality on a Cisco device. (Which one you use depends on your needs and sometimes on the device and IOS load. Some older devices/IOS loads will have different capabilities.) EDIT: You are asking about doing this every one minute, which as you found out in our chat about this question, will bog down your ...


19

IGMP Snooping is a feature for switches to learn what multicast groups are needed on which ports. Routers not handling multicast routing don't care. That said, without an mrouter in the network, you need to configure one (or more) igmp queriers. This ensures group membership reports are flood through the network periodically to keep the forwarding ...


19

Standard maximum payload size for Ethernet is still 1500 bytes. While the maximum frame size has grown slightly, from 1518 to 1522 byte with 802.1Q and further with 802.1AD, the payload, "MAC client data" size, or Maximum Service Data Unit (MSDU) hasn't been changed as per IEEE standards for compatibility reasons - maintaining the payload size enables ...


18

Every time you see .+_, that regular expression represents a single BGP autonomous system. This as-path list denies BGP as paths equal to or longer than 12 paths long. The regular expression works because . represents any character, + is a wild card which optionally repeats the previous character an unlimited number of times, and _ represents the space ...


17

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. Fake IP source addresses in headers can be detected and blocked in commercial network gear; other fake IPv4 headers can be a bit harder to identify. Most people refer to the function to detect fake source IP addresses ...


17

I really don't like how Cisco crippled EIGRP in the ipbase image... at a minimum, you should get some kind of warning that your configuration won't work, but they haven't done that either. As you discovered, eigrp stub prevents EIGRP from advertising downstream routes from the switches connected to your distribution. You can trick EIGRP into advertising ...


17

The SNR margin is the Signal to Noise Ratio margin. Signal to Noise Ratio is the relation between the data signal level and the noise level. Even the most perfect cable absorbs some noise. This "noise" is electromagnetic interference produced by: other cables running near the cable. Faulty connectors. Motors or transformers near the cable. Radio systems. ...


17

That /24 is part of a larger block (161.117.0.0/16) which is assigned to Alibabacom Singapore and announced by AS45102 (which is Alibaba again). My guess is that you're a customer of them and have been assigned a number of IP addresses to use from that /24. However, that does not give you the right to advertise them under your own ASN in BGP. If you do so, ...


17

The sending device uses the subnet mask to determine if the remote host is in it's local network or not. If the IP is within the subnet of the local machine, it uses ARP to determine the MAC address of the remote host. If it's outside, it queries it's local routing table to find the next hop of that IP, and sends out an ARP query to find the MAC address ...


16

bgp bestpath as-path multipath-relax was introduced by CSCea19918. Normally eBGP load-balancing requires the candidate routes to be equal-cost paths; i.e. identical BGP attributes: same Weight same Local-Pref same AS-Path (both the AS numbers, and the AS path length) same Origin same MED different next-hop As you mentioned, this command relaxes the same ...


16

It's important to note that type 2 LSA's are only generated on segments where a DR/BDR has been elected - this includes BMA (Broadcast Multi-Access) and NBMA (Non-Broadcast Multi-Access) networks. The DR is what generates the type 2 LSA. This behavior can be bypassed by configuring your Ethernet interfaces that you're choosing to run OSPF on as point-to-...


16

This question is akin to asking, "Why can't I use a screwdriver to hammer nails?" You could use OSPF in certain inter-AS situations if you REALLY, REALLY wanted to/needed to. There would be many "gotchas" and caveats to keep in mind when utilizing OSPF as an EGP. Just like you could hammer nails with a screwdriver. However, you would risk breaking the ...


16

The reason your router hangs is because you're editing the ACL through the same interface that it's applied to. Usually what happens is you wind up blocking yourself accidentally. Note that I make a suggestion for reorganizing your ACLs at the bottom of this answer, although most people will be interested in this first section (the easiest way to change ...


16

Router describes the general technical function (layer-3 forwarding) or a hardware device intended for that purpose, while gateway describes the function for the local segment (providing connectivity to elsewhere). You could also state that "you set up a router as gateway". Another term is hop which describes the forwarding in between subnets. It's just a ...


16

What is the exact difference between router and gateway? A router is a device/service that provides the function of routing IP packets between networks. A gateway (in network terms) is a router that provides access for IP packets into and/or out of the local network. In Network configuration we set the gateway ip address, but in reality we call the ...


16

You can configure the auto rollback feature to revert to a previous configuration in case you lose access. First, you need to configure configuration archive on the router. For example: archive path flash: write-memory This stores the recent configuration in flash. Then when you're ready to make changes, type configure term revert time 5 <new ...


15

Strictly speaking, when performing (pure) NAT, only IP addresses are translated, and every internal IP address has to be translated to a different external IP address. This can be a static one-on-one mapping in the case of static NAT, or a dynamic mapping with a pool of public addresses. With dynamic NAT, the router selects one IP address from the NAT pool ...


15

I can see two possible explanations: Your WAN address is your public address, but you are yourself using something that makes it look like you are accessing the internet somewhere else. One example is a proxy server which uses the 182-address to access the internet. A website telling you what your public IP is would see the 182 address. Often these sites ...


14

Specifically, you're looking for this: line con 0 logging synchronous line vty 0 15 logging synchronous


14

Assuming you have no IGP configured (such as EIGRP / OSPF / ISIS / RIP), then the simplest explanation is that R3 hasn't got a route to 192.168.1.0/24's next-hop when the iBGP update arrives at R3. loop0(R1)s0/0 <-----------> s0/0(R2)s0/1 <-------------> s0/1(R3) AS 1 AS 2 AS 2 -----...


Only top voted, non community-wiki answers of a minimum length are eligible