Introduction to Routers and Routing

This topic is to discuss the following lesson:

Dear Sir,

I am really thanks for your lessons it’s help full for me

again thanks

My router is not showing the directly connected routes even after configuring the router.What could be the problem?

Router#sh ip int br
Interface              IP-Address      OK? Method Status                Protocol
 
FastEthernet0/0        192.168.1.254   YES manual up                    down
 
FastEthernet0/1        192.168.2.254   YES manual up                    down
 
Vlan1                  unassigned      YES unset  administratively down down
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

Most likely it is a cable problem, or there is a speed/duplex mismatch between your router and the other side where it is plugged in.

Hi,

I would like to know the main differences between Branch Routers and Network Edge Routers, please.

You can find that option on the Cisco Router Selector web page:
http://www.cisco.com/c/dam/assets/prod/routers/cisco-router-selector/index.html

Thanks! :slight_smile:

Hello David!

An edge router is a router that is found at the “edge” of a corporate or an enterprise network, that is, at the location where you connect to the “outside world” be it the Internet or a private or public WAN. Edge routers are usually designed to handle large amounts of traffic for hundreds or even thousands of users. Additionally, edge routers have many functionalities that would be useful at the edge of a large network such as security, content filtering, dual or multi homed connections (connecting to multiple ISPs for redundancy) and others.

Branch routers on the other hand are smaller, cheaper and are used primarily for remote sites (or a corporate branch office, which is where it gets its name from). They can also be used for a standalone office comprised of a small number of end users. The number of users branch routers can accommodate is usually several dozen and won’t often surpass 100. These routers usually have capabilities such as VPNs to connect to the corporate network and some security features. In general, they are a lower end devices as compared to network edge routers.

There really isn’t a solid line that separates the two types of routers. It all depends on the needs you have at each site. These needs however, more often than not, are derived from the number of end users you have at the site you want to provide for.

I hope this has been helpful!

Laz

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Thank you very much! :slight_smile:

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A basic question please: I understand the concept of gateway, i.e. if destination IP address is not in my subnet then go via gateway. But what does that packet look like, is the destination IP address that of the router and another/2nd destination IP address that of the intended end destination? Or asked differently if we have Sw1—Router—Sw2, where all hosts on Sw1 are in 192.168.1.0/24 and all on Sw2 are in 192.168.2.0/24, how will the Sw1 know that a packet with dest IP address for 192.168.2.2 needs to go to the router? -or does it learn MAC addresses in this instance from the router, which I assume is the case?

Hello Johan

Using your topology of Sw1—Router—Sw2, where SW1 has a subnet of 192.168.1.0/24 and SW2 has a subnet of 192.168.2.0/24. Let’s say 192.168.1.1 is the default gateway, that is the Router’s IP address.

Now imagine that Host 1 with an IP address of 192.168.1.10 wants to send a packet to 192.168.2.10. It does the following:

  1. The data is received from the transport layer and encapsulated into an IP packet. The source IP is 192.168.1.10 and the destination IP is 192.168.2.10. These addresses remain the same in the header of the IP packet during the whole trip of the packet from source to destination. They do not change.
  2. The host determines that the destination is not in the same subnet, so it must send the packet to the default gateway. Now as it encapsulates the IP packet into an Ethernet frame, it uses the MAC address of Host 1 as the source MAC address and the MAC address of the default gateway as the destination MAC address.
  3. In order to get the destination MAC address, that of the gateway, it must look up the gateway IP address if 192.168.1.1 (which Host 1 knows because it is configured with a default gateway) in its own ARP table. ARP is used to determine an unknown MAC address that corresponds with the known IP address. If it has an entry for the IP address, it uses that MAC address as the destination. If it does not, it sends an ARP request (broadcast) asking for the MAC address on the network segment that corresponds with this IP address.
  4. Once the MAC address of the default router is determined, the destination MAC address field of the Ethernet frame is populated and the frame is placed on the wire to be sent.
  5. Once the frame is received by the router, it will deencapsulate it to the network layer, look at the destination IP address and reencapsulate it with the appropriate source and destination MAC addresses, that is, its own and Host 2’s respectively.

So essentially, in the Network layer, the source and destination IP addresses remain the same throughout the path of the packet. In the Data Link layer, the source and destination MAC addresses change for each hop of the path to the destination.

You can find some more information about this topic at the following lesson:

I hope this has been helpful!

Laz

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Hey @ReneMolenaar just a minor typo :slight_smile: thanks for the lesson, these refreshers are great

In this lesson we will take a look at the difference between switches and routers and I’ll explain you the basics of routing.

In this lesson we will take a look at the difference between switches and routers and I’ll explain to you the basics of routing.

Thanks @witherford.m, just fixed this.

Rene

Hi Rene/Laz,

I emulated similar topology on GNS3, kindly help to clarify the below points. I have the following topology:

R1(Ge0/0) <-> (Ge0/0)R2(Ge0/1) <-> (Ge0/1)R3

R1-R2: 192.168.1.0/24
R2-R3: 192.168.2.0/24

So when the routers came up and ip address was configured on interfaces, each interface broadcasts gratuitous arp packet. So my question is when I try to ping from R1 to R3, since the R3 ip is in a different subnet R1 tries to send it to its default-gateway. When gratuitous arp has already informed the mac address why does it it have to send an arp request again to find the mac address of the adjacent port.

Apologies if not clear about my question, attaching snippet from the wireshark capture.

In the image you can see 192.168.2.2(R3-Ge0/1) interface has already announced its mac address, however 192.168.2.1(R2-Ge0/1) again sends an arp request? why does that happen?

Thanks and Regards,
Teja

Hello Teja

When a device is connected and turned on, one of the things it may do is to “announce” its existence to its neighbours within the same network segment using a Gratuitous ARP. Whether this actually happens or not depends on the vendor of the network equipment and the software being run on it. The purpose is useful, as it is an attempt to preemptively populate the ARP caches of neighbouring hosts, eliminating the need to initiate a normal ARP request.

This was especially useful “back in the day” (ARP was defined in 1982!) when bandwidth was scarce and every effort for efficiency was necessary. However, there is no rule or standard that obligates any network device to use the information in a Gratuitous ARP to populate its ARP table. Indeed, some, (actually most) vendors will ignore Gratuitous ARPs in such situations altogether, since sending out another ARP request essentially adds no overhead to today’s high speed networks. And this is indeed best practice, as it mitigates against attacks that involve Gratuitous ARPs with modified MAC addresses.

So in your situation, the router simply ignores the information in the Gratuitous ARP because it has been designed to do so.

I hope this has been helpful!

Laz

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