Switching and Routing for Dummies - IP Routing

1

NETMANIAS TECH-BLOG Please visit www.netmanias.com to view more posts

Switching and Routing for Dummies - Part 2. IP

Routing

May 8, 2014 | By Chris Yoo ([email protected]) In the previous post, we learned about L2 (Ethernt) switching. This post is about L3 (IP) routing. This time, we will look into IP routing illustrated in the right figure below.

Again, please take a close look at the network configuration above. In the figure, MAC/IP values are as follows: Server/Router Port Mac address IP address

SVR1 SVR3 R1

Lan1 Lan2 ge1/1 ge2/1

M1 M3 a1 a2

1.1.1.10 2.1.1.30 1.1.1.1 2.1.1.1

http://www.netmanias.com/en/?m=view&id=blog&no=6330

Netmanias Tech-Blog: Evolution of Mobile RAN Architecture in LTE/LTE-Advanced Era

2

IP Routing


3

1. SVR1 sends ARP Request

SVR1 with IP address of 1.1.1.10 is trying to send a packet to the destination, SVR3 with IP address of 2.1.1.30.

Through routing table lookup, SVR1 finds out the destination address 2.1.1.30 is mapped to a default route (0.0.0.0/0), and thus "in order for the packet to reach the destination, it has to go to the gateway 1.1.1.1 via OIF (outgoing Interface) lan1". In case of regular PCs, IP addresses are assigned (leased) through DHCP and gateway addresses are assigned through DHCP Option 3. In case of servers, gateway addresses are set by server administrators. Then, the assigned (or set) addresses are installed in the routing table.

A gateway (also known as default gateway) refers to the first router (no matter how many L2 switches there are in between) connected to a server, e.g. SVR1 here. This gateway is located in the same network that the server (SVR1) belongs to (In the figure, 1.1.1.1, the interface address of the first router (R1) connected to SVR1 (at 1.1.1.10) is the gateway of the server).

As the ARP table does not have a MAC address entry for the gateway whose IP address is 1.1.1.1 (i.e. ARP miss event), SVR1 sends an ARP Request packet with the following fields to lan1 port:

Header Fields

Ethernet Header * Destination MAC = FF:FF:FF:FF:FF:FF (broadcasting, so sending to all nods in the same LAN) * Source MAC = m1, the MAC address of Sender (SVR1)

IP Header

* Sender MAC = m1, the MAC address of the ARP Request packet sender (SVR1) * Sender IP = 1.1.1.10, the IP address of the ARP Request packet sender (SVR1) * Target MAC = 00:00:00:00:00:00 (the value that SVR1 wants to find out) * Target IP = 1.1.1.1, the IP address of the ARP Request packet target (R1)


4

Upon receiving the packet, S1 (Switch 1) performs source MAC learning and records the following in the MAC table: {MAC address m1 is connected to fe1 port}.

By referring the destination MAC address in the received packet, S1 determines an output port, e.g. a broadcast address (FF:FF:FF:FF:FF:FF) in this case. Accordingly, S1 performs flooding to all ports except for the receiving port. As a result, the packet is received by SVR2 and R1 (Router 1).

From the target IP address of the received ARP Request packet, SRV2 learns that the address does not match its own. So, it discards it.

2. R1 responds with ARP Reply

R1 (Router 1) also checks the target IP address of the received ARP Request packet, and notices that its MAC is requested. So, it enters the MAC address corresponding to 1.1.1.1 in the Sender MAC field of an ARP Reply packet, and sends the packet to ge1/1 port. At this time, the packet includes the following fields:

Header Fields

Ethernet Header * Destination MAC = m1, the MAC address of SVR1 to which ARP Reply is to be delivered * Source MAC = a1, the MAC address of Sender (R1)

IP Header

* Sender MAC = a1, the MAC address of the ARP Reply packet sender (R1) * Sender IP = 1.1.1.1, the IP address of the ARP Reply packet sender (R1) * Target MAC = m1, the MAC address of the ARP Reply packet target (SVR1) * Target IP = 1.1.1.10, the IP address of the ARP Reply packet target (SVR1)

S1, upon receiving the packet, learns the source MAC and records the following in the MAC table: {MAC address a1 is connected to fe3 port}.

Then, it sends (unicasts) the packet to fe1 port by referring to the MAC table where m1, the destination MAC address of the received packet, is mapped to fe1.

Next, SVR1 records the received value (a1, the MAC address assigned for 1.1.1.1) in its ARP Table. 3. SVR1 sends IP Packet to R1

SVR1, now ready to send an IP packet to SVR3, sends one with the following fields to lan1 port:

Header Fields

Ethernet Header * Destination MAC = a1, the MAC address of Receiver (R1) * Source MAC = m1, the MAC address of Sender (SVR1)

IP Header * Destination IP = 2.1.1.30, the IP address of Receiver (SVR3) * Source IP = 1.1.1.10, the IP address of Sender (SVR1)

S1, having already learned the source MAC address, skips the source MAC learning and sends the packet to fe3 port, as mapped for the destination MAC address a1 in the MAC table.


5


6

4. R1 sends ARP Request

• Upon receiving the received packet, R1 (router 1) knows that "the destination address 2.1.1.30 is the network immediately connected to itself (because there is no next hop), and the output port is ge2/1" by referring to its FIB (routing table).

• Now, R1 checks the ARP table for the MAC address corresponding to 2.1.1.30, and finds no entry there (ARP Miss). So, it sends an ARP Request packet with the following fields to ge2/1 port.

Header Fields

Ethernet Header * Destination MAC = FF:FF:FF:FF:FF:FF (broadcasting: reaching all nodes in the same LAN) * Source MAC = a2, the MAC address of Sender (R1)

IP Header

* Sender MAC = a2, the MAC address of the ARP Request packet sender (R1) * Sender IP = 2.1.1.1, the IP address of the ARP Request packet sender (R1) * Target MAC = 00:00:00:00:00:00 (the value that R1 wants to find out) * Target IP = 2.1.1.30, the IP address of the ARP Request packet target (SVR3)

• Now, this packet is received by S2 (Switch 2), and the source MAC address of the received packet is learned. Then, the following is recorded in the MAC table at S2: {MAC address a2 is connected to fe3 port}.

• By referring to the destination MAC address of the received packet, S2 determines which output port to send to. In this case, as it is a broadcast-type address (FF:FF:FF:FF:FF:FF), it performs flooding to all the ports except for the receiving port. As a result, the ARP Request packet is received by SVR3 and SVR4.


7

• From the target IP address of the received ARP Request packet, SVR4 notices that the address does not match its own, and hence discards it.

5. SVR3 responds with ARP Reply

• SVR3 also checks the target IP address of the received ARP Request packet, and finds out the address matches its own address. So, it enters the MAC address corresponding to 2.1.1.30 in the Sender MAC field of an ARP Reply packet, and sends the packet to lan1 port. At this time, the packet includes the following fields:

Header Fields

Ethernet Header * Destination MAC = a2, the MAC address of R1 to which ARP Reply is to be delivered * Source MAC = m3, the MAC address of Sender (SVR3)

IP Header

* Sender MAC = m3, the MAC address of the ARP Reply packet sender (SVR3) * Sender IP = 2.1.1.30, the IP address of the ARP Reply packet sender (SVR3) * Target MAC = a2, the MAC address of the ARP Reply packet target (R1) * Target IP = 2.1.1.1, the IP address of the ARP Reply packet target (R1)

• S2, upon receiving the packet, learns the source MAC and records the following in the MAC table: {MAC address m3 is connected to fe3 port}.

• Then, it sends (unicasts) the packet to fe1 port by referring to the MAC table where a2, the destination MAC address of the received packet, is mapped to fe3.

• Next, R1 records the received value (m3, the MAC address assigned for 2.1.1.30) in its ARP Table. 6. R1 sends IP Packet to SVR3

• Now, as SVR3 has learned the MAC address, R1 (Router 1) is ready to route IP packets from SVR1 to SVR2. So, it begins to route the IP packet sent by SVR1 to SVR3, with the following fields:

Header Fields

Ethernet Header * Destination MAC = m3, the MAC address of Receiver (SVR3) * Source MAC = a2, the MAC address of Sender (R1)

IP Header * Destination IP = 2.1.1.30, the IP address of Receiver (SVR3) * Source IP = 1.1.1.10, the IP address of Sender (SVR1)

• S2, having already learned the source MAC address, skips the source MAC learning and sends the packet to fe1 port, as mapped for the destination MAC address m3 in the MAC table, so that it can be delivered on to SVR3.


8

Summary

All devices/servers (to be referred to as servers below) are connected to other servers through switche(es) and router(s). When a server sends a packet, the server first makes a decision on whether to send the packet by Ethernet switching or IP routing. As discussed in the previous post, when a server sends a packet, it uses the MAC address of the destined device as the destination MAC address of the packet if the destination IP address (e.g. 1.1.1.20) is located within the same network that the server belongs to (e.g. 1.1.1.10). On the other hand, if the destination IP address (e.g. 2.1.1.30) is not located in the same network where the sender server (e.g. 1.1.1.10) is located as explained in this post, the sender server uses the MAC address of a gateway (e.g. 1.1.1.1), instead of the destined destination, as the destination MAC address of the packet. Here, the gateway refers to the first L3 hop that is connected to the server, i.e. router (R1). For that reason, the packet is destined to the MAC address of the router, and thus is sent to the router via Ethernet switching regardless of the switch(es) installed in between. Then, the router, after confirming the destination MAC address of the received packet matches its own, performs IP routing (also known as "forwarding") through FIB lookup. So, again the point here is: "The router performs IP routing if the destination MAC address of the received packet matches its own. If not, it simply discards it." Exceptionally, if the destination MAC address is FF:FF:FF:FF:FF:FF, then the packet is received by the control plane of the router, as the ARP Request packet is received and processed by R1 above.

http://www.netmanias.com/en/?m=view&id=blog&no=6330

About NMC Consulting Group (www.netmanias.com) NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2014 NMC Consulting Group. All rights reserved.

9

Carrier WiFi

Data Center Migration

WirelineNetwork

LTE

Mobile Network

Mobile WiMAX

Carrier Ethernet

FTTH

Data Center

Policy Control/PCRF

IPTV/TPS

Metro Ethernet

MPLS

IP Routing

99 00 01 02 03 04 05 06 07 08 09 10 11 12 13

eMBMS/Mobile IPTV

Services

CDN/Mobile CDN

Transparent Caching

BSS/OSS

Cable TPS

Voice/Video Quality

IMS

LTE Backaul

Netmanias Research and Consulting Scope

Visit http://www.netmanias.com to view and download more technical documents.

Future

LTE IP/M

PLS

Carr

ier Eth

ern

et

Networks

Cons

ulti

ng

POC

Trai

ning

Wi-F

i

Infrastructure Services

CDN

Tran

spar

ent

Cach

ing

IMSConcept DesignDRM

eM

BM

S

protocols

Analyze trends, technologies and market

Analysis

ReportTechnical documents

BlogOne-Shot gallery

We design the future



Switching and Routing for Dummies - IP Routing

Technology

Transcript of Switching and Routing for Dummies - IP Routing