The Arrangement Layer
Written on 09:27 by Ahmet Emir
The Arrangement band (also alleged band 3) manages accessory addressing, advance the area of devices
on the network, and determines the best way to move data, which agency that the Arrangement layer
must carriage cartage amid accessories that aren’t locally attached. Routers (layer 3 devices) are
specified at the Arrangement band and accommodate the acquisition casework aural an internetwork.
It happens like this: First, back a packet is accustomed on a router interface, the destination IP
address is checked. If the packet isn’t destined for that accurate router, it will attending up the destination
network abode in the acquisition table. Once the router chooses an avenue interface, the packet
will be beatific to that interface to be affected and beatific out on the bounded network. If the router can’t find
an admission for the packet’s destination arrangement in the acquisition table, the router drops the packet.
Two types of packets are acclimated at the Arrangement layer: abstracts and avenue updates.
Data packets Acclimated to carriage user abstracts through the internetwork. Protocols acclimated to support
data cartage are alleged baffled protocols; examples of baffled protocols are IP and IPv6.
You’ll apprentice about IP acclamation in Chapters 2 and 3 and IPv6 in Chapter 13 .
Route amend packets Acclimated to amend adjoining routers about the networks affiliated to
all routers aural the internetwork. Protocols that accelerate avenue amend packets are alleged routing
protocols; examples of some accepted ones are RIP, RIPv2, EIGRP, and OSPF. Avenue update
packets are acclimated to advice body and advance acquisition tables on anniversary router.
In Figure 1.13, I’ve accustomed you an archetype of a acquisition table. The acquisition table acclimated in a
router includes the afterward information:
Network addresses Protocol-specific arrangement addresses. A router charge advance a acquisition table
for alone acquisition protocols because anniversary acquisition agreement keeps clue of a arrangement with a different
addressing arrangement (IP, IPv6, and IPX, for example). Think of it as a artery assurance in anniversary of the
different languages announced by the association that alive on a accurate street. So, if there were American,
Spanish, and French association on a artery alleged Cat, the assurance would apprehend Cat/Gato/Chat.
FIGURE 1 . 1 3 Acquisition table acclimated in a router
1.0
1.3 2.1
E0 S0
2.2 3.3
S0 E0
3.0
Routing Table
Metric
0
0
1
INT
E0
S0
S0
NET
1
2
3
Routing Table
Metric
1
0
0
INT
S0
S0
E0
NET
1
2
3
1.1
1.2
3.1
3.2
The OSI Reference Model 23
Interface The avenue interface a packet will booty back destined for a specific network.
Metric The ambit to the alien network. Altered acquisition protocols use altered ways
of accretion this distance. I’m activity to awning acquisition protocols in Chapters 6 and 7, but for
now, apperceive that some acquisition protocols (namely RIP) use article alleged a hop calculation (the
number of routers a packet passes through en avenue to a alien network), while others use
bandwidth, adjournment of the line, or alike beat calculation (1/18 of a second).
And as I mentioned earlier, routers breach up advertisement domains, which agency that by
default, broadcasts aren’t forwarded through a router. Do you bethink why this is a good
thing? Routers additionally breach up blow domains, but you can additionally do that application band 2 (Data
Link layer) switches. Because anniversary interface in a router represents a abstracted network, it must
be assigned different arrangement identification numbers, and anniversary host on the arrangement connected
to that router charge use the aforementioned arrangement number. Figure 1.14 shows how a router works in
an internetwork.
FIGURE 1 . 1 4 A router in an internetwork
Here are some credibility about routers that you should absolutely accomplish to memory:
- Routers, by default, will not advanced any advertisement or multicast packets.
- Routers use the analytic abode in a Arrangement band attack to actuate the abutting hop
router to advanced the packet to.
- Routers can use admission lists, created by an administrator, to ascendancy aegis on the types
of packets that are accustomed to access or avenue an interface.
- Routers can accommodate band 2 bridging functions if bare and can accompanying route
through the aforementioned interface.
- Band 3 accessories (routers in this case) accommodate access amid basic LANs (VLANs).
- Routers can accommodate affection of account (QoS) for specific types of arrangement traffic.
Switching and VLANs and are covered in Chapter 8, “LAN Switching and
STP,” and Chapter 9, “Virtual LANs (VLANs).”
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