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konsep IP routing

outing dari kata dasar route yang diserap dalam bahasa indonesia sebagai rute, definisinya adalah rute dari paket IP didalam jaringan dengan serangkaian tugas untuk mengirimkan paket IP dari router ke router sampai ke tujuan akhir sebagaimana sudah ditentukan didalam bagian IP Header. adalah mirip konsep routing antara jaringan IP dengan system transportasi, disini kami akan menerangkan bahwa konsep routing didalam jaringan IP juga mirip dengan pengoperasian pengiriman mail. dan kami akan membandingkan konsep routing IP dengan konsep konsep system lainnya.

a router has directly attached networks that are immediately accessible (in
other words, that do not require any specific routing mechanism to discover). Consider
router R, in the following example. Networks 1.0.0.0, 10.1.1.0, and 10.1.2.0
are directly connected to the router:
hostname R
!
interface Ethernet0
ip address 1.1.1.1 255.0.0.0
!
interface Ethernet1
ip address 10.1.1.4 255.255.255.0
!
interface Ethernet2
ip address 10.1.2.4 255.255.255.0

In fact, the moment these networks are connected to the router they are visible in R’s
routing table. Note in the following output that the command to display the routing
table is show ip route (in EXEC mode). Also note the “C” that is prepended to the
entries in the routing table, indicating that the routes were discovered as directly
connected to the router:
R#show 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, * – candidate default
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
C 1.0.0.0/8 is directly connected, Ethernet0
10.0.0.0/8 is subnetted, 2 subnets
C 10.1.1.0/24 is directly connected, Ethernet1
C 10.1.2.0/24 is directly connected, Ethernet2
Directly connected networks are automatically installed in the routing table if the
interface to the network is up. Figure 1-2 shows router R with its directly connected
networks. (The EXEC command show interface will show the state of the interfaces).
In the previous example, it is assumed that all three interfaces to the directly connected
networks are up. If an interface to a directly connected network goes down,
the corresponding route is removed from the routing table.
If multiple IP addresses are attached to an interface (using secondary addresses), all
the associated networks are installed in the routing table.
Static Routing
ip route 146.1.0.0 255.255.0.0 1.1.1.2
R#sh ip route

1 S 146.1.0.0/16 [1/0] via 1.1.1.2
ip route 146.1.0.0 255.255.0.0 1.1.1.3
The syntax of the static route command is:
ip route network [mask] {address | interface} [distance]
where network and mask specify the IP address and mask of the destination. The
next hop may be specified by its IP address or by the interface on which to send the
packet. To point a static route to an interface (Ethernet0 in this case), use:
ip route 146.1.0.0 255.255.0.0 interface Ethernet0
Static routes are smart to the extent that if the next hop (interface or IP address)
specified goes down, the router will remove the static route entry from the routing
table.
Dynamic Routing
we spoke of the “shorter” or “shortest” path in the context
of both DV and Link State algorithms. Since a router may know of multiple
paths to a destination, each routing protocol must provide a mechanism to discover
the “shorter” or “shortest” path based on one or more of the following criteria: number
of hops, delay, throughput, traffic, reliability, etc. A metric is usually attached to
this combination; lower metric values indicate “shorter” paths. For each routing protocol
discussed in the chapters that follow, we will describe how the route metric is
computed.
A network under a single administrative authority is described as an autonomous system
(AS) in routing parlance. Interior gateway protocols (IGPs) are designed to support
the task of routing internal to an AS. IGPs have no concept of political boundaries
between ASs or the metrics that may be used to select paths between ASs. RIP, IGRP,
EIGRP, and OSPF are IGPs. Exterior gateway protocols (EGPs) are designed to support
routing between ASs. EGPs deploy metrics to select one inter-AS path over
another. BGP is the most commonly used EGP.
The Routing Table
At Grand Central Terminal, a big wall lists all the destinations and their corresponding
track numbers (see Figure 1-4). Passengers find their destination on this wall and
then proceed to the indicated platforms. Similarly, a routing table must contain at
least two pieces of information: the destination network and the next hop toward
that destination. This reflects a fundamental paradigm of IP routing: hop-by-hop
routing. In other words, a router does not know the full path to a destination, but
only the next hop to reach the destination.
Departures
Destination #####time#####tracknumber
New Haven 9:21 22
Cos Cob 9:24 11
Valhalla 9:31 19
Dover Plains 9:42 12
Bronxville 9:18 17

Routes are installed in the routing table as they are learned through the mechanisms
we have been discussing: directly connected networks, static routes, and dynamic
routing protocols. A typical routing table in a Cisco router looks like this:
Router>show 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, * – candidate default
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
2 177.130.0.0/30 is subnetted, 2 subnets
C 177.130.17.152 is directly connected, Serial1
C 177.130.17.148 is directly connected, Serial0
3 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
4 S 10.0.0.0/8 [1/0] via 160.4.115.74
5 S 10.254.101.0/24 [1/0] via 160.4.101.4
6 162.162.0.0/24 is subnetted, 2 subnets
O IA 162.162.101.0 [110/3137] via 11.175.238.4, 02:16:02, Ethernet0
[110/3137] via 11.175.238.3, 02:16:02, Ethernet0
O IA 162.162.253.0 [110/3127] via 11.175.238.4, 02:25:43, Ethernet0
[110/3127] via 11.175.238.3, 02:25:43, Ethernet0
7 O E2 192.188.106.0/24 [110/20] via 11.175.238.33, 20:49:59, Ethernet0

Note that the first few lines of the output attach a code to the source of the routing
information: “C” and “S” denote “connected” and “static”, respectively, as we saw
earlier, “I” denotes IGRP, etc. This code is prepended to each routing entry in the
routing table, signifying the source of that route.
The body of the routing table essentially contains two pieces of information: the destination
and the next hop. So, 177.130.0.0 (line 2) has two subnets, each with a 30-bit
mask. The two subnets are listed in the following two lines.
Line 3 shows an interesting case. 10.0.0.0 has two subnets: 10.0.0.0/8 and 10.254.
101.0/24. Not only are the subnet masks different, but the subnets are overlapping. A
destination address of 10.254.101.1 would match both route entries! So, should a
packet for 10.254.101.1 be routed to 160.4.115.74 or 160.4.101.4? Routing table
lookups follow the rule of longest prefix match. 10.254.101.1 matches 8 bits on line 4
and 24 bits on line 5—the longer prefix wins, and the packet is forwarded to 160.4.
101.4. 162.162.0.0 (line 6) has two subnets, each of which is known via two paths.
192.188.106.0 (line 7) is not subnetted.
What if a route is learnt via multiple sources—say, via OSPF and as a static entry?
Each source of routing information has an attached measure of its trustworthiness,
called administrative distance in Cisco parlance. The lower the administrative distance,
the more trustworthy the source.
Table 1-1 shows the default administrative distances.
Table 1-1. Default administrative distances
Route source Default distance
Connected interface 0
Static route 1
External BGP 20
IGRP 100
OSPF 110
IS-IS 115
RIP 120
EGP 140
Internal BGP 200
Unknown 255

Thus, if a route is known both via OSPF and as a static entry, the static entry, not the
entry known via OSPF, will be installed in the routing table.
Note that distance information and the route metric appear in the output of show ip
route inside square brackets with the distance information first, followed by a “/”
and the route metric: [distance/metric].
Administrative distance is only considered internally within a router; distance information
is not exchanged in routing updates.

to be continued..

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