Download Host-Specific Routing

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Chapter 6
Delivery & Forwarding of IP Packets
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ROUTING METHODS
There are various routing methods:
• Next-Hop Routing – table only holds the address of the next hop
(instead info regarding the entire route) – routing table for each host
• Network-Specific Routing – instead of an entry for each host (on the
same network), only one entry for the network is defined
• Host-Specific Routing – for a specific destination host, you might want
to control the exact route – in this case, the actual Rx is listed in the
routing table and the desired next hop is listed
• Default Routing – instead of listing all of the various networks in the
Internet, Tx host would use one entry called the Default (network
address 0.0.0.0)
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Next-hop routing
Next-Hop Routing – table only holds the address of the next hop
(instead info regarding the entire route)
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Show more routers in better illustrating the routing table
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Network-specific routing
Network-Specific Routing – instead of an entry for each host
on the same physical network, only one entry for the network
is defined
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Host-specific Routing
Host-Specific Routing – for a specific destination host, you might want
to control the exact route – in this case, the actual Rx is listed in the
routing table and the desired next hop is listed
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R1
In this case, you want every packet traveling to Host B to traverse through R3.
For the other hosts on N2 and N3, the Network-specific routing approach is
used.
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Default Routing
•Default Routing – instead of listing all of the various networks in the
Internet, Tx host would use one entry called the Default (network
address 0.0.0.0)
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In this case, R1 sends to a specific network however, R2 sends to the
remainder of the Internet (default)
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Simplified forwarding module in
classful address without subnetting
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Using the next-hop address and interface
number, ARP searches for the physical
address in facilitating the actual hop
For the Classful case, per router, a table was needed for each class –
this made the searching simple
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Configuration for routing for R1, Classful Case
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Simplified forwarding module in classful
address with subnetting
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Recall for the Classful case, subnetting is done within the
organization
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Configuration for the Classful and Subnetting Case
Doesn’t know
what network
is connected
to router here
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Simplified forwarding module in classless
address
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Routing Table for R1 in the Illustrated Configuration –
Classless Case
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Address aggregation
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With the classless approach, routing tables increased – in
reducing the size of some tables, use a router to represent
multiple blocks – address aggregation
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STATIC VERSUS DYNAMIC ROUTING
• Host or router uses a routing table
• Table can be either static or dynamic in nature
• A static routing table contains information entered
manually.
• A dynamic routing table is updated periodically using
one of the dynamic routing protocols such as RIP, OSPF,
or BGP
• Regarding dynamic routing table: if fiber cut or router
failure, the tables are updated
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Router’s Table Logistics
When the router is looking for the route, it:
First check for direct delivery
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Then host-specific delivery,
The network-specific delivery, and
Finally, default delivery
This order can be organized with in the routing table
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Routing Table
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Mask: used to extract the net id of the Rx. For Host-Specific Routing - the mask is
255.255.255.255 and for Default Routing – the mask is 0.0.0.0.
Destination Address: either the destination host address or destination network address
Next-hop Address: next hop router address
Flags
U - The router is up and running. If router is down, the packet discarded
G - The destination is in another network. If G flag present, indirect delivery (if not, direct delivery)
H – If H flag present, destination field contains Host-specific address (if not present, network address)
D – If D flag present, routing info added to host routing table via ICMP’s redirection (cover later)
M - If M flag present, routing info was modified via ICMP’s redirection (cover later)
Reference count: # of users using this route at any moment
Use: # of packets transmitted through this router for the corresponding Rx
Interface: name of the interface
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