- Information maintained by
a router:
- The network IDs
of networks that
the router is
attached to
- This information is used to transmit IP packets directly to IP destinations on the same IP network
- The network IDs
of networks that
the router is
not attached to
- This information is used to transmit IP packets to remote IP destinations on other IP networks
- The network IDs
of networks that
the router is
attached to
- Information maintained by
a node to
forward (transmit)
IP packets:
- A (layer 2)
forwarding table
- Structure of the
layer 2
forwarding table:
destination network ID | Output port ----------------------------+---------------- IP netw ID 1 | port 1 IP netw ID 2 | port 2 .... | ... (and so on)
- This table is used
to check if a
destination is
on the same network
as the IP node
(I.e., destination is immediately reachable)
- If destination is on the same network, the node will send the IP packet to the destination directly
- Structure of the
layer 2
forwarding table:
- A (layer 3)
routing table
- Structure of the
layer 3
forwarding table:
destination network ID | Next IP node ----------------------------+---------------- IP netw ID 1 | IP address 1 IP netw ID 2 | IP address 2 .... | ... (and so on)
- This table is used to pass the packet to the next router if a destination is not directly reachable.
- Structure of the
layer 3
forwarding table:
Example:
Situation:
- The node is
connected to
4 different
IP networks:
- Net6 (represents some IP network address)
- Net12
- Net9
- Net8
- A (layer 2)
forwarding table
- The (level 2) forwarding table
contains IP network IDs
to which the node (host or router)
is
attached to:
Entry in the forwarding table has the form:
(Attached) IP Network ID Port#The forwarding table is used to
- Forward the IP-packets destinated for IP destination on the same IP network as the node itself
- A (level 3) routing table
containing
IP network IDs
of
"remote" networks:
Entry in the routing table has the following form:
(Remote) IP network ID next-Router IP-addrUsage:
- The next-Router IP addr
is the IP address of
a router that
is immediately reacheable
- IP packets destined for a destination of that (remote) IP network will be forwarded to the next-Router IP addr node for further processing
- The next-Router IP addr
is the IP address of
a router that
is immediately reacheable
- Note:
- The name of the
level 2 forwarding table
is some what
unfortunate because:
- We have seen
another
kind of
forwarding table inside
a bridge
previously:
See: click here
- We have seen
another
kind of
forwarding table inside
a bridge
previously:
- The level 2 forwarding table
inside a router contains
different information than
the forwarding table in
a bridge:
Namely:
- The forwarding table in a bridge contains physical addresses (Ethernet)
- The forwarding table in a router contains IP (logical) network IDs (IP domains)
- The name of the
level 2 forwarding table
is some what
unfortunate because:
-
Default route:
- The (level 3) routing table
may (often)
contain a special
default route
entry:
destination network ID | Next IP node ----------------------------+---------------- IP netw ID 1 | IP address 1 IP netw ID 2 | IP address 2 .... | ... (and so on) Default | IP address X
- The
default route
entry will be used
for routing
an IP packet
if:
- its destination network ID is not found in the routing table.
- The (level 3) routing table
may (often)
contain a special
default route
entry:
- Consider the following IP network that consists of
4 IP domains:
The IP networks (domains) are:
- IP network w
- IP network x
- IP network y
- IP network z
(w, x, y and z are IP network prefixes)
Remember that:- Nodes on the same IP network (= IP domain) has the same network prefix !!!
Example:
- The nodes x3, x6 and x7 have the same IP network prefix x
- Content of the
forwarding table and the
routing table
of some of the nodes:
Explanation:
- The tables at node
x7:
Comments:
- x7 is on
IP network x
- Hence the forwarding table of x7 contains IP network x
- There is only
one way
out of
network x:
- Through the router A
So x7's routing table only contains the default router entry
(Most end destinations hosts (computers) that are connected to the Internet by one single router would have this configuration)
- x7 is on
IP network x
- The tables at
router A:
Comments:
-
Router
A is the
only entrance ("gateway")
to the network x
Because the router A is attached to the network x:
- Router A's forwarding table contains the entry for network x
- Because there is
only
one route through the
router B
to
all other networks:
- A's routing table only contains the default router entry (to router B)
(Most routers that connects one network to the Internet would have this configuration)
-
Router
A is the
only entrance ("gateway")
to the network x
- The tables at
router B:
Comments:
- Router B is
attached
to 2 IP networks
(w and y):
- Router B's forwarding table contains rntries for networks w and y
- Router B is
connected to 2 different routers
Router B's routing table indicates that:
- It uses router A to reach network x
- It uses router C to reach all other networks
- Router B is
attached
to 2 IP networks
(w and y):
- The tables at node
x7: