Download Route Selection in Cisco Routers

Route Selection in Cisco Routers
Route Selection
• One of the intriguing aspects of Cisco
routers, especially for those new to routing,
is how the router chooses which route is the
best.
Route Selection
• To understand it completely requires some
knowledge about the way Cisco routers
work.
Processes Involved
• There are three processes involved in
building and maintaining the routing table
in a Cisco router:
Processes Involved
• There are three processes involved in
building and maintaining the routing table
in a Cisco router:
• Various routing processes (Protocols) RIP
EIGRP etc.
Processes Involved
• There are three processes involved in
building and maintaining the routing table
in a Cisco router:
• Various routing processes (Protocols) RIP
EIGRP etc.
• The routing table itself
Processes Involved
• There are three processes involved in
building and maintaining the routing table
in a Cisco router:
• Various routing processes (Protocols) RIP
EIGRP etc.
• The routing table itself
• The forwarding process
Routing Processes
•
•
•
•
•
Dynamic Routing Protocols
IGRP –
EIGRP –
OSPF –
RIP –
• These processes populate the routing table.
The Routing Table
• Accepts information from the routing
processes and also replies to requests for
information from the forwarding process.
Forwarding Process
• Requests information from the routing table
to make a packet forwarding decisions as to
which router interface to forward the packet
to.
Building the routing table
• Let's examine the interaction between the
routing protocols and the routing table to
understand how the routing table is built.
Building the routing table
• As each routing process ( EIGRP, RIP etc..)
receives updates and other information, it
chooses the best path to any given
destination and attempts to install this path
into the routing table.
Building the routing table
• For instance, if EIGRP learns of a path
towards 10.1.1.0/24, and decides this
particular path is the best EIGRP path to
this destination, it tries to install the path it
has learned into the routing table.
Building the routing table
• The router decides whether or not to install
the routes presented by the routing
processes based on the ….
• administrative distance of the route in
question.
Administrative distance
• This is the believability of the route trying
to be inserted into the routing table.
Administrative distance
• This is the believability of the route trying
to be inserted into the routing table.
• Some Routing Processes are more
trustworthy than others.
Administrative distance
• This is the believability of the route trying
to be inserted into the routing table.
• Some Routing Processes are more
trustworthy than others.
• Each Routing Process has a given
Administrative Distance as a number.
Administrative distance
• The smaller the number the more believable
the route.
Administrative distance
• The smaller the number the more believable
the route.
• If two routing processes try to install the
same route into the table one will be
rejected.
Example
• To understand this better, let's look at an
example.
Example
• To understand this better, let's look at an
example.
Example
• Assume a router has four routing processes
running: EIGRP, OSPF, RIP, and IGRP.
• Now, all four of these processes have
learned of various routes to the
10.168.24.0/24 network, and each has
chosen its best path to that network through
its internal metrics and processes .
Example
• Each of these four processes attempts to
install their route toward 10.168.24.0/24
into the routing table
Example
• The routing processes are each assigned an
administrative distance, which is used to
decide which route to install.
Example
• Since the internal EIGRP route has the best
administrative distance, it's installed in the
routing table
Backup Routes
• What do the other protocols? RIP, IGRP,
and OSPF? do with the routes that weren't
installed?
Backup Routes
• What do the other protocols? RIP, IGRP,
and OSPF? do with the routes that weren't
installed?
• What if the most preferred route, learned
from EIGRP, fails?
Backup Routes
• Cisco IOS® Software uses two approaches
to solve this problem:
Backup Routes
•
Cisco IOS® Software uses two approaches
to solve this problem:
1. The first is to have each routing process
attempt to install its best routes
periodically
Backup Routes
•
Cisco IOS® Software uses two approaches
to solve this problem:
1. The first is to have each routing process
attempt to install its best routes
periodically ( RIP, IGRP )
2. The second method is to store the route in
a database and attempt to install it if the
primary route fails. ( EIGRP, OSPF )
Prefix Lengths
• Let's look at another scenario to see how the
router handles another common situation?
varying prefix lengths.
Prefix Lengths
Assume, again, that a router has four routing
processes running on it, and each process
has received these routes:
• EIGRP (internal): 10.168.32.0/26
• RIP:
10.168.32.0/24
• OSPF:
10.168.32.0/19
Prefix Lengths
• Which of these routes will be installed in
the routing table?
• EIGRP (internal): 10.168.32.0/26
• RIP:
10.168.32.0/24
• OSPF:
10.168.32.0/19
Prefix Lengths
• Since EIGRP internal routes have the best
administrative distance, it's tempting to
assume the first one will be installed.
Prefix Lengths
• Since EIGRP internal routes have the best
administrative distance, it's tempting to
assume the first one will be installed.
• However, since each of these routes has a
different prefix length (subnet mask),
they're considered different destinations,
and they will all be installed in the routing
table.
Making Forwarding Decisions
• Which routing table entry show I use to get
to the required destination ?
Making Forwarding Decisions
Let's look at the three routes we just installed
in the routing table, and see how they look
on the router.
....
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
Identifies which Process installed the route.
D = EIGRP, R = RIP, O = OSPF
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
Identifies the Admin
Distance.
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
Identifies the next HOP IP address or
Interface to Forward the packet to.
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
If a packet arrives on a router interface
destined for 10.168.32.1, which route would
the router choose?
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
It depends on the prefix length, or the number of bits set
in the subnet mask.
Longer prefixes are always preferred over shorter ones
when forwarding a packet.
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
It depends on the prefix length, or the number of bits set
in the subnet mask.
Longer prefixes are always preferred over shorter ones
when forwarding a packet.
Making Forwarding Decisions
Pod1_1603#show ip route
D 10.168.32.0/26 [90/25789217] via 10.1.1.1
R 10.168.32.0/24 [120/4] via 10.1.1.2
O 10.168.32.0/19 [110/229840] via 10.1.1.3
Before I tell you which route is preferred lets
break down each route to see what it is
describing !
Routing table entries
•
•
•
•
•
•
•
D 10.168.32.0/26
/26 = 255.255.255.192
256 – 192 = 64
So networks are 0, 64, 128, 192
This route is describing Subnetwork 0
10.168.32.0/26
IP host addresses covered = ( 1 thru 63 )
Routing table entries
•
•
•
•
•
•
R 10.168.32.0/24
/24 = 255.255.255.0
256 – 255 = 1
So networks are 0,1, 2, 3, 4 in 3rd Octet
This route is describing Network 10.168.32.0
IP host addresses covered = ( 1 thru 255 )
Routing table entries
•
•
•
•
•
•
O 10.168.32.0/19
/19 = 255.255.224.0
256 – 224 = 32
So networks are 0,32,64,96,etc.. in 3rd Octet
This route is describing Network 10.168.32.0
IP host addresses covered = ( 1 thru 255 )
Back to our Example
• If a packet arrives on a router interface
destined for 10.168.32.1, which route would
the router choose?
• /26 IP host addresses covered = ( 1 thru 63 )
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
Back to our Example
• If a packet arrives on a router interface
destined for 10.168.32.1, which route would
the router choose?
• /26 IP host addresses covered = ( 1 thru 63 )
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
• This is for Host 1 on network 10.168.32.0
Back to our Example
• If a packet arrives on a router interface destined
for 10.168.32.1, which route would the router
choose?
• /26 IP host addresses covered = ( 1 thru 63 )
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
• This is for Host 1 on network 10.168.32.0
• All entries cover this host, so the longest Prefix is
used.
Back to our Example
• If a packet arrives on a router interface
destined for 10.168.32.100, which route
would the router choose?
• /26 IP host addresses covered = ( 1 thru 63 )
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
• This is for Host 100 on network 10.168.32.0
Back to our Example
• If a packet arrives on a router interface destined
for 10.168.32.100, which route would the router
choose?
• /26 IP host addresses covered = ( 1 thru 63 )
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
• This is for Host 100 on network 10.168.32.0
• /26 Does not cover this host address
• ( 10.168.32.64 / 26 now this does !!!!! But that’s not in
the table )
Back to our Example
• If a packet arrives on a router interface destined
for 10.168.32.100, which route would the router
choose?
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
• This is for Host 100 on network 10.168.32.0
• Both cover host 100
• So longest Prefix is used = /24
Back to our Example
• If a packet arrives on a router interface destined
for 10.168.32.100, which route would the router
choose?
• /24 IP host addresses covered = ( 1 thru 255 )
• /19 IP host addresses covered = ( 1 thru 255 )
• This is for Host 100 on network 10.168.32.0
• Both cover host 100
• So longest Prefix is used = /24
Conclusion
• These examples are to make you think and
to examine just what the routing table entry
is describing.
Conclusion
• Its tempting to just go for the Longest prefix
Conclusion
• That’s OK if the longest prefix describes
your host !!!!
IP Classless
• Where the ip classless configuration
command falls within the routing and
forwarding processes is often confusing.
IP Classless
• In reality, IP classless only affects the
operation of the forwarding processes in
IOS; it doesn't affect the way the routing
table is built.
IP Classless
• If IP classless isn't configured (using the no
ip classless command), the router won't
forward packets to subnets it does not
specifically know about !!!!
• As an example, let's again place three routes
in the routing table and route packets
through the router
IP Classless
router#show ip route....
172.30.0.0/16 is variably subnetted, 2 subnets, 2
masks
D 172.30.32.0/20 [90/4879540]
via
10.1.1.2
D 172.30.32.0/24
[90/25789217]
via
10.1.1.1
S* 0.0.0.0/0
[1/0]
via 10.1.1.3
IP Classless
• Remembering that the
172.30.32.0/24 network = Addresses
172.30.32.1 through 172.30.32.255
( /24 = Network bits = 255.255.255.0 )
( Networks increment in 1’s )
( As such just one network is described )
IP Classless
• Remembering that the
172.30.32.0/24 network = Addresses
172.30.32.1 through 172.30.32.255
( /24 = Network bits = 255.255.255.0 )
( Networks increment in 1’s )
( As such just one network is described )
IP Classless
• Remembering that the
172.30.32.0/24 network = Addresses
172.30.32.0 through 172.30.32.255
( /24 = Network bits = 255.255.255.0 )
( Networks increment in 1’s )
( As such just one network is described )
IP Classless
• and the 172.30.32.0/20 network includes the
addresses
• 172.30.32.0 through 172.30.47.255
•
•
•
•
( /20 = 255.255.240.0 )
( Networks increment in 16’s )
Networks 0,16,32,48,64 etc…..
This mask therefore describes networks 32 - 47
NO IP Classless
• Finally the routing table entry 0.0.0.0/0
• Is the DEFAULT route to take
• However the NO IP CLASSLESS command
affects how this works.
NO IP Classless
• You only take the Default route for a subnet
if your table knows of NO other subnet for
the CLASSFULL MAJOR NETWORK
NO IP Classless
• We can try switching three packets through
this routing table and see what the results
are.
• D 172.30.32.0/20
10.1.1.2
• D 172.30.32.0/24
• S* 0.0.0.0/0
[90/4879540]
via
[90/25789217]via 10.1.1.1
[1/0]
via 10.1.1.3
NO IP Classless
• A packet destined to 172.30.32.1 is
forwarded to 10.1.1.1, since this is the
longest prefix match.
• D 172.30.32.0/20
10.1.1.2
• D 172.30.32.0/24
• S* 0.0.0.0/0
[90/4879540]
via
[90/25789217]via 10.1.1.1
[1/0]
via 10.1.1.3
NO IP Classless
• A packet destined to 172.30.33.1 is
forwarded to 10.1.1.2, since this is the
longest prefix match.
• D 172.30.32.0/20
10.1.1.2
• D 172.30.32.0/24
• S* 0.0.0.0/0
[90/4879540]
via
[90/25789217]via 10.1.1.1
[1/0]
via 10.1.1.3
NO IP Classless
• A packet destined to 10.168.10.1 is
forwarded to 10.1.1.3; since this network
doesn't exist in the routing table, this packet
is forwarded to the default route.
• D 172.30.32.0/20
10.1.1.2
• D 172.30.32.0/24
• S* 0.0.0.0/0
[90/4879540]
via
[90/25789217]via 10.1.1.1
[1/0]
via 10.1.1.3
NO IP Classless
• A packet destined to 172.30.254.1 is
dropped.
• D 172.30.32.0/20
10.1.1.2
• D 172.30.32.0/24
• S* 0.0.0.0/0
[90/4879540]
via
[90/25789217]via 10.1.1.1
[1/0]
via 10.1.1.3
NO IP Classless
• A packet destined to 172.30.254.1 is
dropped.
• Because No table entry described this route
• IP CLASSLESS will not allow the default
route to be taken, because we know some
routes to the MAJOR network 172.30.0.0
• So its dropped !!
No IP Classless
No IP Classless
Blocks access to any 10.x.x.x subnet in the cloud but allows
internet access to any other Major Network / Subnetwork.
Lets Break and then attempt LAB 4