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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.
• If two routing processes try to install the same route into the table one will be rejected.
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:
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
10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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
10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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
10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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
10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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 10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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 10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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
10.1.1.1R 10.168.32.0/24 [120/4] via 10.1.1.2O 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.
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 masksD 172.30.32.0/20 [90/4879540] via
10.1.1.2D 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 [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
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 [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
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 [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
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 [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
NO IP Classless
• A packet destined to 172.30.254.1 is dropped.
• 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
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 !!
Blocks access to any 10.x.x.x subnet in the cloud but allows
internet access to any other Major Network / Subnetwork.
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