When you're preparing to pass the BSCI exam on the way to the coveted Cisco CCNP certification, you can be quickly overwhelmed by the sheer amount of BGP and OSPF knowledge you must demonstrate a mastery of. One set of details that some BSCI and CCNP candidates underestimate are the differences between the OSPF router types.
An OSPF Internal router has one rule - it must have all its interfaces in a single area. It does not mean that area has to be Area 0.
An OSPF Backbone router is a router with at least a single area in the OSPF backbone area, Area 0. A router can be both an Internal and Backbone router if all its interfaces are in Area 0.
An Area Border Router has at least one interface in Area 0 and another interface in a non-backbone area. ABRs are also one of two router types that can perform OSPF route summarization. (To advertise a summary route from one OSPF area to another, use the area range command on the ABR.)
Finally, an ASBR is an OSPF router that is performing route redistribution by injecting routes from another source into the OSPF domain. This is the other OSPF router type that can perform route summarization; to summarize routes being redistributed into OSPF, use the summary-address command on the ASBR.
There are several commands you can use to determine the router types in a given OSPF area. The command "show ip ospf" will display quite a bit of information regarding the local router, and this includes whether that router is acting as an ABR or ASBR. To see the routes to the ABRs and ASBRs from the local router, run "show ip ospf border-routers".
Wednesday, September 15, 2010
Tuesday, September 14, 2010
Cisco CCNP / BSCI Exam Tutorial: ISIS Router Types
To pass the BSCI exam and earn your CCNP, you've got to know ISIS inside and out. There are many similarities between ISIS and OSPF, but one major difference is that ISIS has three different types of routers - Level 1 (L1), Level 2 (L2), and L1/L2.
L1 routers are contained in a single area, and are connected to other areas by an L1/L2 router. The L1 uses the L1/L2 router as a default gateway to reach destinations contained in other areas, much like an OSPF stub router uses the ABR as a default gateway.
L1 routers have no specific routing table entries regarding any destination outside their own area; they will use an L1/L2 router as a default gateway to reach any external networks. ISIS L1 routers in the same area must synchronize their databases with each other.
Just as we have L1 routers, we also have L2 routers. Anytime we're routing between areas (inter-area routing), an L2 or L1/L2 router must be involved. All L2 routers will have synchronized databases as well.
Both L1 and L2 routers send out their own hellos. As with OSPF, hello packets allow ISIS routers to form adjacencies. The key difference here is that L1 routers send out L1 hellos, and L2 routers send out L2 hellos. If you have an L1 router and an L2 router on the same link, they will not form an adjacency.
An ISIS router can act as an L1 and an L2 router at the same time; these routers are L1/L2 routers. An L1/L2 router can have neighbors in separate ISIS areas. The L1/L2 router will have two separate databases, though - one for L1 routes and another for L2 routes. L1/L2 is the default setting for Cisco routers running ISIS. The L1/L2 router is the router that makes it possible for an L1 router to send data to another area.
In the next part of my ISIS tutorial, we'll take a more detailed look at those ISIS hellos!
L1 routers are contained in a single area, and are connected to other areas by an L1/L2 router. The L1 uses the L1/L2 router as a default gateway to reach destinations contained in other areas, much like an OSPF stub router uses the ABR as a default gateway.
L1 routers have no specific routing table entries regarding any destination outside their own area; they will use an L1/L2 router as a default gateway to reach any external networks. ISIS L1 routers in the same area must synchronize their databases with each other.
Just as we have L1 routers, we also have L2 routers. Anytime we're routing between areas (inter-area routing), an L2 or L1/L2 router must be involved. All L2 routers will have synchronized databases as well.
Both L1 and L2 routers send out their own hellos. As with OSPF, hello packets allow ISIS routers to form adjacencies. The key difference here is that L1 routers send out L1 hellos, and L2 routers send out L2 hellos. If you have an L1 router and an L2 router on the same link, they will not form an adjacency.
An ISIS router can act as an L1 and an L2 router at the same time; these routers are L1/L2 routers. An L1/L2 router can have neighbors in separate ISIS areas. The L1/L2 router will have two separate databases, though - one for L1 routes and another for L2 routes. L1/L2 is the default setting for Cisco routers running ISIS. The L1/L2 router is the router that makes it possible for an L1 router to send data to another area.
In the next part of my ISIS tutorial, we'll take a more detailed look at those ISIS hellos!
Cisco CCNP / BCMSN Exam Tutorial: Static VLANs
BCMSN exam success and earning your CCNP certification requires you to add to your knowledge of VLAN configuration. When you studied for your CCNA exam, you learned how to place ports into a VLAN and what the purpose of VLANs was, but you may not be aware that there are two types of VLAN membership. To pass the BCMSN exam, you must know the details of both types.
In this tutorial, we'll take a look at the VLAN type you are most familiar with, the "static VLAN". As you know, VLANs are a great way to create smaller broadcast domains in your network. Host devices connected to a port belonging to one VLAN will receive broadcasts and multicasts only if they were originated by another host in that same VLAN. The drawback is that without the help of a Layer 3 switch or a router, inter-VLAN communication cannot occur.
The actual configuration of a static VLAN is simple enough. In this example, by placing switch ports 0/1 and 0/2 into VLAN 12, the only broadcasts and multicasts hosts connected to those ports will receive are the ones transmitted by ports in VLAN 12.
SW1(config)#int fast 0/1
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 12
% Access VLAN does not exist. Creating vlan 12
SW1(config-if)#int fast 0/2
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 12
One of the many things I love about Cisco switches and routers is that if you have forgotten to do something, the Cisco device is generally going to remind you or in this case actually do it for you. I placed port 0/1 into a VLAN that did not yet exist, so the switch created it for me!
There are two commands needed to place a port into a VLAN. By default, these ports are running in dynamic desirable trunking mode, meaning that the port is actively attempting to form a trunk with a remote switch in order to send traffic between the two switches. The problem is that a trunk port belongs to all VLANs by default, and we want to put this port into a single VLAN only. To do so, we run the switchport mode access command to make the port an access port, and access ports belong to one and only one VLAN. After doing that, we placed the port into VLAN 12 with the switchport access vlan 12 command. Running the switchport mode access command effectively turns trunking off on that port.
The hosts are unaware of VLANs; they simply assume the VLAN membership of the port they're connected to. But that's not quite the case with dynamic VLANs, which we'll examine in the next part of this BCMSN tutorial.
In this tutorial, we'll take a look at the VLAN type you are most familiar with, the "static VLAN". As you know, VLANs are a great way to create smaller broadcast domains in your network. Host devices connected to a port belonging to one VLAN will receive broadcasts and multicasts only if they were originated by another host in that same VLAN. The drawback is that without the help of a Layer 3 switch or a router, inter-VLAN communication cannot occur.
The actual configuration of a static VLAN is simple enough. In this example, by placing switch ports 0/1 and 0/2 into VLAN 12, the only broadcasts and multicasts hosts connected to those ports will receive are the ones transmitted by ports in VLAN 12.
SW1(config)#int fast 0/1
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 12
% Access VLAN does not exist. Creating vlan 12
SW1(config-if)#int fast 0/2
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 12
One of the many things I love about Cisco switches and routers is that if you have forgotten to do something, the Cisco device is generally going to remind you or in this case actually do it for you. I placed port 0/1 into a VLAN that did not yet exist, so the switch created it for me!
There are two commands needed to place a port into a VLAN. By default, these ports are running in dynamic desirable trunking mode, meaning that the port is actively attempting to form a trunk with a remote switch in order to send traffic between the two switches. The problem is that a trunk port belongs to all VLANs by default, and we want to put this port into a single VLAN only. To do so, we run the switchport mode access command to make the port an access port, and access ports belong to one and only one VLAN. After doing that, we placed the port into VLAN 12 with the switchport access vlan 12 command. Running the switchport mode access command effectively turns trunking off on that port.
The hosts are unaware of VLANs; they simply assume the VLAN membership of the port they're connected to. But that's not quite the case with dynamic VLANs, which we'll examine in the next part of this BCMSN tutorial.
Cisco CCNA Exam Tutorial: Route Summarization
Preparing to pass the CCNA exam and earn this important Cisco certification? Route summarization is just one of the many skills you'll have to master in order to earn your CCNA. Whether it's RIP version 2, OSPF, or EIGRP, the CCNA exam will demand that you can flawlessly configure route summarization.
Route summarization isn't just important for the CCNA exam. It's a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately - what I like to call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is binary math; more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we'll only illustrate how to convert the first and second octet from decimal to binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren't used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a "1" for every bit to the left of the line we drew previously, and a "0" for every bit to the right. That gives us the following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the CCNA exam, emphasis is put on knowing how to advertise these summary routes in RIPv2 and EIGRP. For both of these protocols, route summarization happens at the interface level - it's not configured under the protocol. On the interface that should advertise the summary route, use the command "ip summary-address". Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command must specify the AS number - that's what the "100" is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there's no additional value needed in the configuration.
For OSPF, the commands differ. If you're configuring inter-area route summarization, use the "area range" command; if you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR. Neither of these are interface-level commands.
I speak from experience when I tell you that practice makes perfect on the CCNA exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions - you either know how to do it or you don't. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNA. Here's to your success!
Route summarization isn't just important for the CCNA exam. It's a valuable skill to have in the real world as well. Correctly summarizing routes can lead to smaller routing tables that are still able to route packets accurately - what I like to call "concise and complete" routing tables.
The first skill you've got to have in order to work with route summarization is binary math; more specifically, you must be able to take multiple routes and come up with both a summary route and mask to advertise to downstream routers. Given the networks 100.16.0.0 /16, 100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16, could you quickly come up with both the summary address and mask? All you need to do is break the four network numbers down into binary strings. We know the last two octets will all convert to the binary string 00000000, so in this article we'll only illustrate how to convert the first and second octet from decimal to binary.
100 16 = 01100100 00010000
100 17 = 01100100 00010001
100 18 = 01100100 00010010
100 19 = 01100100 00010011
To come up with the summary route, just work from left to right and draw a line where the four networks no longer have a bit in common. For these four networks, that point comes between the 14th and 15th bits. This leaves us with this string: 01100100 000100xx. All you need to do is convert that string back to decimal, which gives us 100 for the first octet and 16 for the second. (The two x values are bits on the right side of the line, which aren't used in calculating the summary route.) Since we know that zero is the value for the last two octets, the resulting summary network number is 100.16.0.0.
But we're not done! We now have to come up with the summary mask to advertise along with the summary route. To arrive at the summary route, write out a mask in binary with a "1" for every bit to the left of the line we drew previously, and a "0" for every bit to the right. That gives us the following string:
11111111 11111100 00000000 00000000
Converting that to dotted decimal, we arrive at the summary mask 255.252.0.0. The correct summary network and mask to advertise are 100.16.0.0 252.0.0.0.
For the CCNA exam, emphasis is put on knowing how to advertise these summary routes in RIPv2 and EIGRP. For both of these protocols, route summarization happens at the interface level - it's not configured under the protocol. On the interface that should advertise the summary route, use the command "ip summary-address". Here are examples of how the above summary route would be configured on ethernet0 in both RIPv2 and EIGRP.
R1(config-if)#ip summary-address rip 100.16.0.0 255.252.0.0
R1(config-if)#ip summary-address eigrp 100 100.16.0.0 255.252.0.0
The main difference between the two is that the EIGRP command must specify the AS number - that's what the "100" is in the middle of the EIGRP command. Since RIPv2 does not use AS numbers, there's no additional value needed in the configuration.
For OSPF, the commands differ. If you're configuring inter-area route summarization, use the "area range" command; if you are summarizing routes that are being redistributed into OSPF, use the summary-address command under the OSPF routing process on the ASBR. Neither of these are interface-level commands.
I speak from experience when I tell you that practice makes perfect on the CCNA exam, especially with binary and summarization questions. The great thing about these questions is that there are no grey areas with these questions - you either know how to do it or you don't. And with practice and an eye for detail, you can master these skills, pass the exam, and become a CCNA. Here's to your success!
Cisco CCNA Exam Tutorial: Loopback Interfaces
As a CCNA candidate, you most likely have some background in PC hardware and workstation support. If so, you're already familiar with loopback interfaces, particularly 127.0.0.1, the loopback address assigned to a PC.
When you're learning all about the different physical interfaces for your CCNA exam - serial, ethernet, and BRI, among others - there's one logical interface you need to know about, and that is - you guessed it! - the loopback interface.
What isn't as immediately apparent is why we use loopback interfaces on routers and switches to begin with. Many of the Cisco router features that can use loopbacks are intermediate and advanced features that you'll learn about in your CCNP and CCIE studies, but these features all come back to one basic concept: If the loopback interface on a router is down, that means the router is unavailable as a whole.
In contrast, a physical interface being down does not mean the router itself is out of commission. A router's ethernet port can go down, but the other physical interfaces on that router are still operational. Since a loopback interface is logical, there's nothing physical that can go wrong with it.
As I mentioned, you'll learn different Cisco router and switch features that utilize loopback interfaces as you climb the Cisco certification ladder. There's one misconception about Cisco loopback interfaces that you want to get clear on now, though. You’re probably familiar with loopback interfaces on a PC, and may even know that the address range 127.0.0.0 is reserved for loopback addressing.
Note that this reserved address range does not apply to loopbacks on Cisco devices, however. If you attempt to assign an address from this range to a Cisco loopback interface, you get this result:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface loopback0
R1(config-if)#ip address 127.0.0.2 255.255.255.0
Not a valid host address - 127.0.0.2
R1(config-if)#ip address 127.1.1.1 255.255.255.0
Not a valid host address - 127.1.1.1
The range 127.0.0.0 is reserved for host loopbacks (such as PCs), not routers or switches. The most commonly used address from this range is 127.0.0.1 – if you can’t ping that on a workstation, that means you can’t ping yourself, which means there’s a problem with the TCP/IP install itself.
Keep these details in mind on the exam and in the workplace, and you’re on your way to CCNA exam success!
When you're learning all about the different physical interfaces for your CCNA exam - serial, ethernet, and BRI, among others - there's one logical interface you need to know about, and that is - you guessed it! - the loopback interface.
What isn't as immediately apparent is why we use loopback interfaces on routers and switches to begin with. Many of the Cisco router features that can use loopbacks are intermediate and advanced features that you'll learn about in your CCNP and CCIE studies, but these features all come back to one basic concept: If the loopback interface on a router is down, that means the router is unavailable as a whole.
In contrast, a physical interface being down does not mean the router itself is out of commission. A router's ethernet port can go down, but the other physical interfaces on that router are still operational. Since a loopback interface is logical, there's nothing physical that can go wrong with it.
As I mentioned, you'll learn different Cisco router and switch features that utilize loopback interfaces as you climb the Cisco certification ladder. There's one misconception about Cisco loopback interfaces that you want to get clear on now, though. You’re probably familiar with loopback interfaces on a PC, and may even know that the address range 127.0.0.0 is reserved for loopback addressing.
Note that this reserved address range does not apply to loopbacks on Cisco devices, however. If you attempt to assign an address from this range to a Cisco loopback interface, you get this result:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface loopback0
R1(config-if)#ip address 127.0.0.2 255.255.255.0
Not a valid host address - 127.0.0.2
R1(config-if)#ip address 127.1.1.1 255.255.255.0
Not a valid host address - 127.1.1.1
The range 127.0.0.0 is reserved for host loopbacks (such as PCs), not routers or switches. The most commonly used address from this range is 127.0.0.1 – if you can’t ping that on a workstation, that means you can’t ping yourself, which means there’s a problem with the TCP/IP install itself.
Keep these details in mind on the exam and in the workplace, and you’re on your way to CCNA exam success!
Cisco CCNA Certification Exam Tutorial: The OSPF RID
OSPF is a major topic on your CCNA exam, as well it should be. OSPF is a widely-used WAN protocol, and you need to learn the fundamentals before moving on to more complicated configurations. One such detail is the OSPF Router ID, or RID.
The RID is the dotted decimal value by which other OSPF routers will identify a given OSPF router. There are some interesting defaults for this value, and a command you should know to hardcode the RID. You had also better know what has to happen for this command to take effect, so let's take a more detailed look at the OSPF RID.
In this example, R1 has an adjacency with R2 and R3 over the 172.12.123.0/24 frame network. R1 is the hub, with R2 and R3 as the spokes. No other interfaces are OSPF-enabled on any of the routers. Running show ip ospf neighbor on R1, we see some unusual values under "Neighbor ID", which is another name for the OSPF RID.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/DROTHER 00:01:57 172.12.123.3 Serial0
2.2.2.2 0 FULL/DROTHER 00:01:57 172.12.123.2 Serial0
Notice the Neighbor ID of each remote address is the loopback address. How can that be if they’re not OSPF-enabled?
When determining the Router ID (RID) of an OSPF-enabled router, OSPF will always use the numerically highest IP address on the router’s loopback interfaces, regardless of whether that loopback is OSPF-enabled.
What if there is no loopback? OSPF will then use the numerically highest IP address of the physical interfaces, regardless of whether that interface is OSPF-enabled.
BOTTOM LINE: An interface does not have to be running OSPF to have its IP address used as the OSPF RID.
The OSPF RID can be changed, but it requires a restart or to reinitialize the OSPF routing process. Use the router-id command to change the default RID of each router as shown, and clear the OSPF process to do so.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#router-id 11.11.11.11
Reload or use "clear ip ospf process" command, for this to take effect
R1#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
After entering the router-id command, the router console informed you that you have to reload the router or reset the OSPF processes for this to take effect. You enter the clear ip ospf process command to do this. Notice that when you’re asked if you really want to do this, the prompt is “no”? That’s because all the OSPF adjacencies on this router will be lost and will have to begin the process again. That’s OK on a practice rack, not good in a production network. Don’t use that one at work.
The OSPF RID is not a complicated concept, but the fact that an interface doesn't have to be OSPF-enabled in order to have its IP address act as the RID takes some getting used to. And remember - when the router or switch asks you a question and the prompted answer is "no", take one step back and make sure you really want to do what you're about to do!
The RID is the dotted decimal value by which other OSPF routers will identify a given OSPF router. There are some interesting defaults for this value, and a command you should know to hardcode the RID. You had also better know what has to happen for this command to take effect, so let's take a more detailed look at the OSPF RID.
In this example, R1 has an adjacency with R2 and R3 over the 172.12.123.0/24 frame network. R1 is the hub, with R2 and R3 as the spokes. No other interfaces are OSPF-enabled on any of the routers. Running show ip ospf neighbor on R1, we see some unusual values under "Neighbor ID", which is another name for the OSPF RID.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/DROTHER 00:01:57 172.12.123.3 Serial0
2.2.2.2 0 FULL/DROTHER 00:01:57 172.12.123.2 Serial0
Notice the Neighbor ID of each remote address is the loopback address. How can that be if they’re not OSPF-enabled?
When determining the Router ID (RID) of an OSPF-enabled router, OSPF will always use the numerically highest IP address on the router’s loopback interfaces, regardless of whether that loopback is OSPF-enabled.
What if there is no loopback? OSPF will then use the numerically highest IP address of the physical interfaces, regardless of whether that interface is OSPF-enabled.
BOTTOM LINE: An interface does not have to be running OSPF to have its IP address used as the OSPF RID.
The OSPF RID can be changed, but it requires a restart or to reinitialize the OSPF routing process. Use the router-id command to change the default RID of each router as shown, and clear the OSPF process to do so.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#router-id 11.11.11.11
Reload or use "clear ip ospf process" command, for this to take effect
R1#clear ip ospf process
Reset ALL OSPF processes? [no]: yes
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
1d05h: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
After entering the router-id command, the router console informed you that you have to reload the router or reset the OSPF processes for this to take effect. You enter the clear ip ospf process command to do this. Notice that when you’re asked if you really want to do this, the prompt is “no”? That’s because all the OSPF adjacencies on this router will be lost and will have to begin the process again. That’s OK on a practice rack, not good in a production network. Don’t use that one at work.
The OSPF RID is not a complicated concept, but the fact that an interface doesn't have to be OSPF-enabled in order to have its IP address act as the RID takes some getting used to. And remember - when the router or switch asks you a question and the prompted answer is "no", take one step back and make sure you really want to do what you're about to do!
Cisco CCNA Certification: Static Routing Tutorial
In studying for your CCNA exam and preparing to earn this valuable certification, you may be tempted to spend little time studying static routing and head right for the more exciting dynamic routing protocols like RIP, EIGRP, and OSPF. This is an understandable mistake, but still a mistake. Static routing is not complicated, but it's an important topic on the CCNA exam and a valuable skill for real-world networking.
To create static routes on a Cisco router, you use the ip route command followed by the destination network, network mask, and either the next-hop IP address or the local exit interface. It's vital to keep that last part in mind - you're either configuring the IP address of the downstream router, or the interface on the local router that will serve as the exit interface.
Let's say your local router has a serial0 interface with an IP address of 200.1.1.1/30, and the downstream router that will be the next hop will receive packets on its serial1 interface with an IP address of 200.1.1.2/30. The static route will be for packets destined for the 172.10.1.0 network. Either of the following ip route statements would be correct.
R1(config)#ip route 172.10.1.0 255.255.255.0 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 172.10.1.0 255.255.255.0 serial0 ( local exit interface)
You can also write a static route that matches only one destination. This is a host route, and has 255.255.255.255 for a mask. If the above static routes should only be used to send packets to 172.10.1.1., the following commands would do the job.
R1(config)#ip route 172.10.1.1 255.255.255.255 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 172.10.1.1 255.255.255.255 serial0 ( local exit interface)
Finally, a default static route serves as a gateway of last resort. If there are no matches for a destination in the routing table, the default route will be used. Default routes use all zeroes for both the destination and mask, and again a next-hop IP address or local exit interface can be used.
R1(config)#ip route 0.0.0.0 0.0.0.0 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 0.0.0.0 0.0.0.0 serial0 ( local exit interface)
IP route statements seem simple enough, but the details regarding the next-hop IP address, the local exit interface, default static routes, and the syntax of the command are vital for success on CCNA exam day and in the real world.
To create static routes on a Cisco router, you use the ip route command followed by the destination network, network mask, and either the next-hop IP address or the local exit interface. It's vital to keep that last part in mind - you're either configuring the IP address of the downstream router, or the interface on the local router that will serve as the exit interface.
Let's say your local router has a serial0 interface with an IP address of 200.1.1.1/30, and the downstream router that will be the next hop will receive packets on its serial1 interface with an IP address of 200.1.1.2/30. The static route will be for packets destined for the 172.10.1.0 network. Either of the following ip route statements would be correct.
R1(config)#ip route 172.10.1.0 255.255.255.0 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 172.10.1.0 255.255.255.0 serial0 ( local exit interface)
You can also write a static route that matches only one destination. This is a host route, and has 255.255.255.255 for a mask. If the above static routes should only be used to send packets to 172.10.1.1., the following commands would do the job.
R1(config)#ip route 172.10.1.1 255.255.255.255 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 172.10.1.1 255.255.255.255 serial0 ( local exit interface)
Finally, a default static route serves as a gateway of last resort. If there are no matches for a destination in the routing table, the default route will be used. Default routes use all zeroes for both the destination and mask, and again a next-hop IP address or local exit interface can be used.
R1(config)#ip route 0.0.0.0 0.0.0.0 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 0.0.0.0 0.0.0.0 serial0 ( local exit interface)
IP route statements seem simple enough, but the details regarding the next-hop IP address, the local exit interface, default static routes, and the syntax of the command are vital for success on CCNA exam day and in the real world.
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