CCNA exam success depends on mastering many technologies that are new to you, and few exam topics have more details than ISDN. ISDN isn't just for your CCNA exam studies, though. While ISDN is dismissed by many, the fact is that there are many small and mid-size networks out there that use ISDN as their backup to frame relay. Some of these companies have spoke networks that use ISDN to connect to their hub as well, so it's a great idea to know ISDN configuration and troubleshooting for your real-world career as well as passing the CCNA. With that in mind, let's take a look at five common ISDN errors and how to avoid them.
With dialer map statements, remember that the phone number you put in the dialer map is the phone number of the remote router, not the local one. Look at it this way - if you want to call a friend on your cell, you don't pick up your cell and dial your own number!
Speaking of dialer map statements, don't forget the all-important broadcast option at the end of the command:
R1(config-if)#dialer map ip 172.12.21.1 name R2 broadcast 5555555
The router will accept that command without the "broadcast" option, but routing protocol updates and hellos would not be able to travel across the line. (This command is also needed in frame relay map statements to allow broadcasts and multicasts to be transmitted.)
PAP is PPP's clear-text authentication scheme, and clear text is a really bad idea. But if you do have to configure it, don't forget that PAP requires additional configuration -the ppp pap sent-username command.
R1(config-if)#ppp pap sent-username R1 password CISCO
Must set encapsulation to PPP before using PPP subcommands
R1(config-if)#
The error message we got while configuring the sent-username command is another important reminder - by default, a BRI line is running HDLC, not PPP. Since HDLC doesn't allow us to use either PAP or CHAP, we'll need to set the link to PPP with the encapsulation ppp command.
R1(config-if)#encapsulation ppp
R1(config-if)#ppp authentication pap
R1(config-if)#ppp pap sent-username R1 password CISCO
But before we configure any of this information, we should configure the ISDN switch-type. Why? Because without the switch-type configuration, it doesn't matter that we avoid the other four errors - the line will not come up. Configure the switch-type with the "isdn switch-type" command, and then verify it with "show isdn status".
R1(config)#isdn switch-type basic-ni
R1#show isdn status
Global ISDN Switchtype = basic-ni (output of this command cut here for clarity)
If you forget this part of the configuration, the output of show isdn status wastes no time in reminding you!
R1#show isdn status
**** No Global ISDN Switchtype currently defined ****
ISDN is an important part of your CCNA studies, and this knowledge still comes in handy in production networks as well. Keep studying, notice the details, run those debugs, and you'll be a CCNA before you know it!
Showing posts with label bri. Show all posts
Showing posts with label bri. Show all posts
Thursday, December 25, 2008
Wednesday, December 24, 2008
Cisco CCNA Certification Exam Tutorial: Configuring Dialer Profiles
The most common method of configuring ISDN is with dialer maps, but dial information can also be configured on a logical interface. To pass the CCNA exam, you must know how to configure and troubleshoot both dialer maps and dialer profiles.
Dialer Profiles allow different dialing information to be configured onto logical interfaces. The logical interfaces may have different dialing destinations, different remote router names, etc., but they’ll be using the same physical interface.
Dialer strings are used on dialer profiles. Note that each logical interface has a different IP address, a different remote router to dial, and a different dialer string, but they will be using the same physical interface to dial out. The commands dialer pool and dialer pool-member are used to link the logical and physical interfaces. The number following each command must match for the logical interface to correctly bind to the physical interface.
R1(config)#interface dialer0
R1(config-if)#ip address 172.16.1.1 255.255.255.0
R1(config-if)#encapsulation ppp
<. The encapsulation type is placed on both the logical and physical interfaces. >
R1(config-if)#dialer remote-name Remote0
R1(config-if)#dialer pool 1
< places logical interface into dialer pool >
R1(config-if)#dialer string 5551212
< number dialed to contact router Remote0 >
R1(config-if)#dialer-group 1
< links logical interface to dialer-list 1 >
R1(config)#interface dialer1
R1(config-if)#ip address 172.16.1.2 255.255.255.0
R1(config-if)#encapsulation ppp
R1(config-if)#dialer remote-name Remote1
R1(config-if)#dialer pool 1
R1(config-if)#dialer string 5551234
R1(config-if)#dialer-group 1
R1(config)#interface bri0
R1(config-if)#no ip address
< With dialer profiles, IP addresses are assigned to logical interfaces. >
R1(config-if)#encapsulation ppp
< The encapsulation type is place on both the logical and physical interfaces.>
R1(config-if)#dialer pool-member 1
< The number associated with this command should match the number configured with the dialer pool number on the logical dialer interfaces. >
R1(config-if)#isdn spid1 0835866101
R1(config-if)#isdn spid2 0835866301
When configuring dialer profiles, the encapsulation type should be placed on both the physical BRI interface and the logical dialer interfaces. The SPIDs are configured on the physical interface as well.
Configuring dialer profiles can be a little tricky at first, and the best way to master this skill is to get real hands-on practice in your own CCNA / CCNP home lab or a rack rental service. Either way, hands-on is the best practice. Best of luck in your CCNA studies!
Dialer Profiles allow different dialing information to be configured onto logical interfaces. The logical interfaces may have different dialing destinations, different remote router names, etc., but they’ll be using the same physical interface.
Dialer strings are used on dialer profiles. Note that each logical interface has a different IP address, a different remote router to dial, and a different dialer string, but they will be using the same physical interface to dial out. The commands dialer pool and dialer pool-member are used to link the logical and physical interfaces. The number following each command must match for the logical interface to correctly bind to the physical interface.
R1(config)#interface dialer0
R1(config-if)#ip address 172.16.1.1 255.255.255.0
R1(config-if)#encapsulation ppp
<. The encapsulation type is placed on both the logical and physical interfaces. >
R1(config-if)#dialer remote-name Remote0
R1(config-if)#dialer pool 1
< places logical interface into dialer pool >
R1(config-if)#dialer string 5551212
< number dialed to contact router Remote0 >
R1(config-if)#dialer-group 1
< links logical interface to dialer-list 1 >
R1(config)#interface dialer1
R1(config-if)#ip address 172.16.1.2 255.255.255.0
R1(config-if)#encapsulation ppp
R1(config-if)#dialer remote-name Remote1
R1(config-if)#dialer pool 1
R1(config-if)#dialer string 5551234
R1(config-if)#dialer-group 1
R1(config)#interface bri0
R1(config-if)#no ip address
< With dialer profiles, IP addresses are assigned to logical interfaces. >
R1(config-if)#encapsulation ppp
< The encapsulation type is place on both the logical and physical interfaces.>
R1(config-if)#dialer pool-member 1
< The number associated with this command should match the number configured with the dialer pool number on the logical dialer interfaces. >
R1(config-if)#isdn spid1 0835866101
R1(config-if)#isdn spid2 0835866301
When configuring dialer profiles, the encapsulation type should be placed on both the physical BRI interface and the logical dialer interfaces. The SPIDs are configured on the physical interface as well.
Configuring dialer profiles can be a little tricky at first, and the best way to master this skill is to get real hands-on practice in your own CCNA / CCNP home lab or a rack rental service. Either way, hands-on is the best practice. Best of luck in your CCNA studies!
Cisco CCNA / CCNP Home Lab Tutorial: Using 2520 Routers
I know from experience that part of the excitement and anxiety of putting together your own CCNA / CCNP home lab is deciding what to buy! While you can make a workable home lab out of almost any combination of Cisco routers and switches, some routers are better suited for home lab work than others because they can fill multiple roles.
My personal favorite is the Cisco 2520. This router has four serial interfaces, making it an ideal frame relay switch. Don't forget that just because you're using a router as a frame switch, you can still use its routing capabilities. One setup I use is to use three of the four serial interfaces for frame switching and the fourth interface as a point-to-point network with another router. All you need is some DTE/DCE cables and you're all set.
The 2520 also comes with one ethernet interface and an ISDN interface, so that gives you even more options. Even if you're not planning to run ISDN in your home lab right now, you may choose to do so in the future - and with a 2520, you've already got the right router to do so. Keep in mind that if you are going to run ISDN in your home lab, you’ll need an ISDN device such as an ISDN simulator in your lab. (ISDN simulators are physical devices and are plentiful on ebay – they’re no relation to “router simulators”.)
Again, I want to reiterate that you can work any Cisco router into a CCNA / CCNP home lab - there's no "right" or "wrong" combination of equipment. But as with anything else, some combinations are better than others, so consider adding some 2520s to your home lab! This router gives you a great combination of interfaces and capabilities, plus the most important factor of all - real hands-on experience during your CCNA and CCNP exam preparation!
My personal favorite is the Cisco 2520. This router has four serial interfaces, making it an ideal frame relay switch. Don't forget that just because you're using a router as a frame switch, you can still use its routing capabilities. One setup I use is to use three of the four serial interfaces for frame switching and the fourth interface as a point-to-point network with another router. All you need is some DTE/DCE cables and you're all set.
The 2520 also comes with one ethernet interface and an ISDN interface, so that gives you even more options. Even if you're not planning to run ISDN in your home lab right now, you may choose to do so in the future - and with a 2520, you've already got the right router to do so. Keep in mind that if you are going to run ISDN in your home lab, you’ll need an ISDN device such as an ISDN simulator in your lab. (ISDN simulators are physical devices and are plentiful on ebay – they’re no relation to “router simulators”.)
Again, I want to reiterate that you can work any Cisco router into a CCNA / CCNP home lab - there's no "right" or "wrong" combination of equipment. But as with anything else, some combinations are better than others, so consider adding some 2520s to your home lab! This router gives you a great combination of interfaces and capabilities, plus the most important factor of all - real hands-on experience during your CCNA and CCNP exam preparation!
Cisco CCNA / CCNP Home Lab Tutorial: The 2503 Router
I know from experience that part of the excitement and anxiety of putting together your own CCNA / CCNP home lab is deciding what to buy! While you can make a workable home lab out of almost any combination of Cisco routers and switches, some routers are better suited for home lab work than others because they can fill multiple roles.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
Cisco CCNA / CCNP Home Lab Tutorial: The 2503 Router
I know from experience that part of the excitement and anxiety of putting together your own CCNA / CCNP home lab is deciding what to buy! While you can make a workable home lab out of almost any combination of Cisco routers and switches, some routers are better suited for home lab work than others because they can fill multiple roles.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
When you buy CCNA or CCNP "lab kits" - bundles of routers and switches - you can get a little confused about whether you're getting a good deal. One router I get asked about quite a bit is the 2503.
2503 routers are fantastic for CCNA and CCNP home labs. They come with two serial interfaces, allowing you to connect one interface directly to another router (you'll need a DTE/DCE cable for that, too) while connecting another to a frame relay switch if you like. If you don't have a frame relay switch, you can connect a 2503 directly to two other routers via the serial interfaces.
You also have an AUI port, which requires a transceiver to operate as your Ethernet interface. Transceivers are pretty cheap and readily available from Cisco resellers and ebay vendors, so pick one up for each 2503 you decide to buy.
2503 routers also come with a BRI interface. Even though you may not have an ISDN simulator right now, you may choose to add one later. That makes a 2503 a great bet for future lab expansion.
All in all, 2503 routers are great for your CCNA / CCNP home lab. They run about $125 each on ebay, or less, so they're also very affordable. There's no better training than training on your own CCNA or CCNP home lab, and you can always sell the equipment later if you like. Basically, you're renting the routers and switches, and the experience you get by working with the real deal is invaluable.
Cisco CCNA / CCNP Exam Tutorial: Testing ISDN Links Without Pings
To earn your Cisco CCNA and CCNP certifications, you've got to master ISDN - and despite what some people say, there's still a lot of ISDN out there that needs to be supported. And when it comes to troubleshooting ISDN, there's a lot to look at. Is the correct ISDN switchtype configured? Are the dialer map statements correct? What about the dialer-group and dialer-list commands? And that's just the start.
I always say that all troubleshooting starts at Layer 1, the Physical layer of the OSI model. The usual method of troubleshooting ISDN is sending pings across the link, but the connection can be tested without using pings or even before assigning IP addresses to the BRI interfaces!
It's a good idea to place these test calls before configuring the interfaces - that way, you know you've got a valid connection before beginning the configuration (and there's a lot of config to go along with ISDN!)
To place a test call without using pings, use the isdn call interface command.
R1#isdn call interface bri0 8358662
R1#
03:54:43: BR0 DDR: Attempting to dial 8358662
03:54:43: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
03:54:44: BR0:1 DDR: dialer protocol up
03:54:45: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up
03:54:49: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2
To tear the test call down correctly, use isdn disconnect interface. IOS Help displays the options with this command.
R1#isdn disconnect interface bri 0 ?
all Disconnect the data call(s) on all b channels
b1 Disconnect the data call on b1 channel
b2 Disconnect the data call on b2 channel
R1#isdn disconnect interface bri 0 all
03:58:36: BR0:1 DDR: disconnecting call
03:58:36: BR0:2 DDR: disconnecting call
03:58:36: %ISDN-6-DISCONNECT: Interface BRI0:1 disconnected from 8358662
R2, call lasted 20 seconds
03:58:36: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
03:58:36: BR0:1 DDR: disconnecting call
03:58:37: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to down
I say "correctly" because the one thing you don't want to do to end an ISDN call, test or otherwise, is just shut the interface. Telcos don't like it, and ISDN lab devices like it even less. Always let the d-channel do its work and tear the call down in an orderly fashion - don't just cut it off by shutting the interface down.
I always say that all troubleshooting starts at Layer 1, the Physical layer of the OSI model. The usual method of troubleshooting ISDN is sending pings across the link, but the connection can be tested without using pings or even before assigning IP addresses to the BRI interfaces!
It's a good idea to place these test calls before configuring the interfaces - that way, you know you've got a valid connection before beginning the configuration (and there's a lot of config to go along with ISDN!)
To place a test call without using pings, use the isdn call interface command.
R1#isdn call interface bri0 8358662
R1#
03:54:43: BR0 DDR: Attempting to dial 8358662
03:54:43: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up
03:54:44: BR0:1 DDR: dialer protocol up
03:54:45: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up
03:54:49: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2
To tear the test call down correctly, use isdn disconnect interface. IOS Help displays the options with this command.
R1#isdn disconnect interface bri 0 ?
all Disconnect the data call(s) on all b channels
b1 Disconnect the data call on b1 channel
b2 Disconnect the data call on b2 channel
R1#isdn disconnect interface bri 0 all
03:58:36: BR0:1 DDR: disconnecting call
03:58:36: BR0:2 DDR: disconnecting call
03:58:36: %ISDN-6-DISCONNECT: Interface BRI0:1 disconnected from 8358662
R2, call lasted 20 seconds
03:58:36: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down
03:58:36: BR0:1 DDR: disconnecting call
03:58:37: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to down
I say "correctly" because the one thing you don't want to do to end an ISDN call, test or otherwise, is just shut the interface. Telcos don't like it, and ISDN lab devices like it even less. Always let the d-channel do its work and tear the call down in an orderly fashion - don't just cut it off by shutting the interface down.
Tuesday, December 23, 2008
Cisco CCNA / CCNP Certification Exam Tutorial: ISDN And Multilink PPP
ISDN is a huge topic on both your Cisco CCNA and BCRAN CCNP exams. While many ISDN topics seem straightforward, it’s the details that make the difference in the exam room and working with ISDN in production networks. Configuring and troubleshooting multilink PPP is just one of the skills you’ll need to pass both of these demanding exams.
With BRI, we've got two B-channels to carry data, and both of them have a 64-kbps capacity. You might think it would be a good idea to have both channels in operation before one reaches capacity, and it is a great idea Problem is, it's not a default behavior of ISDN. The second b-channel will not begin to carry traffic until the first one reaches capacity.
With Multilink PPP (MLP), a bandwidth capacity can be set that will allow the second b-channel to bear data before the first channel reaches capacity. The configuration for MLP is simple, but often misconfigured. We'll use our good friend IOS Help to verify the measurement this command uses.
Enabling MLP is a three-step process:
Enable PPP on the link
Enable MLP with the command ppp multilink
Define the threshold at which the second b-channel should start carrying data with the dialer load-threshold command.
Let's say you wanted the second b-channel to start carrying data when the first channel reaches 75% of capacity. It would make sense that the command to do so would be dialer load-threshold 75... but it's not.
R1(config)#int bri0
R1(config-if)#ppp multilink
R1(config-if)#dialer load-threshold ?
<1-255> Load threshold to place another call
The dialer load-threshold value is based on 255, not 100. To have this command bring the line up at a certain percentage, multiply that percentage in decimal format by 255. Below, I multiplied 255 by .75 (75%) to arrive at 191.
R1(config-if)#dialer load-threshold 191 ?
either Threshold decision based on max of inbound and outbound traffic
inbound Threshold decision based on inbound traffic only
outbound Threshold decision based on outbound traffic only
R1(config-if)#dialer load-threshold 191 either
As illustrated by IOS Help in the above configuration, dialer load-threshold has additional options as well. You can configure the interface to consider only incoming, outgoing, or all traffic when calculating when to bring the next channel up.
Configuring Multilink PPP is just one of the skills you’ll need to earn your CCNA and pass the CCNP BCRAN exam. Don’t underestimate ISDN on Cisco’s certification exams!
With BRI, we've got two B-channels to carry data, and both of them have a 64-kbps capacity. You might think it would be a good idea to have both channels in operation before one reaches capacity, and it is a great idea Problem is, it's not a default behavior of ISDN. The second b-channel will not begin to carry traffic until the first one reaches capacity.
With Multilink PPP (MLP), a bandwidth capacity can be set that will allow the second b-channel to bear data before the first channel reaches capacity. The configuration for MLP is simple, but often misconfigured. We'll use our good friend IOS Help to verify the measurement this command uses.
Enabling MLP is a three-step process:
Enable PPP on the link
Enable MLP with the command ppp multilink
Define the threshold at which the second b-channel should start carrying data with the dialer load-threshold command.
Let's say you wanted the second b-channel to start carrying data when the first channel reaches 75% of capacity. It would make sense that the command to do so would be dialer load-threshold 75... but it's not.
R1(config)#int bri0
R1(config-if)#ppp multilink
R1(config-if)#dialer load-threshold ?
<1-255> Load threshold to place another call
The dialer load-threshold value is based on 255, not 100. To have this command bring the line up at a certain percentage, multiply that percentage in decimal format by 255. Below, I multiplied 255 by .75 (75%) to arrive at 191.
R1(config-if)#dialer load-threshold 191 ?
either Threshold decision based on max of inbound and outbound traffic
inbound Threshold decision based on inbound traffic only
outbound Threshold decision based on outbound traffic only
R1(config-if)#dialer load-threshold 191 either
As illustrated by IOS Help in the above configuration, dialer load-threshold has additional options as well. You can configure the interface to consider only incoming, outgoing, or all traffic when calculating when to bring the next channel up.
Configuring Multilink PPP is just one of the skills you’ll need to earn your CCNA and pass the CCNP BCRAN exam. Don’t underestimate ISDN on Cisco’s certification exams!
Monday, December 22, 2008
Cisco CCNA / CCNP Certification Exam Tutorial: Floating Static Routes
To pass the Cisco CCNA and CCNP certification exams, as well as becoming a world-class networker, you've got to know how and when to use floating static routes. And if you're wondering what makes them "float" -- read on!
In this example, R1 and R2 are running OSPF over a Frame Relay network, 172.12.123.0 /24. They're also connected by a BRI ISDN link, 172.12.12.0 /24. R1 is advertising a loopback network, 1.1.1.1 /32, via OSPF. We want R2 to have a route to that loopback even if the frame goes down - and here, we'll use a floating static route to make that happen.
R2 sees the route to the loopback interface via OSPF, and can ping that interface successfully.
R2#show ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, Serial0
R2#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms
This is when it's important to know your administrative distances.... or at least know where to look to see them! The AD of OSPF is 110, which means we can configure a static route to 1.1.1.1 /32, and as long as the AD of the static route is higher than 110, it won't be used unless the OSPF route leaves the routing table. That's why this kind of route is called a "floating" static route - the route "floats" in the routing table and isn't seen unless the primary route leaves the table.
You learned how to write a static route in your CCNA studies, but you also remember that the default AD of a static route is either 1 or 0... and both of those values are less than 110! To change the AD of a static route, configure the desired distance at the end of the ip route command.
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 ?
<1-255> Distance metric for this route
A.B.C.D Forwarding router's address
name Specify name of the next hop
permanent permanent route
tag Set tag for this route
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 111
The static route has an AD that's only one higher than that of the OSPF route, but that's enough to make the route "float" and not yet be seen in the routing table.
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:06:44, Serial0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, BRI0
C 172.12.123.0 is directly connected, Serial0
Let's see the effect on the routing table when the Serial0 interface is closed.
R2(config)#int s0
R2(config-if)#shutdown
12:04:53: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.1 on Serial0 from FULL to DOWN, Neighbor Down: Interface down or detached
12:04:55: %SYS-5-CONFIG_I: Configured from console by console
12:04:55: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
12:04:56: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
S 1.1.1.1 is directly connected, BRI0
172.12.0.0/24 is subnetted, 1 subnets
C 172.12.12.0 is directly connected, BRI0
The floating static route appears in the table, but the ISDN link will not come up until the BRI interface has traffic to send. Let's ping 1.1.1.1 and see what happens. debug dialer was configured on R2 before sending the ping.
R2#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
12:16:01: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=1.1.1.1)
12:16:01: BR0 DDR: Attempting to dial 8358661
12:16:01: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.!!
12:16:01: BR0:1 DDR: dialer protocol up!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 36/37/40 ms
The link comes up and traffic can still reach 1.1.1.1. Once R2 becomes an OSPF neighbor of R1 again, the OSPF route will again become the primary path and the floating static route leaves the routing table.
R2(config)#int s0
R2(config-if)#no shut
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.12.123.1 1 FULL/DR 00:01:57 172.12.123.1 Serial0
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:16, Serial0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, BRI0
C 172.12.123.0 is directly connected, Serial0
A floating static route is an excellent "back door" that will keep the ISDN link down while allowing that link to serve as a backup route. Just make sure the ISDN link comes down when you expect it to - always check that with show isdn status!
In this example, R1 and R2 are running OSPF over a Frame Relay network, 172.12.123.0 /24. They're also connected by a BRI ISDN link, 172.12.12.0 /24. R1 is advertising a loopback network, 1.1.1.1 /32, via OSPF. We want R2 to have a route to that loopback even if the frame goes down - and here, we'll use a floating static route to make that happen.
R2 sees the route to the loopback interface via OSPF, and can ping that interface successfully.
R2#show ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, Serial0
R2#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms
This is when it's important to know your administrative distances.... or at least know where to look to see them! The AD of OSPF is 110, which means we can configure a static route to 1.1.1.1 /32, and as long as the AD of the static route is higher than 110, it won't be used unless the OSPF route leaves the routing table. That's why this kind of route is called a "floating" static route - the route "floats" in the routing table and isn't seen unless the primary route leaves the table.
You learned how to write a static route in your CCNA studies, but you also remember that the default AD of a static route is either 1 or 0... and both of those values are less than 110! To change the AD of a static route, configure the desired distance at the end of the ip route command.
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 ?
<1-255> Distance metric for this route
A.B.C.D Forwarding router's address
name Specify name of the next hop
permanent permanent route
tag Set tag for this route
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 111
The static route has an AD that's only one higher than that of the OSPF route, but that's enough to make the route "float" and not yet be seen in the routing table.
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:06:44, Serial0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, BRI0
C 172.12.123.0 is directly connected, Serial0
Let's see the effect on the routing table when the Serial0 interface is closed.
R2(config)#int s0
R2(config-if)#shutdown
12:04:53: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.1 on Serial0 from FULL to DOWN, Neighbor Down: Interface down or detached
12:04:55: %SYS-5-CONFIG_I: Configured from console by console
12:04:55: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
12:04:56: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
S 1.1.1.1 is directly connected, BRI0
172.12.0.0/24 is subnetted, 1 subnets
C 172.12.12.0 is directly connected, BRI0
The floating static route appears in the table, but the ISDN link will not come up until the BRI interface has traffic to send. Let's ping 1.1.1.1 and see what happens. debug dialer was configured on R2 before sending the ping.
R2#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
12:16:01: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=1.1.1.1)
12:16:01: BR0 DDR: Attempting to dial 8358661
12:16:01: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.!!
12:16:01: BR0:1 DDR: dialer protocol up!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 36/37/40 ms
The link comes up and traffic can still reach 1.1.1.1. Once R2 becomes an OSPF neighbor of R1 again, the OSPF route will again become the primary path and the floating static route leaves the routing table.
R2(config)#int s0
R2(config-if)#no shut
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.12.123.1 1 FULL/DR 00:01:57 172.12.123.1 Serial0
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:16, Serial0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, BRI0
C 172.12.123.0 is directly connected, Serial0
A floating static route is an excellent "back door" that will keep the ISDN link down while allowing that link to serve as a backup route. Just make sure the ISDN link comes down when you expect it to - always check that with show isdn status!
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