As a CCNA / CCNP candidate you are expected to understand IPv6
During your career as a Cisco network engineer you will have to deal understanding IPv6 address structure.
For your CCNA and CCNP studies you have to at some point confront and understand IPv6. At first glance it can see quite daunting compared to IPv4 that we are all used to, in actual fact IPv6 is quite a simply addressing protocol once you get past the initial shock. In this article we are going to have a look at Neighbour discovery protocol for layer 2 mapping.
When an IPv6 host or router needs to send a packet to some other host on the same network it will first of all look into it’s own local neighbour database to find if it has an IPv6 to MAC mapping, if it finds the right mapping the host will use it, if there is no mapping the host will need to resolve the known layer 3 IPv6 address to a currently unknown Layer 2 MAC address and to do this the host uses the Neighbor Discovery Protocol or NDP to discover the MAC address dynamically.
The Sending host will use a multicast message called a Neighbor Solicitation (NS) icmp message to ask the receiving host for it’s MAC address, the receiving host will reply with a Neighbor Advertisement (NA) icmp message unicast in return with the requested MAC address.
How IPv6 achieves this is all in the construction of the Neighbor Solicitation message which makes use of a special IPv6 destination address called a “Solicited Node Multicast”, this solicited Node Multicast at any given moment represented all of the IPv6 hosts on the link, the last 24 bits of the Solicited Node Address are the last 24 bits of the IPv6 address of the device that a host is requesting the MAC from.
The IPv6 multicast destination address is FF02::1:FF:0/104 the final 24 bits are made up of the last 24 bits of the IPv6 address to which the message is being sent to. For example if a host wanted to discover the MAC address of an IPv6 host addressed as 2222:3333:4444:5555:6666:AAAA:BBBB:CCCC:DDDD/64 then the solicited Node Address will look like the following FF02::1:FF:CC:DDDD/104
When a sending hosts wants to get the MAC address from the IPv6 host of 2222:3333:4444:5555:6666:AAAA:BBBB:CCCC:DDDD/64 it will take the last 24 bits of the known IPv6 address and place them into the remaining 24 bits of the Solicited Node Address and since all IPv6 hosts listen to their own Solicited Node addresses, when they hear their address they will reply with the MAC address.
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Joe Spoto is a senior lecturer at Commsupport networks
CCNA
in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNA Commsupport run free one day training sessions and free on-line webinars, CCNA
Tuesday, 5 October 2010
CCNA – CCNP Training – BGP Weights
As a CCNA / CCNP candidate you are expected to understand the BGP Weight component in BGP path selection.
During your career as a Cisco network engineer may come across BGP and will be required to configure certain aspects of it operation.
BGP is a very tuneable protocol which lets you engineer the paths which BGP chooses to reach prefixes it learned from any peers.
One of the settings within BGP on a cisco router which we can use to tune the path chosen by BGP is called “Weight”
A Cisco router can look at the weight given to a route and determine if to take that route. When a Cisco router receives a BGP update it will set the weight either on a single prefix or on all the routes learnt via a particular neighbour.
The weight feature is a Cisco only feature which is therefore not found on any other BGP implementation on any other routers. Weight is never advertised to a neighboring router so it cannot be learnt from another router Cisco or otherwise. To use the weight feature it has to be set to look at all incoming routes from a neighbour, since this feature is a proprietary one and not supported on any other it is not known to be a path attribute. In any event the BGP update packets have no field in which they could communicate the weight value even if they wished to do so.
The weight value is set on an inbound route and has a range of between 0 and 65,535 (2 to the 16 power minus 1), Higher weight values are preferred over lower values, and the default setting is always zero for learned routes and 32,768 for locally injected routes, it is by the way at the time of writing not possible to change the default of 0.
Imagine two routers, one router has a peering arrangement with a neighbour known as 3.3.3.3, we are going to match all routes advertised to our router from 3.3.3.3 matching 10.10.10.0/24 prefix and apply a weight of 1000
Below we create an access list which is to be referenced in a route map in the next step to match the prefix we want to alter the weigh of.
Router #conf t
Router (config)#access-list 1 permit 10.10.10.0 0.0.0.255
The route map below matches the previously configured access list and sets the weigh to 1000, we must also configure an empty route map statement or other routes will not be seen from the Router, the route map will be named TO-R1
Router (config)#route-map TO-R1 permit 10
Router (config-route-map)#match address 1
Router (config-route-map)#set weight 1000
Router (config)#route-map TO-R1 permit 20
Router (config-route-map)#exit
Now we have to assign the route map under the BGP process in an “in” direction since the10.10.10.0/24 prefix is being advertised in an inbound direction to the router , now this device will give the route to the 10.10.10.0/24 network higher preference via the 3.3.3.3 peer.
Router (config)#router bgp 400
Router (config-router)#neighbor 3.3.3.3 route-map TO-R1 in
R4(config-router)#end
Joe Spoto is a senior lecturer at Commsupport networks
CCNA
in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNA Commsupport run free one day training sessions and free on-line webinars, CCNA
During your career as a Cisco network engineer may come across BGP and will be required to configure certain aspects of it operation.
BGP is a very tuneable protocol which lets you engineer the paths which BGP chooses to reach prefixes it learned from any peers.
One of the settings within BGP on a cisco router which we can use to tune the path chosen by BGP is called “Weight”
A Cisco router can look at the weight given to a route and determine if to take that route. When a Cisco router receives a BGP update it will set the weight either on a single prefix or on all the routes learnt via a particular neighbour.
The weight feature is a Cisco only feature which is therefore not found on any other BGP implementation on any other routers. Weight is never advertised to a neighboring router so it cannot be learnt from another router Cisco or otherwise. To use the weight feature it has to be set to look at all incoming routes from a neighbour, since this feature is a proprietary one and not supported on any other it is not known to be a path attribute. In any event the BGP update packets have no field in which they could communicate the weight value even if they wished to do so.
The weight value is set on an inbound route and has a range of between 0 and 65,535 (2 to the 16 power minus 1), Higher weight values are preferred over lower values, and the default setting is always zero for learned routes and 32,768 for locally injected routes, it is by the way at the time of writing not possible to change the default of 0.
Imagine two routers, one router has a peering arrangement with a neighbour known as 3.3.3.3, we are going to match all routes advertised to our router from 3.3.3.3 matching 10.10.10.0/24 prefix and apply a weight of 1000
Below we create an access list which is to be referenced in a route map in the next step to match the prefix we want to alter the weigh of.
Router #conf t
Router (config)#access-list 1 permit 10.10.10.0 0.0.0.255
The route map below matches the previously configured access list and sets the weigh to 1000, we must also configure an empty route map statement or other routes will not be seen from the Router, the route map will be named TO-R1
Router (config)#route-map TO-R1 permit 10
Router (config-route-map)#match address 1
Router (config-route-map)#set weight 1000
Router (config)#route-map TO-R1 permit 20
Router (config-route-map)#exit
Now we have to assign the route map under the BGP process in an “in” direction since the10.10.10.0/24 prefix is being advertised in an inbound direction to the router , now this device will give the route to the 10.10.10.0/24 network higher preference via the 3.3.3.3 peer.
Router (config)#router bgp 400
Router (config-router)#neighbor 3.3.3.3 route-map TO-R1 in
R4(config-router)#end
Joe Spoto is a senior lecturer at Commsupport networks
CCNA
in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNA Commsupport run free one day training sessions and free on-line webinars, CCNA
Friday, 2 July 2010
CCNA Training Commsupport – OSPF LSA types Part 1
As a CCNA / CCNP candidate you are expected to understand how to set and interpret the different types of OSPF LSA’s.
During your career as a Cisco network engineer you will have to deal with setting and manipulating the many different types of OSPF LSA types.
Within an OSPF Area every router must maintain the same link state data base. The link state data base is constructed by the routers by the LSA (Link State Advertisements) it receives from other routers in the area.
From the Link State Data Base (LSDB) the router will run the shortest path first algorithm to calculate the best path based on cost to any network.
OSPF LSA types can be confusing but they all play a vital role within their particular scope of operation . For example some LSA types remain within an area, or advertise routes between areas and advertise routes from outside of OSPF completely.
For the CCNA exam your are expected to know of LSA type 1 and 2
LSA Type 1: Router LSA
Each and every router operating OSPF will generate it’s own Type 1 LSA to present itself. It will create an individual Type 1 LSA for each area it connects to.
The Type 1 LSA will contain the Router ID along with a list of all the interface IP addresses on the router that are within the area that the LSA is advertising into, example if a router is a member of two Areas 0 and 1 the LSA advertised into area 1 will contain a list of all interface IP addresses that are also members of area 1 but none which are members of Area 0. This type of LSA will flood within the same area, the neighbors receiving the Type 1 LSA will forward it to their neighbors until all routers in the area have a copy the advertisement.
Info which is listed inside the LSA
1. Lists the routers interface ip address/mask and interface cost (If no DR has been elected)
2. List the neighbors RID if there is no DR
3. Lists the IP address of the DR and a notation that the link attaches to a transit network.
4. The advertising router’s RID
LSA type 1 never traverse Area border routers. ABR’s create multiple LSA type 1’s, one per area.
During your career as a Cisco network engineer you will have to deal with setting and manipulating the many different types of OSPF LSA types.
Within an OSPF Area every router must maintain the same link state data base. The link state data base is constructed by the routers by the LSA (Link State Advertisements) it receives from other routers in the area.
From the Link State Data Base (LSDB) the router will run the shortest path first algorithm to calculate the best path based on cost to any network.
OSPF LSA types can be confusing but they all play a vital role within their particular scope of operation . For example some LSA types remain within an area, or advertise routes between areas and advertise routes from outside of OSPF completely.
For the CCNA exam your are expected to know of LSA type 1 and 2
LSA Type 1: Router LSA
Each and every router operating OSPF will generate it’s own Type 1 LSA to present itself. It will create an individual Type 1 LSA for each area it connects to.
The Type 1 LSA will contain the Router ID along with a list of all the interface IP addresses on the router that are within the area that the LSA is advertising into, example if a router is a member of two Areas 0 and 1 the LSA advertised into area 1 will contain a list of all interface IP addresses that are also members of area 1 but none which are members of Area 0. This type of LSA will flood within the same area, the neighbors receiving the Type 1 LSA will forward it to their neighbors until all routers in the area have a copy the advertisement.
Info which is listed inside the LSA
1. Lists the routers interface ip address/mask and interface cost (If no DR has been elected)
2. List the neighbors RID if there is no DR
3. Lists the IP address of the DR and a notation that the link attaches to a transit network.
4. The advertising router’s RID
LSA type 1 never traverse Area border routers. ABR’s create multiple LSA type 1’s, one per area.
Thursday, 24 June 2010
Once again Joe at Commsupport has shown the industry that anything is possible by launching the worlds first free CCNA live on-line CCNA course.
The course is run online using the latest in web based conferencing services, the students will be able to speak to the instructor asking and answering questions. The CCNA course will cover all the CCNA training that a CCNA candidate requires to become a comfortable with the CCNA course syllabus.
To register for the free on-line CCNA course. Only 30 students on each on-line course, so hurry
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at
Commsupport please visit us at CCNA
Commsupport run free one day training sessions and free on-line
webinars, CCNA
The course is run online using the latest in web based conferencing services, the students will be able to speak to the instructor asking and answering questions. The CCNA course will cover all the CCNA training that a CCNA candidate requires to become a comfortable with the CCNA course syllabus.
To register for the free on-line CCNA course. Only 30 students on each on-line course, so hurry
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at
Commsupport please visit us at CCNA
Commsupport run free one day training sessions and free on-line
webinars, CCNA
Wednesday, 16 June 2010
CCNA Training – OSPF Costs
CCNA Training – OSPF Costs
As a CCNA / CCNP candidate you are expected to understand how to set and interpret the OSPF cost function on your Cisco devices
During your career as a Cisco network engineer you will have to deal with setting and manipulating the OSPF costs on an interface.
OSPF uses a metric called “Cost” to calculate the metric of path. The cost is a cumulative value which is an incremental metric.
The cost is as a default based on the bandwidth of the interface. The Higher the interface bandwidth the lower the cost that is associated to that interface, to see the cost that is assigned to any given interface which is participating in OSPF issue the following command:
Router# show ip ospf interface
The output of this command will show the current cost given to this interface. The costs of the interface is calculated by taking the bandwidth of the interface and dividing this number by a value known as the “auto-cost reference-bandwidth”.
This auto-cost reference-bandwidth is an integer used to calculate a standard metric across OSPF and is set to 100,000,000. The cost is calculated as follows:
100,000,000/BW
If the interface bandwidth is 10Mbps, then the resulting cost would be 10:
100,000,000/10,000,000 = 10
If the interface bandwidth is 100Mbps, then the resulting cost would be 1:
100,000,000/100,000,000 = 1
The next example reveals some deficiencies with the auto-cost reference-bandwidth set at 100,000,000. If we had a 1Gbp interface it’s cost will be calculated by OSPF to be the exact same number as that for the 100Mbps interface.
100,000,000/1000,000,000 = 1
The same would apply for higher speed interfaces such as 10Gbps interfaces which would also be interpreted by OSPF as having a cost of 1, since OSPF cannot define interface costs as an integer part of a decimal number i.e. 0.5 or 0.125.
So that OSPF may be able to calculate the cost of an interface based on the bandwidth of the interface with great accuracy we will need to change the value of the auto-cost reference-bandwidth value from 100,000,000 to a greater value.
To change the auto-cost reference-bandwidth use the following commands:
router(config)#router ospf 1
R2_4(config-router)#auto-cost reference-bandwidth 10000
% OSPF: Reference bandwidth is changed.
Please ensure reference bandwidth is consistent across all routers.
The auto-cost reference-bandwidth now has a value of 10000,000,000 (The value is set in Mbps), therefore now the OSPF process will view the 100Mbps interface as having a cost of 100, 1Gbps having a Cost of 10 and 10Gbps a cost of 1, another point worth mentioning is that the router will prompt you to make sure that the auto-cost reference-bandwidth value is set consistently across all your routers.
Note: The cost given to a path is added to the metric only when the route is received inbound on an interface, not outbound, therefore should you wish to alter the cost of the path you must do this on the inbound interface.
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNACommsupport run free one day training sessions and free on-line webinars, CCNP
As a CCNA / CCNP candidate you are expected to understand how to set and interpret the OSPF cost function on your Cisco devices
During your career as a Cisco network engineer you will have to deal with setting and manipulating the OSPF costs on an interface.
OSPF uses a metric called “Cost” to calculate the metric of path. The cost is a cumulative value which is an incremental metric.
The cost is as a default based on the bandwidth of the interface. The Higher the interface bandwidth the lower the cost that is associated to that interface, to see the cost that is assigned to any given interface which is participating in OSPF issue the following command:
Router# show ip ospf interface
The output of this command will show the current cost given to this interface. The costs of the interface is calculated by taking the bandwidth of the interface and dividing this number by a value known as the “auto-cost reference-bandwidth”.
This auto-cost reference-bandwidth is an integer used to calculate a standard metric across OSPF and is set to 100,000,000. The cost is calculated as follows:
100,000,000/BW
If the interface bandwidth is 10Mbps, then the resulting cost would be 10:
100,000,000/10,000,000 = 10
If the interface bandwidth is 100Mbps, then the resulting cost would be 1:
100,000,000/100,000,000 = 1
The next example reveals some deficiencies with the auto-cost reference-bandwidth set at 100,000,000. If we had a 1Gbp interface it’s cost will be calculated by OSPF to be the exact same number as that for the 100Mbps interface.
100,000,000/1000,000,000 = 1
The same would apply for higher speed interfaces such as 10Gbps interfaces which would also be interpreted by OSPF as having a cost of 1, since OSPF cannot define interface costs as an integer part of a decimal number i.e. 0.5 or 0.125.
So that OSPF may be able to calculate the cost of an interface based on the bandwidth of the interface with great accuracy we will need to change the value of the auto-cost reference-bandwidth value from 100,000,000 to a greater value.
To change the auto-cost reference-bandwidth use the following commands:
router(config)#router ospf 1
R2_4(config-router)#auto-cost reference-bandwidth 10000
% OSPF: Reference bandwidth is changed.
Please ensure reference bandwidth is consistent across all routers.
The auto-cost reference-bandwidth now has a value of 10000,000,000 (The value is set in Mbps), therefore now the OSPF process will view the 100Mbps interface as having a cost of 100, 1Gbps having a Cost of 10 and 10Gbps a cost of 1, another point worth mentioning is that the router will prompt you to make sure that the auto-cost reference-bandwidth value is set consistently across all your routers.
Note: The cost given to a path is added to the metric only when the route is received inbound on an interface, not outbound, therefore should you wish to alter the cost of the path you must do this on the inbound interface.
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNACommsupport run free one day training sessions and free on-line webinars, CCNP
Tuesday, 15 June 2010
CCNA Training – Recovering the IOS in ROMMON
CCNA Training – Recovering the IOS in ROMMON
As a CCNA / CCNP candidate you are expected to understand how to recover the IOS on a router when the router is in ROMMON mode
During your time as a network engineer you will come across Cisco routers that will have lost their IOS (Internetwork Operating System software).
Cisco router may lose their IOS by various means. The main reasons that the Cisco router may lose is IOS are:
1. User deletes the contents of flash and in the process also removes the IOS
2. The IOS becomes corrupt and fails to load
3. The user fails to upgrade the Cisco router and then reboots the router causing the router to boot into ROMMON
4. The user misspells the word start in the command “copy run start” causing the router to ask the user if they wish to delete the contents of flash, this last reason is all too common and causes no end of grief, we suggest that instead of using the “copy run start” command that you use the command “wri mem” which is an abbreviation of the command “write memory”.
When your router boots and drops into ROMMON do not think that the IOS is missing, it could be that the router has experienced a bad boot process, at the “ROMMON>” prompt type the “reset” command, watch the router reboot and watch for any messages from the router stating that the router is unable to find a file in flash.
So, we find ourselves in with a router that has booted into ROMMON what do we do next, the following steps will assist in recovering the routers IOS
1. Type the following command into the Router:
ROMMON>tftpdnld
At this point you will receive an output onto the console
2. Make sure that you have a laptop connected to the router either directly or via a switch with a TFTP program open and showing an address that will be an address within the same subnet that you will assign to the router in a moment.
3. Enter an address for the router
ROMMON>IP_ADDRESS=10.1.1.4
4. Enter the subnet for the routers interface
ROMMON>IP_SUBNET_MASK=255.255.255.0
5. Enter a default gateway for the router, at this point it is good practice to simply type in the address of the laptop.
DEFAULT_GATEWAY=10.1.1.100
6. Enter the IP address of the laptop
TFTP_SERVER=10.1.1.100
7. Enter the name of the file that you are attempting to load into the router. Make sure that the TFTP application on the laptop has been “browsed” to the location where this IOS image resides.
TFTP_FILE=c1841-advsecurityk9-mz.124-15.T1.bin
8. Press return, at this point the router will ask you is you wish to continue with the download type “yes”, the routers Ethernet LED will now turn on and attempt to locate the TFTP server. If you are successful you will see a series of exclamation marks issue across the screen.
Once the router has downloaded the complete IOS to flash it will perform a lengthy checksum when complete will present the ROMMON> prompt to you, at this point type reset forcing the router to reboot and load the new IOS into RAM and booting correctly.
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNACommsupport run free one day training sessions and free on-line webinars, CCNP
As a CCNA / CCNP candidate you are expected to understand how to recover the IOS on a router when the router is in ROMMON mode
During your time as a network engineer you will come across Cisco routers that will have lost their IOS (Internetwork Operating System software).
Cisco router may lose their IOS by various means. The main reasons that the Cisco router may lose is IOS are:
1. User deletes the contents of flash and in the process also removes the IOS
2. The IOS becomes corrupt and fails to load
3. The user fails to upgrade the Cisco router and then reboots the router causing the router to boot into ROMMON
4. The user misspells the word start in the command “copy run start” causing the router to ask the user if they wish to delete the contents of flash, this last reason is all too common and causes no end of grief, we suggest that instead of using the “copy run start” command that you use the command “wri mem” which is an abbreviation of the command “write memory”.
When your router boots and drops into ROMMON do not think that the IOS is missing, it could be that the router has experienced a bad boot process, at the “ROMMON>” prompt type the “reset” command, watch the router reboot and watch for any messages from the router stating that the router is unable to find a file in flash.
So, we find ourselves in with a router that has booted into ROMMON what do we do next, the following steps will assist in recovering the routers IOS
1. Type the following command into the Router:
ROMMON>tftpdnld
At this point you will receive an output onto the console
2. Make sure that you have a laptop connected to the router either directly or via a switch with a TFTP program open and showing an address that will be an address within the same subnet that you will assign to the router in a moment.
3. Enter an address for the router
ROMMON>IP_ADDRESS=10.1.1.4
4. Enter the subnet for the routers interface
ROMMON>IP_SUBNET_MASK=255.255.255.0
5. Enter a default gateway for the router, at this point it is good practice to simply type in the address of the laptop.
DEFAULT_GATEWAY=10.1.1.100
6. Enter the IP address of the laptop
TFTP_SERVER=10.1.1.100
7. Enter the name of the file that you are attempting to load into the router. Make sure that the TFTP application on the laptop has been “browsed” to the location where this IOS image resides.
TFTP_FILE=c1841-advsecurityk9-mz.124-15.T1.bin
8. Press return, at this point the router will ask you is you wish to continue with the download type “yes”, the routers Ethernet LED will now turn on and attempt to locate the TFTP server. If you are successful you will see a series of exclamation marks issue across the screen.
Once the router has downloaded the complete IOS to flash it will perform a lengthy checksum when complete will present the ROMMON> prompt to you, at this point type reset forcing the router to reboot and load the new IOS into RAM and booting correctly.
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNACommsupport run free one day training sessions and free on-line webinars, CCNP
Monday, 14 June 2010
DHCP IP Helper address in Cisco devices
DHCP IP Helper address in Cisco devices
As a CCNA / CCNP candidate you are expected to understand the purpose and function of the IP Helper address .
The CCNA / CCNP’s exam will ask of you to determine the purpose of the IP Helper command, what it contains and the role it plays in your network.
The purpose of the IP helper command for the purposes of DHCP (Dynamic Host Configuration Protocol) is to assist a host within a broadcast domain to acquire an IP address from a DHCP server that resides within another broadcast domain.
Before we take a detailed view of how the IP helper command works we will firstly need to look at what defines a broadcast domain. A broad is a message which is generated by host or hosts when they wish to either announce their presence to the other hosts on the network or they need to discover some entity on the network. The two most common types of broadcast messages that are issued from a host are ARP (Address Resolution Protocol) messages to resolve a known IP address to a Layer 2 MAC address or a DHCP discover message.
Both the ARP message and the DHCP discover message will received by all of the host within the VLAN or Subnet of the host that generated the broadcast message, if the broadcast message is received by a router interface the default action of the router interface is to discard the message unless it itself is the intended recipient of the broadcast, say for example the generating host was ARP’ing for it’s default gateways’ MAC address.
The router will not by default forward the broadcast of any nature across to another subnet. Imagine if the router did this and forwarded all broadcasts from one network into another network, your router at home or at the office is connected to the internet, imagine if your router as to forward by default your broadcasts from one subnet, i.e. yours over into the public network, and multiplied that by a few million times one for every router that is connected to the internet, well that would be a lot of traffic, so again by default it is not in the interest of performance that broadcasts are forwarded from one subnet to another.
So where does that leave us in regards to the client sending out DHCP Discover messages in the attempt of getting a valid host IP address. If no DHCP server resides within the same subnet that the client resides in because the DHCP server lives in another subnet in then there is no chance of the client getting an IP address.
We in this case have to assist the client in getting an IP address from the DHCP server which lives in another network and to do this we need to use the “IP helper-address” command which is placed on the same interface that is the default gateway for the host requesting the IP address. The command would be as follows
Router(config-if)#ip helper-address x.x.x.x
The x.x.x.x would represent the address of the DHCP server. It is important that the router knows how to reach the subnet that the DHCP resides within, unless this is the case the router will never be able to forward the DHCP discover messages which are generated by the client to the DHCP server.
When the routers’ interface receives the DHCP discover message from the client it will relay the DHCP discover message over to the DHCP server placing it’s own interface IP address in a field called the “Relay agent”, this is done so that the DHCP server knows from which of its address scopes to draw a valid address from. It is important to bear in mind at this point that the original DHCP discover message which is a broadcast is relayed over to the DHCP server as a unicast message.
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNACommsupport run free one day training sessions and free on-line webinars, CCNP
As a CCNA / CCNP candidate you are expected to understand the purpose and function of the IP Helper address .
The CCNA / CCNP’s exam will ask of you to determine the purpose of the IP Helper command, what it contains and the role it plays in your network.
The purpose of the IP helper command for the purposes of DHCP (Dynamic Host Configuration Protocol) is to assist a host within a broadcast domain to acquire an IP address from a DHCP server that resides within another broadcast domain.
Before we take a detailed view of how the IP helper command works we will firstly need to look at what defines a broadcast domain. A broad is a message which is generated by host or hosts when they wish to either announce their presence to the other hosts on the network or they need to discover some entity on the network. The two most common types of broadcast messages that are issued from a host are ARP (Address Resolution Protocol) messages to resolve a known IP address to a Layer 2 MAC address or a DHCP discover message.
Both the ARP message and the DHCP discover message will received by all of the host within the VLAN or Subnet of the host that generated the broadcast message, if the broadcast message is received by a router interface the default action of the router interface is to discard the message unless it itself is the intended recipient of the broadcast, say for example the generating host was ARP’ing for it’s default gateways’ MAC address.
The router will not by default forward the broadcast of any nature across to another subnet. Imagine if the router did this and forwarded all broadcasts from one network into another network, your router at home or at the office is connected to the internet, imagine if your router as to forward by default your broadcasts from one subnet, i.e. yours over into the public network, and multiplied that by a few million times one for every router that is connected to the internet, well that would be a lot of traffic, so again by default it is not in the interest of performance that broadcasts are forwarded from one subnet to another.
So where does that leave us in regards to the client sending out DHCP Discover messages in the attempt of getting a valid host IP address. If no DHCP server resides within the same subnet that the client resides in because the DHCP server lives in another subnet in then there is no chance of the client getting an IP address.
We in this case have to assist the client in getting an IP address from the DHCP server which lives in another network and to do this we need to use the “IP helper-address” command which is placed on the same interface that is the default gateway for the host requesting the IP address. The command would be as follows
Router(config-if)#ip helper-address x.x.x.x
The x.x.x.x would represent the address of the DHCP server. It is important that the router knows how to reach the subnet that the DHCP resides within, unless this is the case the router will never be able to forward the DHCP discover messages which are generated by the client to the DHCP server.
When the routers’ interface receives the DHCP discover message from the client it will relay the DHCP discover message over to the DHCP server placing it’s own interface IP address in a field called the “Relay agent”, this is done so that the DHCP server knows from which of its address scopes to draw a valid address from. It is important to bear in mind at this point that the original DHCP discover message which is a broadcast is relayed over to the DHCP server as a unicast message.
Joe Spoto is a senior lecturer at Commsupport networks CCNA in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks, if you want to find out more about what we do at Commsupport please visit us at CCNACommsupport run free one day training sessions and free on-line webinars, CCNP
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