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
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