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First of all, may I welcome you to my site. My name is Chris and I'm from the UK and work as a Systems Engineer for Cisco. This blog was initially created to post up my subnetting technique but has now got more stuff to do with attaining Cisco certifications. Either way I really hope that the content is sufficent for your needs and I look forward to hearing your feedback. If you find that the content really helps you please feel free to donate using the PayPal link on the right.
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CCIE - Warm-Up Phase - OSPF
Wednesday 20 April 2011
This is in relation to INE Workbook 1 and their 48 week program to get up to speed with passing the CCIE exam.
What have I learned?
6.1 - A network statement of "network 0.0.0.0 0.0.0.0 area 0" gets changed to "network 0.0.0.0 255.255.255.255 area 0" which is basically a single network statement that covers all interfaces.
6.2 - By default Frame Relay defaults to an OSPF network type of non-broadcast. This means that a DR/BDR is elected, however, hellos are sent as UNICASTS therefore you must configure static neighbor commands. This needs to be done only on one end of the link but it may be best practice to configure it at both ends for clarity. In addition, as a DR is elected it does not change the next-hop value when it sends out Type 2 LSAs hence spoke routers need a path to the next-hop value via the PVC to the hub (e.g. frame-relay map ip 155.1.0.4 105 -- this will send traffic to 155.1.0.4 via DLCI 105 which is the PVC to R5 which is the hub for this network).
6.4 - Point-to-point uses multicast hellos and no need for DR/BDR election or neighbor statements.
6.5 - Point-to-multipoint. As this is over Frame Relay and the initial Frame Relay maps did not include the "broadcast" keyword no adjacencies formed. Point-to-multipoint uses multicast hellos so you have to specify the "broadcast" keyword. There are also no DR/BDR elections so the hub (R5 in this case) changes the next-hop to itself (155.1.0.5). As each router (R1 to R4) has a Frame Relay map to R5 full-reachability is made.
6.6 - Point-to-multipoint non-broadcast. The only difference here is that it sends hellos as unicast and requires a neighbor statement.
6.7 - Changing the network type on Loopback interfaces to point-to-point advertises the network with its correct mask (in this example /24) rather than the default Loopback type which advertises them as a stub with a /32 even if they are not configured as /32.
6.8 - The reference bandwidth is configured in Mbps.
6.10 - OSPF Path selection with Bandwidth - Change the bandwidth on an interface that RECEIVES the LSA and that is downstream from the device you wish to be affected (e.g. this example requires R6 to route via R1 so change the bandwidth value on R1 Se0/0 to a greater value to affect the overall cost. Note that R1, R4, and R6 share an Ethernet segment)
6.11 - Changing the cost per neighbor. To find out what cost OSPF would assign a bandwidth value change the bandwidth on an interface and use "show ip ospf interface
6.12 - If a router connects two areas together and neither of those areas are Area 0 then the ABR will not act as an ordinary ABR and will not forward Inter-Area routes as all Inter_Area routes must be advertised via Area 0. Therefore you need to use a Virtual Link.
6.13 - OSPF Path Selection with Non-Backbone Transit Areas. After the virtual link was formed and I adjusted some costs on R4, routes to SW2 were being routed via Area 1 (specifically, via R4) and not to R1 even though that link is effectively Area 0 as it is a virtual link. To allow the use of the virtual link you must use "no capability transit" command at BOTH ends of the virtual link.
6.14 - OSPF virtual links will not come up if the cost used to reach the other end of the virtual link is 65535 which is the maximum cost associated with an interface. You must therefore change the OSPF cost at BOTH ENDS of the virtual link.
Posted byChris Bloomfield at 16:58
Labels: CCIE, ine, ine workbook 1, ospf, virtual link
