Subnetting Made Easy And Other Cisco Tidbits

CCIE - Warm-Up Phase - BGP
Tuesday, 26 April 2011

Hi,

I am pretty happy with BGP but still there are a few things that I need to get into my head.

7.5 - Disable Connected Check - this allows eBGP peers to use an IP address to peer that is not directly connected to the neighbor (e.g. a loopback interface). The difference between this and ebgp-multihop is that the TTL is not adjusted for disable-connected-check and that eBGP sessions will not form over transit routers.

7.8 - The bgp cluster-id is applied to the route reflector only.

7.9 - The confederation AS is used in the "router bgp" command and the bgp-confederation identifier refers to the proper AS. You must also specify bgp confederation peers otherwise the adjacency won't come up. In addition, confederation peers are treated as eBGP neighbors and should use directly connected interfaces, otherwise need to use things like update-source and ebgp-multihop.

7.11 - Instead of using next-hop-self you can create a route-map to "set ip next-hop" and apply that to the neighbor (e.g. neighbor 1.2.3.4 route-map ROUTE_MAP_NAME in/out"

7.12 - iBGP synchronization - Do not advertise a BGP route unless it is also learned by the IGP running in the network. In essence this requires the edge router to redistribute the eBGP routes into the IGP in order for iBGP to advertise the routes. Be careful when redistributing routes into IGP. Use an AS PATH access-list:

ip as-path access-list 1 permit ^_54
!
route-map MATCH_AS_PATH permit 10
match as-path 1
!
router eigrp 100
redistribute bgp 100 metric 100000 1000 255 1 1500 route-map MATCH_AS_PATH

Posted byChris Bloomfield at 08:52 0 comments  

Labels: bgp, CCIE, ine

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 | inc Cost" to find out that value. Assign that value to the neighbor using "neighbor cost ". Don't forget to change the bandwidth back to the correct value!

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 0 comments  

Labels: CCIE, ine, ine workbook 1, ospf, virtual link

RIP - Adjusting default timers
Saturday, 9 April 2011

In my last post about RIP I mentioned quite a few different bits of RIP. Here I just want to focus on changing the timers and how to override the timers.

You can adjust your timers using the timers basic update invalid holddown flush under the routing process:

router rip
version 2
timers basic 30 90 120 120

This will set the update to 30 secs, invalid to 90 secs, holddown to 120 secs, and flush to 120 secs.

What would happen if you need to change that behaviour on just one link? Well, we can configure the update timer under the interface:

interface fa0/0
ip rip advertise 30

This then changes the advertise time to 30 seconds but note that this doesn't change the other timers, only the advertise time!

Posted byChris Bloomfield at 07:56 0 comments  

Labels: RIP, RIPv1, RIPv2