Cisco CCNP / BSCI Tutorial: The Role Of The OSPF ASBR
To cross the BSCI examination and earn your CCNP certification, you’ve got to grasp the (many) details of OSPF. You might need thought there have been quite a few OSPF details in your CCNA studies, but you’ll now construct on that foundation on the best way to incomes your CCNP.
One such element is the function of the Autonomous System Border Router (ASBR) in OSPF. The title itself raises some eyebrows, since you discovered in your CCNA research that OSPF would not use autonomous systems! Simply as an OSPF Space Border Router borders a number of OSPF areas, the ASBR borders the whole OSPF area and one other supply of routes. This may be another dynamic routing protocol, or immediately related networks that are not being advertised into OSPF by the network command.
Let’s say we now have a router working both OSPF and RIP version 2. By default, the RIP process won’t include any OSPF-discovered routes, and vice versa. The 2 separate routing processes are just that – separate. If we wish the opposite OSPF routers to know about the RIP routes, route redistribution must be configured. When the RIP routes are redistributed into OSPF, that router is then an ASBR.
Within the below example, RIP subnets have been redistributed into OSPF. A seed metric is just not mandatory when redistributing routes into OSPF. The command “show ip ospf” confirms that this router is now an ASBR.
R1(config)router ospf 1
R1(config-router)redistribute rip subnets
R1show ip ospf
Routing Process “ospf 1” with ID 1.1.1.1
Supports solely single TOS(TOS0) routes
Supports opaque LSA
It’s an autonomous system boundary router
The ASBR may perform route summarization on the routes being injected into OSPF with the summary-tackle command. (To configure OSPF inter-area summarization, use the area vary command.) By mastering route summarization and route redistribution, you’re properly on your strategy to passing the BSCI examination and earning your CCNP certification!
You keep in mind out of your CCNA research that when a port goes by means of the transition from blocking to forwarding, you are looking at a 50-second delay earlier than that port can really start forwarding frames. Configuring a port with PortFast is one technique to get around that, but once more, you may solely use it when a single host machine is discovered off the port. What if the device linked to a port is one other swap?
A swap might be linked to two other switches, giving that native switch a redundant path to the basis bridge, and that’s nice – we at all times desire a backup plan! Nevertheless, STP will only permit one path to be available, but when the obtainable path to the basis switch goes down, there will probably be a 50-second delay as a result of STP timers MaxAge and ForwardDelay earlier than the at the moment blocked path will probably be available.
The delay is there to forestall switching loops, and we can’t use PortFast to shorten the delay since these are switches, not host devices. What we can use is Uplinkfast.
The ports that SW3 may doubtlessly use to reach the foundation switch are collectively known as an uplink group. The uplink group includes the ports in forwarding and blocking mode. If the forwarding port within the uplink group sees that the link has gone down, one other port within the uplink group will probably be transitioned from blocking to forwarding immediately. Uplinkfast is pretty much PortFast for wiring closets. (Cisco recommends that Uplinkfast not be used on switches within the distribution and core layers.)
Some additional particulars regarding Uplinkfast:
The actual transition from blocking to forwarding mode takes about three seconds.
Uplinkfast cannot be configured on a root switch.
Uplinkfast is configured globally. You’ll be able to’t run Uplinkfast on some ports or on a per-VLAN foundation – it is all or nothing.
The original root port will become the root port once more when it detects that its hyperlink to the root switch has come again up. This doesn’t take place immediately. The change uses the next components to find out how lengthy to attend earlier than transitioning again to the forwarding state:
( 2 x FwdDelay) + 5 seconds
Uplinkfast will take rapid action to make sure that the change upon which it’s configured can not grow to be the foundation switch. First, the switch precedence shall be set to 49,152, which implies that if all different switches are still at their default precedence, they’d all should go down earlier than this switch can presumably change into the basis switch. Moreover, the STP Port Cost shall be elevated by 3000, making it highly unlikely that this change will be used to succeed in the foundation switch by any downstream switches.
And also you simply know there’s acquired to be at least one possibility with this command, right? Let’s run IOS Help and see.
SW2(config)spanning-tree uplinkfast ?
max-update-price Price at which station handle updates are despatched
When there’s a direct link failure, dummy multicast frames are sent to the MAC vacation spot 0100.0ccd.cdcd. The max-update-fee value determines what number of of these frames shall be despatched in an a hundred-millisecond time period.
Mastering the small print of UplinkFast, BackboneFast, BPDU Guard, and Loop Guard are vital to your success on the CCNP exams, and one or more of these features are in use on almost each community within the world. Study these options for achievement in both the exam room and the real world!
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