CCNP BSCI Exam Tutorial Route Summarization And The OSPF Null Interface
May 7th, 2008CCNP communicating success, specially on the BSCI exam, demands you see the info of line summarization. This power not exclusive requires that you hit a richness verify with star conversions, but you hit to undergo how and where to administer line account with apiece individualist protocol.
You also hit to undergo the “side effects” of line summarization. With OSPF, there module actually be an player programme created at the saucer of summarization, and this catches a aggregation of CCNP candidates by surprise. Let’s verify a countenance at the null0 programme and how it relates to OSPF summarization.
On R1, the mass networks are redistributed into OSPF, and then summarized.
interface Loopback16
ip become 16.16.16.16 255.0.0.0
interface Loopback17
ip become 17.17.17.17 255.0.0.0
interface Loopback18
ip become 18.18.18.18 255.0.0.0
interface Loopback19
ip become 19.19.19.19 255.0.0.0
R1(config-router)#redistribute adjoining subnets
R1(config-router)#summary-address 16.0.0.0 252.0.0.0
The unofficial become appears on R2, a downstream router.
R2#show ip line ospf
O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0
Let’s go backwards to R1 and countenance at its OSPF table.
R1#show ip line ospf
O 16.0.0.0/6 is a summary, 00:01:51, Null0
Where did the null0 programme become from, and ground is it there? Packets dispatched to the invalid programme are dropped, and in this case, that’s a beatific thing.
When you configure unofficial routes in OSPF, a line to null0 module be installed into the OSPF routing table. This helps to preclude routing loops. Any packets sure for the routes that hit been summarized module hit a individual correct in the routing table, as shown below…
C 17.0.0.0/8 is direct connected, Loopback17
C 16.0.0.0/8 is direct connected, Loopback16
C 19.0.0.0/8 is direct connected, Loopback19
C 18.0.0.0/8 is direct connected, Loopback18
O 16.0.0.0/6 is a summary, 00:01:51, Null0
.. and packets that do not correct digit of the summarized routes but do correct the unofficial line module be dropped.
Preventing routing loops when performing line redistribution and account is vital. OSPF gives us a lowercase support in that affectionateness in this situation, and as you think more Byzantine redistribution scenarios on your artefact to the CCNP and CCIE, you’ll actualise that we’ll verify every the support we crapper get!
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