network cisco ccna gns3 certification arteq

network cisco ccna gns3 certification arteq
a network runs through it

Search insearchofthecert

Monday, January 28, 2013

null0 default bgp...

the following example is from r2 in the last tshoot lab 10...  the routing is static... pay particular attention to the default route, and the advertised network in bgp... r2 below in the lab represents the isp:

R2#sh run | b router
router bgp 65502
 no synchronization
 bgp log-neighbor-changes
 neighbor remote-as 65501
 neighbor ebgp-multihop 2
 neighbor update-source Loopback0
 no auto-summary
ip route Null0
ip route
ip route

on r1, the customer (enterprise), all 0's is the advertised bgp network... is the internal enterprise network... r1's is connected to

R1#sh tcp brie all
TCB       Local Address           Foreign Address        (state)
84D5CF00        ESTAB

R1#sh ip bgp | b Network
   Network          Next Hop            Metric LocPrf Weight Path
*>              0             0 65502 i

the effectiveness of the default to null0 in conjunction with the statics is clearly shown here... of course within the confines of the lab it is meaningless but we have our imagination hats on, correct? 

from doyle:

Configuring a Default Route to BGP Neighbors
router bgp 100
neighbor remote-as 300
neighbor remote-as 100
neighbor remote-as 200
ip route Null0

A default route to the Null0 interface is created statically, and the route is advertised with the network command...

 any destination address that cannot be matched to a more-specific route matches the static route and is dropped.

from halabi:

Less-Specific Routes of a Network’s Own Aggregate

A specific rule of routing states that, for the sake of preventing routing loops, a network must not follow a less-specific route for a destination that matches one of its own aggregated routes. A routing loop occurs when traffic circles back and forth between network elements, never reaching its final destination. Default routes are a special case of this rule. A network should not follow the default route to reach destinations that are part of its aggregated advertisements. This is why routing protocols that handle aggregation of routes should always keep a bit bucket (Null0 route in Cisco parlance) to the aggregate route itself. Traffic sent to the bit bucket will be discarded, which prevents
potential looping situations.

this is wildly important...

No comments:

Post a Comment