Scenario:
Let's say you have a class C subnet that you break off into two smaller
networks:
1.1.1.0/24 = 1.1.1.0/25 + 1.1.1.128/25
Problem:
You are announcing this through BGP, but your provider only allows you to
advertise full class C addresses. The problem lies in that you have to
announce this network as 1.1.1.0/24 but still get traffic to 1.1.1.0/25 &
1.1.1.128/25.
Solution:
So, there are two ways to do this:
1) Use an aggregate-address with a suppress map to filter this.
router bgp <as>
network 1.1.1.0 mask 255.255.255.128
network 1.1.1.128 mask 255.255.255.128
aggregate-address 1.1.1.0 255.255.255.0 suppress-map mysupressmap
!
route-map mysuppressmap permit 10
match ip address 20
!
access-list 20 permit 1.1.1.0 0.0.0.128
access-list 20 permit 1.1.1.128 0.0.0.128
!
int vlan 1
ip add 1.1.1.1 255.255.255.128
no shut
!
int vlan 2
ip add 1.1.1.129 255.255.255.128
no shut
2) Use null routes
router bgp <as>
network 1.1.1.0 mask 255.255.255.0
!
ip route 1.1.1.0 255.255.255.0 null 0
!
int vlan 1
ip add 1.1.1.1 255.255.255.128
no shut
!
int vlan 2
ip add 1.1.1.129 255.255.255.128
no shut
Question:
Which way do you all feel is better?
I read in Boson's test that the preferred way is to use aggregate addresses,
but this seems to be a much more complex way to do what a simple null route
can accomplish? Thought / comments?
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