WB1 7.29 BGP Aggregation

7.29 BGP Aggregation
• Remove the BGP configuration made for the previous task.
• Configure R2 with four new Loopback interfaces with the IP addresses 10.0.0.1/24, 10.0.1.1/24, 10.0.2.1/24, and 10.0.3.1/24.
• Advertise an aggregate route for these networks that does not overlap any address space.
 
Note
 
There are three basic ways to do summarization in BGP:

1) Create a summary prefix in IGP and advertise it into BGP using the network command. Most often this is accomplished by creating a static route to Null0 in the routing table of the advertising router. This is a common way to advertise local prefixes into BGP. However, you cannot summarize external BGP prefixes using the method.
2) Auto-summarization. As discussed in the separate task, summarizes networks to their classful boundaries. Only applies to redistributed prefixes or when using the classful network command. Not used in modern networks.3) Summarization of prefixes found in BGP tables using the aggregate-address command. This is the most flexible way to do summarization, as it may be applied to any paths learned by the BGP speaker.
 
For example, if you issue the command aggregate-address 192.168.0.0 255.255.0.0 then at least one subnet like 192.168.1.0/24 must be in the BGP table (Loc-RIB) and not the router’s routing
table (RIB).
 
If you didn’t specify any additional options to the command, it will create a new prefix in the BGP table, with an empty AS_PATH. It would look like the new prefix was originated in the local AS. The new prefix will automatically have the weight value of 32768 and get a special attribute called ATOMIC_AGGREGATE assigned.
 
BGP attaches another attributecalled AGGREGATOR to the summarized prefix. This attribute specifies the AS number and the BGP router-ID of the aggregating router. Just like the ATOMIC_AGGREGATE, the new attribute is also informational. 

For every aggregate, the BGP process will install an automatic static route to Null0 for the new prefix, in order to prevent routing loops. Keep in mind that the original (specific) prefixes are still advertised, unlike in IGP, where summarization
automatically suppresses more specific prefixes.

Configuration
 
R2:
interface Loopback 100
ip address 10.0.0.1 255.255.255.0
!
interface Loopback 101
ip address 10.0.1.1 255.255.255.0
!
interface Loopback 102
ip address 10.0.2.1 255.255.255.0
!
interface Loopback 103
ip address 10.0.3.1 255.255.255.0
!
router bgp 200
network 10.0.0.0 mask 255.255.255.0
network 10.0.1.0 mask 255.255.255.0
network 10.0.2.0 mask 255.255.255.0
network 10.0.3.0 mask 255.255.255.0
aggregate-address 10.0.0.0 255.255.252.0
 
----------------------------------------------
 
Rack1R2# show ip bgp 10.0.0.0/22
BGP routing table entry for 10.0.0.0/22, version 239
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
        1    2
  Local, (aggregated by 200 150.1.2.2)
    0.0.0.0 from 0.0.0.0 (150.1.2.2)
      Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best
Rack1R2#

Rack1R2#show ip route 10.0.0.0 255.255.252.0
Routing entry for 10.0.0.0/22
  Known via "bgp 200", distance 200, metric 0, type locally generated
  Routing Descriptor Blocks:
  * directly connected, via Null0
      Route metric is 0, traffic share count is 1
      AS Hops 0
Rack1R2#