ALU - BGP Aggregation Part 1

Foreward: An ALU is an Autonomous Learning Unit, you can read this post to learn more about them.

 

Paste the following configuration into R2:

interface Loopback64

 ip address 24.0.64.2 255.255.255.0

interface Loopback65

 ip address 24.0.65.2 255.255.255.0

interface Loopback66

 ip address 24.0.66.2 255.255.255.0

interface Loopback67

 ip address 24.0.67.2 255.255.255.0

 

R2 is in AS 200, R3 is in AS 300.  R2 and R3 should peer via their respective Serial interfaces.  Advertise all four Loopback subnets via BGP.  Advertise a summary address to R3.  Do not advertise the 24.0.65.0/24 subnet to R3.  Your "show ip bgp" output on R2 should match the following output:

 

R2#show ip bgp | begin Network

   Network        Next Hop   Metric LocPrf Weight Path

*> 24.0.64.0/24   0.0.0.0         0         32768 i

*> 24.0.64.0/22   0.0.0.0                   32768 i

s> 24.0.65.0/24   0.0.0.0         0         32768 i

*> 24.0.66.0/24   0.0.0.0         0         32768 i

*> 24.0.67.0/24   0.0.0.0         0         32768 i

 

 

  

 

Highlight below for the solution:

 

Solution:

R2:

router bgp 200

 no synchronization

 bgp log-neighbor-changes

 network 24.0.64.0 mask 255.255.255.0

 network 24.0.65.0 mask 255.255.255.0

 network 24.0.66.0 mask 255.255.255.0

 network 24.0.67.0 mask 255.255.255.0

 aggregate-address 24.0.64.0 255.255.240.0 suppress-map SUPPRESS

 neighbor 1.1.23.3 remote-as 300

 no auto-summary

 

ip prefix-list BGP65 seq 5 permit 24.0.65.0/24

route-map SUPPRESS permit 10

 match ip address prefix-list BGP65

 

 

 

Confirmation:

 

R2#sh ip bgp neighbor 1.1.23.3 advertised-routes

BGP table version is 11, local router ID is 24.0.67.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

 

   Network       Next Hop  Metric LocPrf Weight Path

*> 24.0.64.0/24  0.0.0.0        0         32768 i

*> 24.0.64.0/22  0.0.0.0                  32768 i

*> 24.0.66.0/24  0.0.0.0        0         32768 i

*> 24.0.67.0/24  0.0.0.0        0         32768 i

 

Total number of prefixes 4

 

 

R3:

R3#show ip bgp | begin Network

   Network        Next Hop   Metric LocPrf Weight Path

*> 24.0.64.0/24   1.1.23.2        0             0 200 i

*> 24.0.64.0/22   1.1.23.2        0             0 200 i

*> 24.0.66.0/24   1.1.23.2        0             0 200 i

*> 24.0.67.0/24   1.1.23.2        0             0 200 i

 

 

Explanation:

Unlike most other routing protocols, BGP does not suppress more specific prefixes when summarizing.  Using the "suppress-map" option in the aggregate-address command allows you to selectively suppress certain prefixes specified via a route-map.  Alternatively, you can use the "summary-only" keyword to suppress all prefixes.  As I will detail in my next post, you can use a combination of the "summary-only" option and the "unsuppress-map" on a neighbor statement to suppress all more specific prefixes and selectively permit some through to a certain neighbor.

 

 

DocCD items to reference:

Configuration Guide

aggregate-address

show ip bgp neighbors [advertised-routes]