Thursday, February 12, 2009

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

interface Loopback65

 ip address

interface Loopback66

 ip address

interface Loopback67

 ip address


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 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

*>         0         32768 i

*>                   32768 i

s>         0         32768 i

*>         0         32768 i

*>         0         32768 i





Highlight below for the solution:




router bgp 200

 no synchronization

 bgp log-neighbor-changes

 network mask

 network mask

 network mask

 network mask

 aggregate-address suppress-map SUPPRESS

 neighbor remote-as 300

 no auto-summary


ip prefix-list BGP65 seq 5 permit

route-map SUPPRESS permit 10

 match ip address prefix-list BGP65






R2#sh ip bgp neighbor advertised-routes

BGP table version is 11, local router ID is

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

*>        0         32768 i

*>                  32768 i

*>        0         32768 i

*>        0         32768 i


Total number of prefixes 4




R3#show ip bgp | begin Network

   Network        Next Hop   Metric LocPrf Weight Path

*>        0             0 200 i

*>        0             0 200 i

*>        0             0 200 i

*>        0             0 200 i




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


show ip bgp neighbors [advertised-routes]

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