Thursday, March 14, 2013

BGP - aggregate-address summary-only

In my previous blogtorial we utilized the aggregate-address command to summarize routes in BGP. In this blogtorial we will take a look at one of the optional parameters we can use with the aggregate-address command to manipulate routes.

 R2(config-router)#aggregate-address 11.11.11.0 255.255.255.0 ?  
  advertise-map Set condition to advertise attribute  
  as-set     Generate AS set path information  
  attribute-map Set attributes of aggregate  
  nlri      Nlri aggregate applies to  
  route-map   Set parameters of aggregate  
  summary-only  Filter more specific routes from updates  
  suppress-map  Conditionally filter more specific routes from updates  
  <cr>  

The optional parameter we will look at in this blogtorial is the summary-only option. In my previous post we saw that the aggregate-address command advertised the aggregate route along with the more specific prefixes. What if we wanted to suppress those more specific prefixes and advertise only the summary? summary-only command suppresses all of the more specific prefixes and advertises only the aggregate route to the neighbors.

Before I apply the summary-only parameter three routes are being advertised to R3 -- the specific routes (/25s) and the aggregate route (/24).

 R2#show ip bgp nei 2.2.2.3 advertised-routes  
 BGP table version is 16, local router ID is 2.2.2.2  
 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  
 *> 11.11.11.0/25  1.1.1.1         0       0 1 i  
 *> 11.11.11.0/24  0.0.0.0              32768 i  
 *> 11.11.11.128/25 1.1.1.1         0       0 1 i  
 Total number of prefixes 3  

After I apply the summary-only parameter only the aggregated route is being advertised.

 R2#conf t  
 R2(config)#router bgp 2  
 R2(config-router)#aggregate-address 11.11.11.0 255.255.255.0 summary-only  
 R2(config-router)#^Z  
 R2#show ip bgp nei 2.2.2.3 advertised-routes  
 BGP table version is 18, local router ID is 2.2.2.2  
 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  
 *> 11.11.11.0/24  0.0.0.0              32768 i  
 Total number of prefixes 1  

As you can see on R3 there is only one route in the table instead of all 3 routes.

 R3#show ip bgp 11.11.11.0 255.255.255.0  
 BGP routing table entry for 11.11.11.0/24, version 27  
 Paths: (1 available, best #1, table Default-IP-Routing-Table)  
  Not advertised to any peer  
  2, (aggregated by 2 2.2.2.2)  
   2.2.2.2 from 2.2.2.2 (2.2.2.2)  
    Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best  

Many more articles to come so stay tuned.

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