Basing an Aggregate on Selected More Specific Routes
In some situations, you might want to advertise an aggregate with the AS_SET but do noi want the aggregate to inherit all the attributes of all the aggregated routes. In Figure 3-14, Sugarbush receives all the routes from AS 100 and AS 500 and advertises an aggregate to Burke.
Figure * 14 lor Sugarbush to Advertise the Aggregate with an AS_SET, the Aggregate Must Not Inherit the NO_EXPORT COMMUNITY Attribute from 192.168.197.0/24
Figure * 14 lor Sugarbush to Advertise the Aggregate with an AS_SET, the Aggregate Must Not Inherit the NO_EXPORT COMMUNITY Attribute from 192.168.197.0/24
The problem with the setup in Figure 3-14 is that AS 500 is advertising 192.168.197.0/24 with the COMMUNITY attribute of NO_EXPORT. When Sugarbush uses the AS_SET option, the aggregate inherits the NO_EXPORT attribute, as demonstrated in Example 3-68. Note that the NO_EXPORT attribute is given to the aggregate locally, not added to the advertisement of the aggregate. As a result, Sugarbush acts on the attribute and does not advertise the aggregate route.
Example 3-68 The show ip bgp community no-export Command Displays All Routes with the NOJEXPORT
COMMUNITY Attribute; in This Case, the Aggregate Route Has Inherited the Attribute from One of the Aggregated Addresses, 192.168.197.0/24
Sugarbush#show ip bgp community no-export
BGP table version is 19, local router ID is 172.20.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 192.168.192.0/21 0.0.0.0 32768 {100,500} ?
s> 192.168.197.0 192.168.1.1 0 0 500 i Sugarbush#
The last option of the aggregate-address command to be discussed, advertise-map, enables you to choose the routes upon which to base the aggregate. In the case of the internetwork in Figure 3-14, if Sugarbush does not consider 192.168.197.0/24 when forming the aggregate, the aggregate does not inherit that route's attributes. Example 3-69 shows the configuration for Sugarbush using the advertise-map option of the aggregate-address command.
Example 3-69 Configuring Sugarbush to Choose the Routes Upon Which to Base the Aggregate router bgp 200
aggregate-address 192. .192.0 255,255*248.0 saar»ary-oii2,y advertise-map
neighbor 192.168.1.1 remote-as 500 neighbor 192.168.1.250 remote-as 300 neighbor 192.168.1.254 remote-as 100
i access-list 1 deny 192.168.197.0 access-list 1 permit any i route-map ALL0W_R0UTE permit 10 match ip address 1
The advertise-map option in the configuration in Example 3-69 points to a route map named ALLOW_ROUTE, which identifies the more-specific routes on which the aggregate is based. The route map in turn points to access list 1, which rejects 192.168.197.0 and permits all other routes. Because Sugarbush now ignores 192.168.197.0/24 when forming the aggregate, the aggregate route does not inherit the NO_EXPORT attribute, as demonstrated in Example 3-70.
Example 3-70 After Sugarbush Is Reconfigured with the advertise-map Option, the Aggregate Route No Longer Has the NO_EXPORT Attribute
Sugarbush#show ip bgp community no-export
BGP table version is 18, local router ID is 172.20.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path s> 192.168.197.0 192.168.1.1 0 0 500 i
Sugarbush#
Limiting the more-specific prefixes on which an aggregate is based can present some vulnerabilities. In the network of Figure 3-14 and the associated configurations, suppose the link between Stowe and Sugarbush fails. The aggregate is based only on the prefixes from AS 100 and so is no longer advertised. As a result, the destinations within AS 500 are no longer reachable from AS 300 and beyond.
Continue reading here: Managing BGP Connections
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