Route-map Nesting

Route-maps can be used to filter incoming updates from a BGP peer. The match route-map command within a route-map allows another route-map to be called as a subroutine. The nested route-map is evaluated, and if it returns permit, it is treated as a match; if it returns deny, it is treated as a no-match. A route-map must exist to be nested.

Topology

In the below example, router R2 sets the metric to 300 for all the routes except those with the prefix 20.1.2.0/23. Router R1 then filters incoming route advertisements by matching metric 300 and matching the nested route-map CHILD.

In the CHILD route-map, the sequence 10 denies prefix 10.1.1.0/24, while sequence 20 denies prefix 1.1.1.0/24 and permits all other routes. Since 20.1.2.0/23 is the only route from R2 with a metric different from 300, it is denied.

Figure 25. Route-map Nesting

The nested route-map is referred to only by the name, so the first route-map in the sequence with the same name will be evaluated, and if no match is found, the next in the sequence will be evaluated. This continues until a match is found or the sequence ends. A maximum of 8 route-maps can be nested within a route-map instance at the same time. Note that more than 4 levels of nesting are not allowed.

Configure Route-map

R1

R1#configure terminal

Enter configure mode.

R1(config)#ip prefix-list ABC

Create an entry in the prefix-list. The ABC parameter is the name of the prefix-list.

R1(config-ip-prefix-list)#seq 5 deny 1.1.1.0/24 ge 26 le 28

The sequence number of the prefix entry is specified as 5. deny specifies that the packets must be rejected.

The minimum and maximum prefix lengths to be matched are 26 and 28.

R1(config-ip-prefix-list)#seq 10 permit any

The sequence number of the prefix-list entry is specified as 10. The permit parameter any denotes accepting all packets of any length.

R1(config-ip-prefix-list)#exit

Exit prefix-list mode.

R1(config)#ip prefix-list ABC2

Create an entry in the prefix-list. The ABC2 parameter is thename of the prefix-list.

R1(config-ip-prefix-list)#seq 5 permit 10.1.1.0/24

The sequence number of the prefix entry is specified as 5. The parameter permit specifies that the packets with the prefix 10.1.1.0/24 will be accepted.

R1(config-ip-prefix-list)#exit

Exit prefix-list mode

R1(config)#route-map PARENT permit 10

Create the PARENT route-map.

R1(config-route-map)#match route-map CHILD

Match route-map CHILD as a subroutine. If a match is returned, this match condition is satisfied.

R1(config-route-map)#match metric 300

 Match only the routes with the metric 300.

R1(config-route-map)#exit

Exit Route-map mode and return to Configure mode.

R1(config)#route-map CHILD deny 10

Create an entry with the sequence number 10 and a deny action for the route-map that is being nested.

R1(config-route-map)#match ip address prefix-list ABC2

 Set the match criteria based on the prefix-list ABC2. As the prefix 10.1.1.0/24 will be a match, and the action of the route-map is “deny”, the routes with the prefix 10.1.1.0/24 will be denied. If an entry does not have a match, the next route-map in the sequence will be evaluated.

R1(config-route-map)#exit

Exit Route-map mode and return to Configure mode.

R1(config)#route-map CHILD permit 20

Create an entry with the sequence number 20 and a permit action for the route-map that is being nested.

R1(config-route-map)#match ip address prefix-list ABC

Set the match criteria based on the prefix-list ABC. Every prefix that is matched by the prefix-list will be accepted.

R1(config-route-map)#exit

Exit Route-map mode and return to Configure mode.

R1(config)#router bgp 10

Define the routing process, and establish a TCP session. The number 10 specifies the AS number of R1.

R1(config-router)#neighbor 192.168.10.11 remote-as 11

Define BGP neighbors, and establish a TCP session. 192.168.10.11 is the IP address of the neighbor (R2), and 11 is the neighbor's AS number.

R1(config-router)#address-family ipv4 unicast

Enter address-family ipv4 unicast mode.

R1(config-router-af)#neighbor 192.168.10.11 activate

Activate the neighbor in the IPv4 address family.

R1(config-router-af)#neighbor 192.168.10.11 route-map PARENT in

Apply a route map to routes. 192.168.10.11 specifies the IP address of the BGP neighbor. The name of the route map is PARENT, and the in parameter specifies that the access list applies to incoming advertisements.

R1(config-router-af)#exit-address-family

 Exit address-family mode.

R1(config-router)#commit

Commit the candidate configuration to the running configuration.

R2

R2(config)#ip prefix-list PLIST

Create an entry in the prefix-list. The name of the prefix-list is PLIST.

R2(config-ip-prefix-list)#seq 5 deny 20.1.2.0/23

The sequence number or position of the prefix entry is specified as 5.

deny specifies that the packets with the prefix 20.1.2.0/23 are to be rejected.

R2(config-ip-prefix-list)#seq 10 permit any

The sequence number or position of this prefix entry is specified as 10. The permit parameter any specifies that all packets of any length are accepted.

R2(config-ip-prefix-list)#exit

Exit the prefix-list mode .

R2(config)#route-map TEST permit 10

Enter Route-map mode to set the match operation.

R2(config-route-map)#match ip address prefix-list PLIST

Set the match criteria based on the prefix-list PLIST

R2(config-route-map)#set metric 300

Configure the set action. If the match condition is satisfied, the metric is set to 300.

R2(config-route-map)#exit

Exit Route-map mode and return to Configure mode.

R2 (config)#interface lo

Enter loopback interface mode.

R2(config-if)#ip address 1.1.1.1/27 secondary

Specify a secondary interface address.

R2(config-if)#ip address 1.1.2.1/24 secondary

Specify a secondary interface address

R2(config-if)#ip address 10.1.1.1/24 secondary

 Specify a secondary interface address

R2(config-if)#ip address 20.1.1.1/24 secondary

 Specify a secondary interface address

R2(config-if)#ip address 20.1.2.1/23 secondary

 Specify a secondary interface address

R2(config-if)#ip address 30.1.1.1/28 secondary

 Specify a secondary interface address

R2(config-if)#exit

Exit loopback interface mode.

R2(config)#router bgp 11

Define the routing process, and establish a TCP session. The number 11 specifies the AS number of R2.

R2(config-router)#neighbor 192.168.10.10 remote-as 10

Define BGP neighbors, and establish a TCP session. 192.168.10.10 is the IP address of the neighbor (R1), and 10 is the neighbor’s AS number.

R2(config-router)#address-family ipv4 unicast

Enter address-family IPv4 unicast mode

R2(config-router-af)#neighbor 192.168.10.10 activate

Activate the neighbor in the IPv4 address family.

R2(config-router-af)#neighbor 192.168.10.10 route-map TEST out

Apply a route map to routes. 192.168.10.10 specifies the IP address of the BGP neighbor. The TEST parameter is the name of the route map, and out specifies that the route-map applies to outgoing advertisements.
R2(config-router-af)#network 1.1.1.0/27 Specify the network to be advertised by the BGP routing process.
R2(config-router-af)#network 1.1.2.0/24 Specify the network to be advertised by the BGP routing process.

R2(config-router-af)#network 10.1.1.0/24

 Specify the network to be advertised by the BGP routing process.

R2(config-router-af)#network 20.1.1.0/24

 Specify the network to be advertised by the BGP routing process.

R2(config-router-af)#network 20.1.2.0/23

 Specify the network to be advertised by the BGP routing process.

R2(config-router-af)#network 30.1.1.0/28

 Specify the network to be advertised by the BGP routing process.

R2(config-router-af)#exit-address-family

Exit router mode.

R2(config-router)#commit

Commit the candidate configuration to the running configuration.

Validation

Here is the validation:

Copy 
#sh ip bgp
BGP table version is 2, local router ID is 172.16.0.14
Status codes: s suppressed, d damped, h history, a add-path, b back-up, * valid,
 > best, i - internal,
              l - labeled, S Stale, x-EVPN 
Origin codes: i - IGP, e - EGP, ? - incomplete
Description : Ext-Color - Extended community color

    Network          Next Hop            Metric    LocPrf   Weight Path   Ext-Co
lor
*>   1.1.2.0/24       192.168.10.11        300      100       0   11 i        -
*>   20.1.1.0/24      192.168.10.11        300      100       0   11 i        -
*>   30.1.1.0/28      192.168.10.11        300      100       0   11 i        -

Total number of prefixes 3