Last Updated on July 31, 2021 by Admin 1
The exhibit contains portions of RouterA’s BGP configuration and IP routing table.
Which IP network addresses, that were not learned using BGP, will be present in BGP advertisements from RouterA?
- No IGP networks will be advertised because synchronization is disabled.
The auto-summary command can affect which networks, identified by using the network command, will be advertised. Using the existing BGP configuration, the router will not announce the 172.16.16.0/24 subnet. Instead, it will announce the classful address 172.16.0.0/16 when the IP routing table maintained by the IGP contains any subnet of that classful address.
The network command directly affects what network is advertised in BGP. If the network command does not also include a network mask, and if auto-summary is enabled, the classful address of 172.16.0.0/16 is advertised any time that the router learns about a 172.16.0.0 subnet via its Interior Gateway Protocol (IGP), such as OSPF or EIGRP. In the exhibit, the routing table does contain entries of the 172.16.16.0/24 and 172.16.24.0/24 subnets that were learned by using the IGP.
If auto-summary is disabled by using the no auto-summary command, only networks in the routing table that are exact matches to the network commands are advertised. For example, to have the router announce only the 172.16.16.0/24 subnet learned via its IGP, you should alter the network command’s IP address and include the subnet mask as follows:
network 172.16.16.0 mask 255.255.255.0
A combination of network statements and route statements can be used to advertise a subset of networks that exist. Examine the output shown below:
router bgp 68410
network 192.168.24.0 255.255.252.0
neighbor 172.16.8.5 remote-as 68441
ip route 192.168.24.0 255.255.252.0 null 0
The router is configured to advertise a summary route to the network 192.168.24.0 255.255.252.0. Consider the following networks:
If this router was connected to those networks, and received a packet destined for 192.168.25.1, it would successfully route the packet because the summary address (where the summarization is the result of the mask 255.255.252.0) is designed to include all of the subnets above except for 192.168.32.0/24. Therefore, all subnets except 192.168.32.0/24 will be advertised by the network and ip route statements with the summary mask.
Note: Whenever changes are made to a routing policy or to an access list that is used by a routing policy, the change will not be reflected in the routing tables of the receiving routers until the BGP session has been cleared with the clear ip bgp command.
The BGP synchronization rule specifies that networks will not be advertised or used via iBGP unless it also has been learned through an IGP. If synchronization is disabled, iBGP will advertise a network without also learning it through an IGP.
Layer 3 Technologies
Configure and verify eBGP (IPv4 and IPv6 address families)