Last Updated on August 7, 2021 by Admin 3
You administer the networks shown above.
RouterA is connected to network A, RouterB is connected to network B, and so on. RouterB and RouterD are iBGP peers of RouterC; RouterE and RouterF are eBGP peers of RouterC. RouterA and RouterC are OSPF neighbors.
RouterC, which is not configured as a route reflector, receives routes from all of the other routers on the network. You have issued the network command on each router to advertise their respective networks. You have also issued the redistribute command on RouterC to redistribute the OSPF routes from RouterA into BGP.
Routes to which of the following networks will RouterC advertise to RouterF? (Select the best answer.)
- only network C
- only networks B, C, and D
- only networks A, B, C, and D
- only networks A, B, C, D, and E
- networks A, B, C, D, E, and F
RouterC will advertise only networks A, B, C, D, and E to RouterF. RouterC and RouterF are external Border Gateway Protocol (eBGP) peers, which are BGP routers that belong to different autonomous systems (ASes). An eBGP peer advertises the following routes to another eBGP peer:
-Routes learned through internal BGP (iBGP)
-Routes learned through eBGP
-Routes learned through redistribution
-Routes originated by a network statement
The only route that RouterC will not advertise to RouterF is network F, because RouterC originally learned of the route from RouterF. When RouterF advertises network F to RouterC, RouterF adds the AS number (ASN) to the AS_PATH. Routes with an AS_PATH that contains the ASN of a BGP peer are not advertised back to that peer.
If RouterF were in AS 100, RouterF and RouterC would be iBGP peers. The BGP split horizon rule states that routes learned through iBGP are not advertised to iBGP peers. Therefore, an iBGP peer advertises the following routes to another iBGP peer:
-Routes learned through eBGP
-Routes learned through redistribution
-Routes originated by a network statement
Because iBGP routes are not advertised to iBGP peers, one of the following actions must be taken to enable routers running iBGP to communicate:
-Configure a full mesh.
-Configure a confederation.
-Configure a route reflector.
A full mesh configuration enables each router to learn each iBGP route independently without passing through a neighbor. However, a full mesh configuration requires the most administrative effort to configure. A confederation enables an AS to be divided into discrete units, each of which acts like a separate AS. Within each confederation, the routers must be fully meshed unless a route reflector is established. A route reflector can be used to pass iBGP routes between iBGP routers, eliminating the need for a full mesh configuration. However, it is important to note that route reflectors advertise best paths only to route reflector clients. Additionally, if multiple paths exist, a route reflector will always advertise the exit point that is closest to the route reflector.