Basics of BGP

1:If the weight attribute is used, is a higher or lower weight preferred?

** The weight attribute is proprietary to Cisco and is determined locally on the router. The preference in selection is to the highest weight on the router.

2: What is an alternative to using BGP as the method of connection to the ISP?

** The alternative method, suggested by Cisco, is to use a default route into the ISP and for the ISP to configure static routes into your autonomous system.

3: What does the command clear ip bgp * achieve, and why should it be used cautiously?

** The command resets BGP peer associations, clears the BGP routing table, and reestablishes BGP connections to the neighbors. It should be used cautiously because the loss of connections will drop packets.

4: Give three situations in which you should not use BGP to connect to the Internet.

** It is ill-advised to use BGP in certain conditions:

- When the company has only one connection into the Internet

- When there are limited resources on the network

- When the user is not familiar with BGP configurations or policy-based routing

- When the routers do not understand or need to have greater understanding of each other

5: Explain the use of the command neighbor 10.10.10.10 remote-as 250.

** The command tells the router the IP address of the BGP neighbor and the autonomous system to which the neighbor belongs. This information allows the router to create a TCP session with the neighboring router and exchange BGP routing information.

6: Explain briefly the purpose of the community attribute.

** The community attribute is used to identify prefixes that have some common similarity, without regard to geographic location.

7: In the route selection process, place the following in order of preference: origin code, weight, local preference, and MED. State the method of selection for the individual attributes themselves.

** The correct order is: highest weight, highest local preference, lowest origin code, and lowest MED.

8: What is a mandatory attribute?

** A mandatory attribute is an attribute that is well known. It contains information required in BGP messages in order to maintain the BGP network. These attributes are required and therefore recognized by all BGP implementations.

9: What type of routing protocol is BGP classified as, and what does this mean?

** BGP is classified as an External Gateway Protocol (EGP), as opposed to OSPF, EIGRP, RIP, and so on, which are known as Interior Gateway Protocols (IGPs). If required, BGP can send a summary of the networks known within an organization to maximize security and minimize bandwidth overhead. It is used to convey routing information between autonomous systems.

10: Explain how static routes can be used as an alternative to configuring BGP to connect to another autonomous system.

** A static route could be configured in conjunction with a default route pointing in the other direction. This is a simple solution when there is not a great deal of information to be exchanged between the autonomous systems. A common example is an organization connecting to an ISP.

A static route is a route that has been manually configured. It has the lowest administrative distance of either 0 or 1 by default, depending on the configuration. This means that it will always take precedence over dynamically learned routes and that it must be redistributed into a routing protocol for other routers to make use of it.

11: Explain how a default route can be used as an alternative to configuring BGP to connect to another autonomous system.

** A default route is a route used when there is no entry for the remote network in the routing table. It is used to connect to the Internet and other routing domains when it is not practical to know all the available networks. It is sufficient to have an exit point from your network identified.

12: State two attributes of BGP.

** The following are BGP attributes: next hop, AS_Path, local preference, Multiple Exit Discriminator (MED), community, atomic aggregate, aggregator, and origin.

13: State four message types of BGP.

** The four message types of BGP are:

- Update messages: Contain paths to destination networks and their attributes. Routes that are no longer available or withdrawn routes are included in updates.


- Open messages: Used to establish connections with peers.


- Keepalives: Sent periodically between peers to maintain connections and verify paths held by the router sending the keepalive.


- Notification: Used to inform the receiving router of errors.


14: What is policy-based routing?

** Policy-based routing is the means by which traffic can be forced to take a different route from that determined by the dynamic routing protocol. It is defined on a hop-by-hop basis in that the policy is stated on a router and determines which next hop will be used. The decisions can be based on source and/or destination.

15: What do the letters MED represent? Give a brief explanation of what MED does.

** The Multiple Exit Discriminator is an optional, nontransitive BGP attribute. It is sent only to external BGP peers and is used to influence routers in another autonomous system on the path to take into the autonomous system if multiple paths are available. The lower the value of the attribute, the higher the likelihood that the path will be chosen. By default, a router compares only the MED from routers that are in the same autonomous system as each other but in a different autonomous system from the determining router.

16: What is a community in BGP?

** A community is a group of networks that share a common property. The commonality is defined by the optional transitive attribute, and it has no physical boundaries. A network can be a member of more than one community. When the community is defined, decisions or filtering can be made based on the group instead of the individual.

17: Give two reasons why BGP peer groups are useful.

** A peer group is a group of BGP neighbors that share characteristics. The use of peer groups simplifies the configuration of BGP because one configuration effectively configures every router in the peer group. They are also more efficient because updates are generated once per peer group instead of on a per-router (peer) basis, reducing the resources required to support BGP.

18: What is the difference between a peer and a neighbor?

** In external BGP, there is no difference between a neighbor and a peer. A peer is the BGP term for a neighbor. Both terms refer to a router that is directly connected, with which routing information is exchanged. In iBGP, these routers are not necessarily physically adjacent, but they are the next logical hop router running BGP.

19: In BGP, describe the purpose of the network command.

** The network command permits BGP to advertise a network if it is present in the routing table. It is not responsible for starting the BGP process on an interface; instead, it identifies which networks the router originates.

20: Explain the command neighbor {ip-address | peer-group-name} next-hop-self.

** On a broadcast multiaccess network such as Ethernet, the next-hop address is the IP address of the advertising router. This command forces BGP to advertise itself as the next-hop router instead of letting the protocol determine the address to be used. This avoids problems seen on NBMA networks or nonmeshed environments, such as X.25 and Frame Relay.