BGP Q&A

1: What is the most important difference between BGP-4 and earlier versions of BGP?

A: BGP-4 is classless. Earlier versions are classful.

2: What two problems was CIDR developed to alleviate?

A: CIDR was developed to alleviate the explosion of Internet routing tables and to slow the depletion of Class B network addresses.

3: What is the difference between classful and classless IP routers?

A: Classful IP routers perform routing table lookups on the major class network address first and then match the subnet. Classless IP routers ignore the  class of the destination address and try to make a longest match on the address prefix.

4: What is the difference between classful and classless IP routing protocols?

A: Classful IP routing protocols advertise only a network or subnet address, without any information about the prefix length. As a result, routers receiving the advertisement must make certain assumptions about the address prefix. Classless IP routing protocols include information that allows the receiving router to parse the address prefix. As a result, VLSM and summarization are possible with classless protocols.

5: Given the addresses 172.17.208.0/23, 172.17.210.0/23, 172.17.212.0/23, and 172.17.214.0/23,

summarize the addresses with a single aggregate, using the longest possible address mask.

A: 172.17.208.0/21

6: What is an address prefix?

A: An IP address prefix is the part of an IP address that a router considers when making routing decisions. In a classful environment, the prefix is a major class network address or one of its subnets. In a classless environment, the prefix can be any number of leading bits in the 32-bit address.

8: Explain how summarization helps hide network instabilities.

A: Member addresses, or destination addresses that are summarized by an aggregate address, are not advertised past the summarization point. So if the state of one of the member addresses changes,the change is not advertised past the summarization point.

9: Explain how summarization can cause asymmetric traffic patterns.

A: Summarization hides the details of the internetwork behind the summarization point. If a summary address is advertised by more than one router, the routers beyond the summarization points select only the closest summarizing router.

10: Is asymmetric traffic undesirable?

A: The answer is subjective. Asymmetric traffic can make baselining and troubleshooting more difficult, and if the internetwork is geographically large, delay-sensitive traffic can be affected. On the other hand, the benefits of summarization might outweigh these problems.

11: What is a NAP?

A: A network access point is a LAN or switch through which service providers may interconnect. From the perspective of Internet traffic flow, NAPs are the hierarchically highest points in the Internet topology.

12: What is a route server?

A: A route server is a server with which routers may peer via some routing protocol. Each router sends its updates to the route server rather than to the other peers. The route server applies the appropriate routing policies and then sends the updates to the other peers. Route servers are useful when many routers must peer across a common data link, as in a NAP, by reducing the number of peering sessions each router must establish. This can be especially

important if the routers are using a unicast protocol such as BGP, in which a separate packet must be sent to each peer. A route server is not a router, because it performs no packet forwarding.

13: What is a provider-independent address space, and why can it be advantageous to have one?

A: A provider-independent address space is assigned by the regional IP address registry rather than as part of a service provider's CIDR block. It proves useful if an AS is multihomed to different service providers. It is also useful because it is portable. That is, the owner of the address space can change

ISPs without having to re-address.

14: Why can it be a problem to have a /21 provider-independent address space?

A: Some national service providers do not accept IP prefixes longer than a /19. As a result, a /21 might not be advertised to all parts of the Internet.

15: What is a routing policy?

A: A routing policy is a predefined set of rules for handling incoming and outgoing routes. Typical tools for setting routing policies are redistribution, route filters, and route maps.

16: What is the underlying protocol that BGP uses to reliably connect to its neighbors?

A: BGP uses TCP port 179.

17: What are the four BGP message types, and how is each one used?

A: The four BGP message types are Open, Keepalive, Update, and Notification. Open messages are used to initially identify a BGP speaker to its neighbor and begin a peering session. Keepalives maintain the peer connection. Updates are used to advertise routes, and Notification messages advise peers of errors.

18: In what state or states can BGP peers exchange Update messages?

A: BGP peers can exchange Update messages only when both are in the Established state.

19: What is NLRI?

A: Network Layer Reachability Information is the IP address prefix or prefixes advertised in a BGP

Update.

20: What is a path attribute?

A: A path attribute is a characteristic of a BGP route.

21: What are the four categories of BGP path attributes?

A: The four categories of BGP path attributes are Well-known Mandatory, Well-known Discretionary,Optional Transitive, and Optional Nontransitive.

22: What is the purpose of the AS_PATH attribute?

A: The AS_PATH attribute describes the AS numbers that a received Update has crossed after it left the originating router. This information can be used to determine the shortest inter-AS path, and it is also used to detect routing loops.

23: What are the different types of AS_PATH?

A: AS_PATH types are AS_SEQUENCE, AS_CONFED_SEQUENCE, AS_SET, and AS_CONFED_SET.

AS_SEQUENCE is an ordered set of AS numbers, and AS_SET is an unordered set of AS numbers.

AS_CONFED_SEQUENCE and AS_CONFED_SET are the same as AS_SEQUENCE and AS_SET but are used only within BGP confederations.

24: What is the purpose of the NEXT_HOP attribute?

A: The NEXT_HOP attribute describes the IP address of the next-hop router that packets should be forwarded to in order to reach the destination advertised as the NLRI in a BGP Update.

25: What is the purpose of the LOCAL_PREF attribute?

A: If multiple IBGP speakers are advertising the same route within an AS, the LOCAL_PREF attribute can be used to identify the preferred route. The higher the LOCAL_PREF value, the more preferred the route.

26: What is the purpose of the MULTI_EXIT_DISC attribute?

A: When multiple links exist between two autonomous systems, EBGP speakers can use the MED to inform the neighboring AS of the preferred link for incoming traffic.

27: What attribute or attributes are useful if a BGP speaker originates an aggregate route?

A: THE ATOMIC_AGGREGATE informs downstream routers that a loss of route information has occurred due to aggregation. The AGGREGATOR attribute identifies the router that originated the aggregate.

28: What is a BGP administrative weight?

A: A BGP administrative weight is a Cisco-specific parameter that can be assigned to routes within a single router. The higher the weight, the more preferable the route. Weights are local to the router and are not advertised to peers.

29: Given an EBGP route and an IBGP route to the same destination, which route will a BGP router

prefer?

A: If the weights, LOCAL_PREFs, AS_PATH lengths, ORIGIN codes, and MEDs are equal, EBGP routes are preferred over IBGP routes.

30: A router has two IBGP routes to the same destination. Path A has a LOCAL_PREF of 300 and three AS numbers in the AS_PATH. Path B has a LOCAL_PREF of 200 and two AS numbers in the AS_PATH. Assuming no other differences, which path will the router choose?

A: LOCAL_PREF has a higher priority in the BGP decision process than AS_PATH, so path A is chosen.

31: What is route dampening?

A: Route dampening is a mechanism by which BGP routes are assigned a penalty for changing state. The more often the state changes (the route flaps), the greater the accumulated penalties. If the penalties exceed a certain threshold, the route is suppressed for a time. As a result, unstable routes have less adverse effect on the BGP internetwork.

32: Define the penalty, suppress limit, reuse limit, and half-life as they apply to route dampening.

A: The penalty is a value assigned to a route by the route-dampening mechanism each time the route changes state. The suppress limit is a threshold that, if exceeded by a route's accumulated penalties, signifies that the route should not be advertised. Reuse limit is a threshold that, if a suppressed route's accumulated penalties falls below it, signifies that the route can again be advertised. The half-life is the rate at which a route's accumulated penalties are reduced. At the end of each half-life, the penalty is reduced by half.

33: What is IGP synchronization, and why is it important?

A: IGP synchronization is a rule whereby a BGP router cannot advertise a transit route to an EBGP peer unless the route is found in the IGP routing table. If a BGP router forwards a transit packet to an IBGP peer via an IGP router, and the IGP router does not know the route, the packet is dropped.

34: Under what circumstances can you safely disable IGP synchronization?

A: You can safely turn off IGP synchronization if the IBGP peers in an AS are fully meshed, or when the AS is not a transit AS.

35: What is a BGP peer group?

A: A BGP peer group is a group of BGP peers that have been identified on a single router to share common routing policies. Peer groups simplify configuration by allowing route policies to be applied to the group rather than to each individual member.

36: What is a BGP community?

A: A BGP community is a group of routes that share common routing policies. They work by setting a common COMMUNITY attribute in the routes; peers receiving those routes can recognize the COMMUNITY attribute and apply the appropriate policy.

37: What is a route reflector? What is a route reflection client? What is a route reflection cluster?

A: A route reflector is similar to a route server in that it permits IBGP routers to peer with it rather than with each other. Routes from one peer are advertised, or reflected, to the other peers. As a result, the number of peering sessions is reduced from what would be required if the IBGP peers were fully meshed. Route reflectors differ from route servers in that the route reflector is also a router. A route reflection client is an IBGP router that has  peered with a route reflector. A route reflection cluster is a route reflector and its clients. A cluster can have more than one route reflector, but all the clients in the cluster must be peered with all the route reflectors in the cluster.

38: What is the purpose of the ORIGINATOR_ID and the CLUSTER_LIST path attributes?

A: The ORIGINATOR_ID and CLUSTER_LIST attributes prevent routing loops when route reflectors are being used.

39: What is a BGP confederation?

A: A BGP confederation is a large AS that has been subdivided into a group of smaller autonomous systems for easier manageability.

40: Can route reflectors be used within confederations?

A: Yes.

41: What is the purpose of the next-hop-self function? Are there any reasonable alternatives to using this function?

A: next-hop-self tells a router to change the NEXT_HOP attribute of routes received from an external peer to its own IP address. This function is used when the IGP has no knowledge of the external next-hop address. An alternative method is to run the IGP passively on the external link so that it knows the subnet on which the external next-hop address resides.