Answers to Chapter 1 Review Questions

■■■ '-Äßswer:

Destination Address

Next-Hop Address

172.20.3.5

172.20.6.2

172.20.1J7

172.20.5.2

172.21.255.254

172.20.5.2

mi6J0.5t

by an by an

21 What are the four categories of BGP path attributes?

Answer: 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?

Answer: 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?

Answer: A&JPATH types are AS_SEQUENCE AS_CONFED_SEQUENCE ASjSET/and AS_CONFED_SET AS_SEQUENCE is an ordered set of AS nujnbers 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 NEXTJHOP attribute?

Answer: 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?

Answer: 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?

Answer: 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?

Answer: 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?

Answer: 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?

Answer: 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 LOC AL_PREF of 200 and two AS numbers in the AS_PATH Assuming no other differences which path will the router choose?

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

31 What is route dampening?

Answer: 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

Answer: 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?

Answer: 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?

Answer: 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?

Answer* 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?

Answer* 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?

Answer* 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 ORIGINATORJD and the CLUSTER_LIST path attributes?

Answer* The ORIGINATORJD and CLUSTER_LIST attributes prevent routing loops when route reflectors are being used.

39 What is a BGP confederation?

Answer* 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? Answer* Yes.

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

Answer* next-hop-self tells a router to change the NEXTJHOP 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.