Chapter 16

"Do I Know This Already?"

1.	A and B
2.	C
3.	A and E
4.	B
5.	D
6.	B
7.	A and C
8.	B
9.	B
10.	C
11.	C

12.	B
13.	A
14.	B

Q&A

BGP attributes include weight, local preference, AS-path, Origin, MED, Community, and Cluster ID.

The Cisco-proprietary weight attribute selects the exit when there are multiple paths to the same destination. The higher the weight value, the better the path. Weight is local to the router and the attribute is not propagated to other routers. Weight values range from zero to 65,535. The default weight is zero, unless this router sources the route (in which case the weight defaults to 32,768).

Local preference is used inside an AS to select outbound paths. It has a range from 0 to 4,294,967,295 and higher values are preferred in selecting routes. The default value is 100.

The AS-Path attribute is a list of AS that the route has passed through (the shortest path is preferred).

Origin describes how the route was learned. It can be i (native to BGP), e (EGP), or ? (redistributed).

Multi-exit discriminator (MED) is advertised to external neighbors to try to influence path selection into an AS. MED is an optional, nontransitive attribute and lower MED is preferred. The default MED is zero.

Router(config-router)#neighbor {ip-address peer-group-name} weight weight

Router(config-router)#bgp default local-preference value

Answer:

router bgp AS
  neighbor ip-address route-map question9 out
access-list 9 permit 192.168.25.0 0.0.0.255
route-map question9 permit 10
  match ip address 9
  set metric 90
route-map question9 permit 20

10.

Order	Preference	Description
0. Synchronized	TRUE	Use only routes that meet the synchronization requirement
1. Weight	Highest	Administrative override
2. Local Preference	Highest	Used internally to pick path out of AS
3. Self Originated	TRUE	Used to prefer paths originated on this router
4. AS-Path	Shortest	Minimize AS-hops
5. Origin	i<?	Prefer stability
6. MED	Lowest	Used external to come in
7. External	EBGP<IBGP	Hot Potato routing
8. IGP Cost	Lowest	Look for more information
9. EBGP Peering	Oldest	Prefer stability
10. RID	Lowest	External path preferred over internal path