7 - More BGP & Policy

Question 1

AS-Path Attribute

Answer 1

Sequence of ASes that have been traversed. Used for loop detection

Question 2

Does shorter path always mean shorter

Answer 2

No, there may be an unknown internal structure which causes the route to be longer

Question 3

Next Hop attribute meaning

Answer 3

Next hop to reach a network. Usually a local network is the next hop in an eBGP session.

Question 4

Next Hop Attribute

Recursive route lookup

How do BGP and iGP interact in this case?

Answer 4

BGP looks into IGP to find next hop info
BGP is not permitted to use a BGP route as the next hop

Question 5

What is Community Attribute stored as?

bits and the format of those bits

Answer 5

32 bit number (two 16 bits separated by colon)

Question 6

What is the first half of the Community attribute?

Answer 6

Usually an AS number

Question 7

Who determines the meaning of the community attribute’s second half?

Answer 7

ISP determines meaning of second half

Question 8

What messages carry community attribute?

Answer 8

BGP protocol messages

Question 9

Why use policies?

Answer 9

Steer traffic through preferred paths

Prefix filtering

Enforce customer ISP agreements

Question 10

What is the decision process for updates in a router (considering policies)

Answer 10

All inputs have the in-policy filter applied.
Best path is selected
Final update has out-policy filter

Question 11

BGP “in” process

Answer 11

receive path info from peers
results of selection placed in BGP table
“best path” flagged

Question 12

BGP “out” process

Answer 12

builds update using info from RIB
may modify update based on config

Question 13

What are Prefix Lists used for?

Answer 13

Filtering routes.

Can be combined with route maps

Question 14

Prefix Lists: Notation?

Answer 14

CIDR Address/mask notation

Question 15

Prefix list: ordering

Answer 15

First relevant rule wins

Question 16

Prefix list: exclusive or not?

Answer 16

Implicit “deny all” at the end of them.

Like Access Control Lists

Question 17

Prefix lists: IPv6, any different to IPv4?

Answer 17

No

Question 18

Filter list:

Answer 18

Regular expression match against input string

Question 19

Route Maps

Answer 19

Program for Cisco IOS.
if else conditions

Question 20

Route Map match clauses

Answer 20

AS-path
Community
IP Address

Question 21

Set Clauses

Answer 21

AS-path prepend
Community
Local-preference
…