Chap 12 - Advanced BGP (part 1) Flashcards
What is BGP’s default behavior when multihoming?
To install only the best path to the RIB which means only one path for a network prefix is used when forwarding network traffic to a destination.
What are 3 factors an organization considers when choosing a second service provider?
- Cost
- Circuit availability for remote locations
- Separation of the control plane
What’s the risk of using more than one service provider?
The organization’s AS might become a Transit AS.
How can transit routing be avoided?
By applying outbound BGP route policies that only allow for local BGP routes to be advertised to other AS’s.
How can transit routing be avoided at branch sites?
Configure route filtering at each branch so that branch sites do not advertise what they learn from the WAN.
If transit routing is to be used what are 4 restrictions?
- Data Centers
- Proper routing design accomodates outages
- Bandwidth is sized accordingly
- The routing pattern is bi-directional and predictable
How do Standard ACLs define packets?
Based on the source network only.
What 5 things can Extended ACLs filter on?
- Source
- Destination
- Protocol
- Port
- Other packet attributes
How are Standard ACLs identified?
- Range 1 to 99
- Range 1300 to 1999
- Named
How are Extended ACLs identified?
- Range 100 to 199
- Range 2000 to 2699
- Named
What are the 2 parts that a prefix match specification contains?
- High order bit pattern (network)
- High order bit count (mask)
What does the following prefix match? 10.168.0.0/13 ge 24
- Bit count is 13
- Bit pattern has to match the first 13 bits of 10.168.0.0
- First octet must be 10.
- First 5 bits of the second octet must be binary 10101 to match 168
- Network mask has to be /24 or greater
What is the regex to start a string?
^ Caret
What is the regex to end a string?
$ Dollar sign
What is the regex to match a space.
_ Underscore
What is the regex to match a single character?
[] Match whatever character is inside the brackets
What are the 4 components of a route map?
- Sequence number
- Conditional matching criteria
- Processing action
- Optional action
In a route map what is the Conditional Matching Criteria
Identifies prefix characteristics like network, BGP Path Attribute, next hop, etc.
In a route map what is the Processing action?
Permit or Deny
In a route map what is Optional Action?
Can include modification, addition, or removal of route characteristics
What are 6 Match Options for a route map?
- match as-path (acl)
- match ip address
- match ip address prefix-list (prefix_list)
- match local-preference (local_pref)
- match metric (metric)
- match tag (tag)
In a route map with multiple Match Variables how many variables must match for the prefix to qualify?
Only one
In a route map with multiple Match Options ( statements ) how many must be met for the prefix to qualify?
All of them.
In route maps what are 7 Optional Actions?
- set as-path prepend
- set ip next-hop
- set local-preference
- set metric
- set origin
- set tag
- set weight
In a route map with multiple sequences by default how are they processed?
Route map processing stops when the first sequence has been matched.
In a route map what does the keyword ‘continue’ do?
It allows the route map to continue processing even if there has been a match.
When are inbound route policies executed?
Inbound route policies are executed when the route is in the Adj-RIB-in and before it gets to the Loc-RIB table.
When are outbound route policies executed?
Outbound route policies are executed before the route gets to the Adj-RIB-out.
What are 4 methods of filtering routes inbound or outbound?
- Distribute list
- Prefix list
- AS Path ACL filtering
- Route maps
What does a Distribute List do?
- Prefixes are filtered based on an ACL.
- Has an implicit deny.
What is a Prefix List?
- A prefix list is a list of prefix-matching specifications permitting or denying processed in a top down fashion.
- Has an implicit deny
What is an AS Path ACL and what numbering range does it have?
- It’s a list of regex commands that permit or deny based on the current AS-Path value.
- Has an implicit deny
- 1 - 500
What do Route Maps do?
Conditional matching on a variety of prefix attributes and taking a variety of actions.
Can a BGP neighbor use a distribute list and a prefix list at the same time?
No
What are two methods of clearing BGP connections?
- Hard reset - tears down the BGP session, removes routes from peer, the most disruptive
- Soft reset - invalidates BGP cache, request a fully advertisement from peer
What are BGP Communities?
- 32-bit field used for tagging routes or modifying routing policy on upstream or downstream routers
- Optional Transitive BGP Attribute
- Traverses from AS to AS
What are 3 well-known Communities?
- Internet - identifies routes that should be advertised on the Internet
- No_Advertise - don’t advertise to any BGP peer
- No_Export - only advertise to iBGP peers
What range of numbers are used for Standard Community Lists?
1 to 99
What range of numbers are used for Extended Community Lists?
100 to 500
What is the difference between a Standard Community List and an Extended Community List?
- Standard Community Lists match on well-known communities or private communities.
- Extended Community Lists match using regex commands
For BGP Communities what is the ‘new format’ and how many bits for each number?
Two 16 bit numbers as in 100:12345
Do IOS router display Communities by default?
No
What is the format for a Private BGP Community?
AS number separated with a colon followed by a number assigned by the AS.
What is the command that will change the display of a Community to the new format?
ip bgp community new-format
What 4 events cause BGP to calculate a new best path?
- BGP next-hop reachability change
- Failure of an interface that connects to BGP peer
- Redistribution change
- Reception of a new or removed paths for a route.
What is the first thing BGP looks at when calculating the best path?
Longest match ( /25 would be preferred over a /24)
Display the current BGP table, based on routes that meet a specified AS path regex pattern
show ipv4 unicast bgp regexp
Display the current BGP table, based on routes that meet a specified BGP community
show bgp community
What are the number ranges for standard and extended community lists?
- Standard is 1 - 99
- Extended is 100 - 500
