BGP: Regular Expressions for AS-PATH Filtering

Symbol	Description
|	logical OR
.	match any
[x..y]	match one in range
^	match beginning of string
$	match end of string
_	match any delimiter/white space
( )	group as a single atom
*	match 0 or more instances of previous atom
?	match 0 or 1 instance of previous atom
+	match 1 or more instances of previous atom
\	escape character; if followed by a number n, points to the nth atom

Example 1: Advertise routes with empty AS-PATH (internal routes)

router bgp 123  neighbor 5.6.7.8 remote-as 387  neighbor 5.6.7.8 filter-list 1 out ! ip as-path access-list 1 permit ^$ !match "blank" atom at the "start" and "end" of string !

Example 2: Accept only default routes, preferring primary route based on AS PATH:

	AS387 (primary ISP)
	/
AS123
	\
	AS462 (backup ISP)

router bgp 123  neighbor 1.2.3.4 remote-as 462  neighbor 1.2.3.4 route-map FILTER in  neighbor 5.6.7.8 remote-as 387  neighbor 5.6.7.8 route-map FILTER in ! route-map FILTER permit 10 ! default routes from primary ISP (AS 387) accepted are preferred (larger weight)  match ip prefix-list DEFAULT_ONLY  match as-path 10  set weight 150 ! route-map FILTER permit 20 ! default routes from backup ISP are accepted, with lower preference than primary ISP routes  match ip prefix-list DEFAULT_ONLY  set weight 100 ! ip as-path access-list 10 permit _387$ ip prefix-list DEFAULT_ONLY seq 10 permit 0.0.0.0/0 !

Example 3: AS PATH Filtering with AS Path Prepending:

- customer in AS123 is performing AS-PATH pre-pending

AS123
10.0.0.1	\
	\
	AS462

router bgp 387  neighbor 10.0.0.1 remote-as 213  neighbor 10.0.0.1 filter-list 10 in ! ip as-path access-list 10 permit ^123(_123)*$ ! accepts "123", "123 123", or "123 123 123"

Example 4: AS PATH Filtering with AS Path Prepending, multiple customers:

- multiple customers performing AS PATH Prepending

Customer 1
	\
Customer 2 -	AS387
	/
Customer 3

!

router bgp 387  neighbor 10.0.0.1 remote-as 123  neighbor 10.0.0.1 filter-list 10 in  neighbor 20.0.0.1 remote-as 456  neighbor 20.0.0.1 filter-list 10 in  neighbor 30.0.0.1 remote-as 789  neighbor 30.0.0.1 filter-list 10 in ! ip as-path access-list 10 permit ^([0..9]+)(_\1)*$ ! accepts repeating instances of "123", "456", and "789" ! does not accept strings non-repeating strings (e.g. "123 123 100") !

Notes:
atom 1 = at least one instance of a number at the beginning of the string;
atom 2 = 0 or more instances of a whitespace and atom 1 until the end of the string.

BGP Path Attributes and Route Selection

BGP Path Attributes

Mandatory Well-Known  Origin (i, e, ?)  AS-Path (sequence of AS-Numbers to access network/IP)  Next-Hop (ip address)

Discretionary Well-Known  Local Preference (for routing policy)  Atomic Aggregate (flags route if aggregated)

Optional Non-Transitive  Multi-Exit Discriminator (MED) - multiple entry pts to one AS  Originator-ID - for route reflector environment  Cluster-List - for route reflector environment

Optional Transitive  Aggregator - IP address & AS of routers that aggregated routes  Community - for route tagging

BGP Route Selection Order:

1. Prefer highest weight (local to router)
2. Prefer highest local preference (global within AS)
3. Prefer routes that the router originated
4. Prefer shorter AS paths
5. Prefer lowest origin code (IGP < EGP < Incomplete)
6. Prefer lowest MED
7. Prefer external (EBGP) paths over internal (IBGP)
1. For IBGP paths, prefer nonreflected routes (no originator-ID) over reflected routes
2. For reflected routes, prefer shorter cluster-list
8. For IBGP paths, prefer path through closest IGP neighbor
9. For EBGP paths, prefer oldest (most stable) path
10. Prefer paths from router with the lower BGP router-ID

BGP: Multihomed Customer to Single ISP in Load-Sharing Setup with Static Routes

Outbound Traffic (CE to PE):
- each customer router uses closest CE as exit point;
- CE routers must be collocated to have load-sharing;

Inbound/Return Traffic (PE to CE) (pre-IOS 12.2):
- true load-sharing is impossible to achieve with multiple PEs;
- per BGP route selection, only one route will be the best route (to the customer network)
- can be optimized by dividing the customer address space

Customer Network: 11.2.3.0 /24
Customer Network "division" assigned to PE1: 11.2.3.0/25
Customer Network "division" assigned to PE2: 11.2.3.128/25

BGP: Multihomed Customer to Single ISP in Primary-Backup Setup with Static Routes

Scenario:
Multihomed customer connected to a single service provider on multiple permanent links;
Customer network using OSPF; provider network using BGP; CE-PE via static routing;
Floating static routes configured as backup on both CE and PE;

Floating Static Routes in BGP:
Once active, the floating static routes will be permanently installed in BGP; static routes are locally sourced which is preferred;

• admin distance cannot be used in route-maps; hence, use communities
• tag floating static routes; tags mapped to specific communities;
• use route maps to modify the weight and/or local preference;
• default weight: 32768