MPLS Label Distribution Parameters

MPLS Label Distribution Parameter Options:

Label Space Options	Per-Interface	Per-Plaform
Label Distribution	Unsolicited downstream	Downstream-on-Demand
Label Allocation	Independent Control Mode	Ordered Control Mode
Label Retention	Liberal Label Retention	Conservative Label Retention

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Label Space Options

Per-Platform Labels

• LFIB: [Label | Action | Next Hop ]
• one label assigned to a destination network and announced to all neighbors
• locally unique, valid on all incoming interfaces
• smaller LFIB, FIB; faster label exchange
• less secure than per-interface labels; label-spoofing
• default in Frame-Mode MPLS

Per-Interface Labels

• LFIB:[IN intf | IN Label | NH intf | NH Label]
• one label for each destination, each device, each interface
• secure; prevents label-spoofing; labeled packets/ATM cells only accepted from interface where label is assigned
• default in Cell-Mode MPLS

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Label Distribution

Unsolicited Downstream

• label advertised to all neighbor LSRs, regardless of whether upstream or downstream
• Frame-Mode

Downstream-On-Demand

• label advertised only upon request of upstream LSR
• cell-mode

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Label Allocation 

Independent Control

• LSR can assign a label for a prefix w/o outgoing/downstream label
• for edge LSRs only (requires L3 capabilities)
• faster label propagation
• for unsolicited downstream / frame-mode

Ordered Control

• local label allocated and propagated only
• if exist(nexthop label) propagae/allocate label, else request label from nexthop
• for downstream-on-demand/cell-mode

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Label Retention

Liberal Label Retention

• all received label stored in LIB, even if not from next-hop LSR
• improves convergence speed; allows easy failover for link failures
• frame-mode

Conservative Label Retention

• only received labels from nexthop LSRs are  stored; others are ignored
• downstream-on-demand/cell-mode

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Standard Parameter Sets for Cisco Platforms

Parameter	Routers, frame interfaces	Routers, ATM interfaces	ATM switches
Label Space	per-platform	per-interface	per-interface
Label Distribution	unsolicited downstream	downstream-on-demand	downstream-on-demand
Label Allocation	independent control	independent control	ordered control
Label Retention	liberal label retention	conservative OR liberal	conservative

BGP Route Dampening

At a Glance: designed to reduce router processing load caused by unstable routes; prevents sustained routing oscillations without affecting well-behaved routes; RFC 2439: BGP Route Flap Dampening; minimizes BGP updates by suppressing unstable routes.

Route-Dampening Operation:

• EBGP route flaps = 1000 penalty points
• IBGP route flaps not dampened;
• penalty not user-configurable;
• penalty decays via exponential decay algorithm
• if penalty > suppress limit, route is dampened
• if penalty < reuse limit, dampened route is propagated
• if penalty < 1/2 reuse limit, flap history forgotten
• route is never dampened more than the maximum suppress time limit
• unreachable route with flap history is in history state (still in BGP table to retain flap history)
• penalty is applied to an individual path, not the prefix

---

Configuring Route Dampening:

router(config-router)# bgp dampening [half-life reuse suppress max-suppress-time] [route-map route-map-name]
half-life	time for penalty to halve (default 15 minutes)
reuse	penalty points when dampened route is reused (default 750)
suppress	penalty points when route is suppressed (default 2000)
max-suppress-time	maximum time a route is suppressed (default 1hour; maximum 255 minutes)

router(config-route-map)# set dampening half-life reuse suppress max-suppress-time

- used for less aggressive dampening of routes towards root DNS servers - dampening of smaller prefixes more aggressively - selective dampening based on BGP neighbors and route-map match criteria

Other Commands:

clear ip bgp ip_addr flap-statistics [{regexp regexp} | {filter-list listname} | {ipaddr mask} ]

clear ip bgp dampening [ipaddr mask]

show ip bgp dampened-paths

show ip bgp flap-statistics [{regexp regexp}|{filter-list listname}|{ipaddr mask [longer-prefix]}]

debug ip bgp dampening

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