1 © 2000, Cisco Systems, Inc. Session # Presentation_ID Border Gateway Protocol

1© 2000, Cisco Systems, Inc.

Session #Presentation_ID

Border Gateway Protocol

AgendaAgenda

• BGP Fundamentals

• BGP Attributes

• Controlling the flow of BGP updates

• Practical Design Examples

Autonomous System (AS)Autonomous System (AS)

AS 100AA

• Collection of networks with same policy

• Single routing protocol

• Usually under single administrative control

• Identified by AS number (1 – 65535) Private ASNs from 64512 – 65535

ARINARIN

• A unique routing policy (its policy differs from its border gateway peers)

• A multi-homed site

ASN Registration Guidelines

http://www.arin.net

What is an IGP?What is an IGP?

• Interior Gateway Protocol

• Within an Autonomous System

• Carries information about internal prefixes

• Examples—OSPF, RIP, EIGRP…

What is an EGP?What is an EGP?

• Exterior Gateway Protocol

• Used to convey routing information between Autonomous Systems

• Decoupled from the IGP

• Current EGP is BGP

Interior vs. Exterior Routing Protocols

Interior vs. Exterior Routing Protocols

• Interior

Automatic discovery

Generally trust your IGP routers

Routes go to all IGP routers

• ExteriorSpecifically configured peers

Connecting with outside networks

Set administrative boundaries

Why do we need an EGP?Why do we need an EGP?

• Scaling to large network

Hierarchy

Limit scope of failure

Fast convergence

No manual reconfig (static routes) - high maintenance

• Complex Routing Policies

Control reachability to prefixes by selecting outbound paths and announcing internal routes

NJEDgeNJEDge

Member RemoteNetwork

Alternate ISP

NJEDge IntranetNJEDge Internet

VerizonATM

What is BGP?What is BGP?

• Border Gateway Protocol, currently version 4 – defined in RFC 1771

• Distance-vector routing protocol running over TCP port 179

• Supports classless routing

• Actually two protocols – iBGP and eBGP

Internal BGPInternal BGP

AS 3847When BGP speakers in the same AS form a BGP connection for the purpose of exchanging routinginformation, they are said to be running IBGP or internal BGP.

IBGP speakers are usually fully-meshed.

B

A

c

External BGPExternal BGP

When BGP speakers in different ASs form a BGP connection for the purpose of exchanging routinginformation, they are said to be running EBGP or external BGP.

EBGP peers are usually directly connected.

AS 109

AS 110

131.108.0.0

A

B

150.10.0.0

131.108.10.0.1

.2

AgendaAgenda


• BGP Attributes



BGP AttributesBGP Attributes

• AS-path

• Origin

• Next-hop

• Weight

• Local preference

• Multi Exit Discriminator (MED)

• Community

BGP AttributesBGP Attributes

1880

AS-Path

141.253.10.0/24

A

690B

200

C

1. Router A sends update for 141.253.10.0/24 with AS_PATH: 18801. Router A sends update for 141.253.10.0/24 with AS_PATH: 1880

2. Router B sends update for 141.253.10.0/24 with AS_PATH: 690 1880

2. Router B sends update for 141.253.10.0/24 with AS_PATH: 690 1880

3.Router C sends update for 141.253.10.0/24 with AS_PATH: 200 690 1880

3.Router C sends update for 141.253.10.0/24 with AS_PATH: 200 690 1880

4.Router A will detect its own AS number and will discard the update

4.Router A will detect its own AS number and will discard the update

AS-PathAS-Path

AS3847207.240.0.0/16 AS1673

140.222.0.0/16

AS701192.67.95.0/24 AS3561

204.70.0.0/15

192.67.95.0/24 3847 701 i140.222.0.0 3847 1673 i204.70.0.0/15 3847 3561 i207.240.0.0/16 3847 i

AS6201

E

C

FG

D

B

A

show ip bgp

AS-PathAS-Path

•Sequence of ASNs a route has traversed.

• Provides a mechanism for loop detection

• Shortest AS path preferred

• Policies may be applied based on AS path

OriginOrigin

Order of preference:

• IGP (i)

Route is interior to the originating AS

Set with the Network statement under router BGP

• EGP (e)

Route learned via EGP

• Incomplete (?)

Route redistributed from IGP

RouterB# show ip bgp

table version is 24, local router ID is 203.250.15.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path *> 192.208.10.0 192.208.10.5 0 0 300 i

OriginOrigin

AS 300

AS 200

192.208.10.5

AB

192.208.10.6

Next HopNext Hop

160.10.0.0/16

150.10.0.0/16

150.10.1.1 150.10.1.2

AS 100

AS 300AS 200

150.10.0.0/16 150.10.1.1160.10.0.0/16 150.10.1.1

AA BB

• Next hop IP address to reach a network

• For EBGP usually the IP of the neighbor specified by the neighbor remote-as command

Next HopNext Hop

160.10.0.0/16

150.10.0.0/16

150.10.1.2

AS 100

AS 300

AS 200AA BB

CC

150.10.0.0/16 150.10.1.1160.10.0.0/16 150.10.1.1

150.10.1.1

• With IBGP Next Hop does not change

• Ensure that router C can reach 150.10.1.1 via an IGP

WeightWeight

• Cisco proprietary

• Local to router, not propagated in any routing updates

• Value 0-65535 (default if originated by router - 32768, other - 0)

• Highest weight preferred

• Rarely used

router bgp 300

neighbor 1.1.1.1 remote-as 100 neighbor 1.1.1.1 weight 2000 neighbor 2.2.2.2 remote-as 200 neighbor 2.2.2.2 weight 1000

WeightWeight

Local PreferenceLocal Preference

AS 400

AS 200

160.10.0.0/16

AS 100

AS 300

160.10.0.0/16 150> 160.10.0.0/16 200

150 200 EE

BB

CC

AA

DD

• Path with highest local pref is preferred (default = 100)

• Unlike weight, local pref advertised to routers within the same AS (IBGP)

• Often used attribute

• “Powerful” attribute, comes before AS-Path length in the BGP selection algorithm


router bgp 400

neighbor 3.3.3.4 remote-as 300 neighbor 128.213.11.1 remote-as 400 bgp default local-preference 200


Multi-Exit Discriminator (MED)Multi-Exit Discriminator (MED)

AS 201

AS 200

192.68.1.0/24

CC

AA BB

192.68.1.0/24 1000192.68.1.0/24 2000

• Lowest MED preferred

• Used to convey the relative preference of entry points into an AS (Local Pref is outbound)

• Influences best path selection – after AS_PATH evaluation

• Comparable if paths are from same AS. Use bgp always-compare-med command to compare all MEDs

• Advertised to external neighbors

• Usually based on IGP metric


router bgp 300

neighbor 3.3.3.2 remote-as 100 neighbor 3.3.3.2 route map SETMEDOUT out neighbor 1.1.1.1 remote-as 300

route-map SETMEDOUT permit 10 set metric 200


• BGP attribute

• Used to group destinations

• Useful in applying routing policies

• Represented as number(s) that get “stamped” on BGP routes

• Each destination could be member of multiple communities

• Community attribute carried across Autonomous Systems

CommunitiesCommunities

router bgp 200

network 160.10.0.0 neighbor 3.3.3.1 remote-as 300 neighbor 3.3.3.1 send-community neighbor 3.3.3.1 route-map SETCOMMUNITY out

route-map SETCOMMUNITY permit 10 match ip address 1 set community no-export route-map SETCOMMUNITY permit 20

access list 1 permit 0.0.0.0 255.255.255.255

CommunitiesCommunities

AS 200

AS 300

3.3.3.2 A

B

3.3.3.1

AS 100

160.10.0.0

C

AgendaAgenda


• BGP Attributes



BGP Path Selection AlgorithmBGP Path Selection Algorithm

1. Do not consider IBGP path if not synchronized

2. Do not consider path if no route to next hop

3. Highest weight (local to router)

4. Highest local preference (global within AS)

5. Shortest AS path

BGP Path Selection AlgorithmBGP Path Selection Algorithm

6. Lowest origin code

IGP < EGP < incomplete

7. Multi-Exit Discriminator

Considered only if paths are from the same AS

8. Prefer EBGP path over IBGP path

9. Path with shortest next hop metric wins

10. Lowest router-id

router bgp 256 neighbor 3.3.3.4 remote-as 300 route-map SETLOCALIN in neighbor 128.213.11.1 remote-as 256

ip as-path 7 permit ^300$

route-map SETLOCALIN permit 10 match as-path 7

set local-preference 200

route-map SETLOCALIN permit 20

Route MapsRoute Maps

• Route-maps are Cisco’s mechanism to select and modify routes with if/then style algorithms.

• For route-maps with the keyword “permit”, if the prefix being examined passes the match statement, the set commands are executed and the route-map is exited.

• If the match statement is not passed, the next sequence number is executed.

• If there are no more sequence numbers, the prefix is filtered/dropped.

Route MapsRoute Mapsroute-map SETLOCALIN permit 10 match as-path 7 set local-preference 200

route-map SETLOCALIN permit 20

ip as-path 7 permit ^300$

. Period matches any single character, including white space.

* Asterisk matches 0 or more sequences of the pattern.

+ Plus sign matches 1 or more sequences of the pattern.

? Question mark matches 0 or 1 occurrences of the pattern

^ Caret matches the beginning of the input string.

$ Dollar sign matches the end of the input string.

_ Underscore matches a comma (,), left brace ({), right brace (}) left parenthesis, right parenthesis, the beginning or end of the input string, or a space.

][ Square brackets designate a range of single character patterns.

- Hyphen separates the endpoints of a range.

These are much like standard vi regular expressions.

Cisco Regular ExpressionsCisco Regular Expressions

Cisco Regular ExpressionsCisco Regular Expressions

3847

6201

D

A

C

BE

701

F

6202

G

The following configuration could be used on router B to accept routes from AS6201 & 6202 and deny all others.

ip as-path access-list 10 permit ^6201$ip as-path access-list 10 permit ^6201_6202$ip as-path access-list 10 deny .*

Router A

router bgp 100

network 170.10.0.0 neighbor 2.2.2.2 remote-as 200 neighbor 2.2.2.2 route-map SETPATH out

route-map SETPATH permit 10 set as-path prepend 100 100

AS-Path PaddingAS-Path Padding

AS 400

AS 200

AS 100

AS 300

A

• A way to group in a configuration template a set of neighbors having the same outbound policy.

• Peer-groups allow:

easier configuration (and maintenance) of BGP neighbors

better cpu/memory usage when generating updates

• By grouping neighbors with common policy together, routers can save CPU by creating once a route object and then advertising that object to multiple peers.

• Also, saves typing :)

Peer GroupsPeer Groups

router bgp 300

neighbor EXTERNALMAP peer-group neighbor EXTERNALMAP route-map SETMED neighbor EXTERNALMAP filter-list 1 out neighbor EXTERNALMAP filter-list 2 in neighbor 2.2.2.2 remote-as 100 neighbor 2.2.2.2 peer-group EXTERNALMAP neighbor 4.4.4.2 remote-as 600 neighbor 4.4.4.2 peer-group EXTERNALMAP neighbor 1.1.1.2 remote-as 200 neighbor 1.1.1.2 peer-group EXTERNALMAP neighbor 1.1.1.2 filter-list 3 in

Peer GroupsPeer Groups

• Three ways to configure route aggregation

Redistribute static

Network mask command

Aggregate-address command

AggregationAggregation

router bgp 200

neighbor 3.3.3.1 remote-as 300 redistribute static

ip route 160.0.0.0 255.0.0.0 null 0


Redistribute Static

router bgp 200

network 160.0.0.0 mask 255.0.0.0 neighbor 3.3.3.1 remote-as 300

ip route 160.0.0.0 255.0.0.0 null 0


Network Mask

router bgp 200

network 160.0.0.0 neighbor 3.3.3.1 remote-as 300

aggregate-address 160.0.0.0 255.0.0.0


Aggregate-address

router bgp 300

neighbor 3.3.3.3 remote-as 200 neighbor 2.2.2.2 remote-as 100 network 160.10.0.0 aggregate-address 160.0.0.0 255.0.0.0 suppress-map CHECK

route-map CHECK permit 10 match ip address 1

access-list 1 deny 160.20.0.0 0.0.255.255 access-list 1 permit 0.0.0.0 255.255.255.255


Suppress-map

AgendaAgenda


• BGP Attributes



Multi-homing with two ISPsMulti-homing with two ISPs

141.253.10.0/24

A

AS 100

ISP A

AS 200

CNJEDge Internet

B

10.10.10.10

20.20.20.20

AS 300

1.0.0.0 / 82.0.0.0 / 8

Risk of your AS becoming a transit ASRisk of your AS becoming a transit AS

router bgp 300

network 1.0.0.0 network 2.0.0.0 neighbor 10.10.10.10 remote-as 100 neighbor 10.10.10.10 route-map localonly out neighbor 20.20.20.20 remote-as 200 neighbor 20.20.20.20 route-map localonly out

ip as-path access-list 10 permit ^$

route-map localonly permit 10 match as-path 10

Configuration to Receive Full Internet Routing Table

Configuration to Receive Full Internet Routing Table

Load Sharing when Multi-homed to Two ISPsLoad Sharing when Multi-homed to Two ISPs

AS 100

AS 300ISP A Network

Internet

AS 400

10.10.10.0/24 10.10.20.0/24

EE

BB

CC

AA

DD

IBGP

NJEDge Internet

Member Network

router eigrp 10 network 175.220.0.0

router bgp 200 neighbor 1.1.1.1 remote-as 300 neighbor 2.2.2.2 remote-as 200 neighbor 1.1.1.1 distribute-list 1 out redistribute eigrp 10

access-list 1 permit 175.220.0.0 0.0.255.255

Redistributing IGP into BGPRedistributing IGP into BGP

Requires careful use of access lists to prevent routes from being injected back into BGP

router bgp 200 network 175.220.0.0 neighbor 1.1.1.1 remote-as 300 neighbor 2.2.2.2 remote-as 200

Redistributing IGP into BGP(Preferred)

Redistributing IGP into BGP(Preferred)

• Works for networks learned through IGP or static routes

• Use with aggregate-address command if necessary

Redistributing BGP into IGPRedistributing BGP into IGP

• Normally avoided because too many routes would be injected into the IGP

• Common design is to redistribute one or two routes and make them exterior routes

• Or, have your BGP router generate default for your autonomous system

• When redistributing from BGP into IGP, only routes learned using EBGP get redistributed

Cisco RoutersCisco Routers

• Can’t run full BGP -

2500

4000/4000M/4500/4500M

• Can run full BGP (64 MB)-

2600

3620/3640/3660

4700M

7206

7000

7500

Best PracticesBest Practices

• Peer IBGP routers using loopback address

neighbor 1.1.1.1 update-source loopback0

• BGP soft-reconfig

Allows config changes w/o clearing neighbor Inbound: neighbor 1.1.1.1 soft-reconfiguration inbound Outbound: no configuration necessary clear ip bgp 1.1.1.1 soft (in/out)

Route Refresh Capability IOS 12.0(1.0.4)S and later

• bgp dampening command

Suppress flapping routes (high CPU utilization) For EBGP only Alternate paths still usable Use judiciously!

• bgp log-neighbor-changes

Used to log neighbor up/down events and resets

Documents

1 © 2000, Cisco Systems, Inc. Session # Presentation_ID Border Gateway Protocol