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© 2001, Cisco Systems, Inc. All rights reserved.

© 2001, Cisco Systems, Inc. All rights reserved. BGP_Overview-2

BGP Overview

© 2001, Cisco Systems, Inc. All rights reserved. BGP_Overview—Page-3

Objectives

Upon completion of this lesson, you will be able to perform the following tasks:

• Explain the need for BGP and typical BGP usage scenarios

• Describe basic BGP technical characteristics

• Describe BGP path attributes

• Describe BGP session establishment and routing information exchange

• Configure basic BGP on Cisco router

• Monitor and troubleshoot basic BGP setup

Introduction to BGP



Objectives

Upon completion of this section, you will be able to perform the following tasks: • Explain the need for interdomain routing

• Describe the need for external routing protocol

• List the basic BGP technical characteristics

• Identify typical BGP usage scenarios

• Explain BGP limitations


Interdomain Routing

AS65001AS65001AS65000AS65000

• An autonomous system (AS) is a collection of networks under a single technical administration

• Interior routing protocol (IGP) is run inside an autonomoussystem resulting in optimum intra-AS routing

OSPFEIGRP

• Exterior routing protocol (EGP) is run between autonomoussystems to enable routing policies and improve security

BGP


Interdomain Routing Goals

Scalability

• Internet has over 80,000 routes and is still growing

Secure routing information exchange

• Routers from another autonomous system cannot be trusted

• Tight filters are required, authentication is desirable

Routing policies support • Routing between autonomous systems might not

always follow the optimum path


Routing Policies - Case Study

Q: Assuming standard IGP route selection rules, how will the traffic between AS1 and AS20 flow?

Q: Will AS 2 allow this traffic?Q: How would you solve this problem with OSPF or EIGRP?

Company B (AS 2)Company B (AS 2)Company A (AS 1)Company A (AS 1)

Service ProviderService Provider(AS 10)(AS 10)

Company X (AS 20)Company X (AS 20)

64 kbps

64 kbps

2 Mbps

2 Mbps

2 Mbps


BGP Characteristics

Distance-vector protocol with enhancements:

• Reliable updates

• Triggered updates only

• Rich metrics (called path attributes)

Designed to scale to huge internetworks


Reliable Updates

• Uses TCP as transport protocol

• No periodic updates

• Periodic keepalives to verify TCP connectivity

• Triggered updates are batched and rate-limited (every 5 seconds for internal peer, every 30 seconds for external peer)


Common BGP Usages

• Customer connected to one Internet Service Provider (ISP)

• Customer connected to several Service Providers

• Service Provider networks (transit autonomous systems)

• Service providers exchanging traffic at an exchange point (CIX, GIX, NAP …)

• Network cores of large enterprise customers


Single-homed Customers

Large customer or small ISP connecting to the Internet

InternetInternet

Customer or smallCustomer or smallService ProviderService Provider

Service ProviderService Provider

BGP


Customer Connected to One ISPUsage Guidelines

Use BGP between the customer and the Service Provider:• Customers multi-homed to the same Service Provider

• Customer that needs dynamic routing protocol with the Service Provider to detect failures

• Hint: Use private AS number for these customers

• Smaller ISPs that need to originate their routes in the Internet

Use static routes in all other cases• Static routes are always simpler than BGP


Multi-homed Customers

Customer connecting to several service-providers (multi-homed customer)4

InternetInternet

Multi-homedMulti-homedCustomerCustomer

Service Provider #2Service Provider #2BGP

Service Provider #1Service Provider #1

BGP


Multi-homed Customer Usage Guidelines

• BGP is almost mandatory for multi-homed customers

• Multi-homed customers have to use public AS numbers

• Multi-homed customers should use provider-independent address space


Transit Autonomous System

• Using BGP to exchange routes is mandatory for transit autonomous systems (provider networks carrying customer traffic)

InternetInternet

Multi-homedMulti-homedCustomerCustomer

Service ProviderService Provider(Transit AS)(Transit AS)

BGP

Another ServiceAnother ServiceProviderProvider

BGP

Simple CustomerSimple Customer StaticRouting

BG

P


BGP Limitations

BGP and associated tools cannot express all routing policies• You cannot influence the routing policies of

downstream autonomous systems

RFC 1771: “BGP does not enable one AS to send traffic to neighbor AS intending that the traffic take a different route from that taken by traffic originating in the neighbor AS”


RFCs Covering Baseline BGP4

1930 Guidelines for creation, selection, and registration of an Autonomous System

1774 BGP-4 Protocol Analysis

1773 Experience with the BGP-4 protocol

1772 Application of the BGP in the Internet

1771 A Border Gateway Protocol 4 (BGP-4)

1745 BGP4/IDRP for IP---OSPF interaction

1675 BGP MIB


RFCs Covering Additions to BGP4

1965 Autonomous System Confederations for BGP

1997 BGP Communities Attribute

1998 An Application of the BGP Community Attribute in Multi-home Routing

2385 Protection of BGP sessions via the TCP MD5 Signature Option

2439 BGP Route Flap Damping

2796 BGP Route Reflection An alternative to full mesh IBGP


RFCs Covering BGP Extensions

2842 Capabilities Advertisement with BGP-4

2858 Multiprotocol extensions for BGP-4

2918 Route Refresh Capability for BGP-4


Protocol Development Considerations

BGP was designed to perform well in

• Interdomain Routing application

• Huge internetworks with large routing tables

• Environments that require complex routing policies

Some design tradeoffs that were made

• BGP uses TCP for reliable transport - CPU intensive

• Scalability is the top priority - slower convergence


Summary

After completing this section, you should be able to perform the following tasks:

• Explain the need for interdomain routing

• Describe the need for external routing protocol

• List the basic BGP technical characteristics

• Identify typical BGP usage scenarios

• Explain BGP limitations


Review Questions

• What is an autonomous system?

• What are the design goals of an interdomain routing protocol?

• When should you use BGP between a customer and a Service Provider?

• Which transport mechanism is used to exchange the BGP routing updates?

• What is BGP optimized for?

BGP Path Attributes



Objectives

Upon completion of this section, you will be able to perform the following tasks:

• Describe the concept of BGP path attribute

• Explain the difference between well-known and optional BGP attributes

• List common mandatory and optional BGP attributes

• Describe the functionality of AS-Path, Origin and Next-Hop attributes


BGP Path Attributes

• BGP metrics are called path attributes

• BGP attributes are categorized as well-known and optional

• Well-known attributes must be recognized by all compliant implementations

• Optional attributes are only recognized by some implementations (could be private), expected not to be recognized by everyone


Well-known BGP Attributes

• Well-known attributes are divided into mandatory and discretionary

• Well-known mandatory attributes must be present in all update messages

• Well-known discretionary attributes are optional - they could be present in update messages

• All well-known attributes are propagated to other neighbors


Optional BGP Attributes

• Optional BGP attributes are transitive or non-transitive

• Optional transitive attributes

–Propagated to other neighbors if not recognized, Partial bit set to indicate that the attribute was not recognized

• Optional non-transitive attributes

–Discarded if not recognized• Recognized optional attributes are propagated to

other neighbors based on their meaning (not constrained by transitive bit)


Well-known Mandatory Attributes

• Origin

–Specifies the origin of a BGP routeIGP Route originated in an IGPEGP Route originated in EGPUnknown Route was redistributed into BGP

• AS_Path

–Sequence of AS numbers through which the network is accessible

• Next_Hop

– IP address of the next-hop router


Well-known Discretionary Attributes

Local preference

• Used for consistent routing policy within AS

Atomic aggregate

• Informs the neighbor AS that the originating router aggregated routes


Optional Attributes

• Non-transitive attributes

–Multi_Exit_Disc

• Used to discriminate between multiple entry points to a single autonomous system

• Transitive attributes

–Aggregator

• Specifies IP address and AS number of the router that performed route aggregation

–Communities

• Used for route tagging


AS-Path

• The AS-path attribute is empty when a local route is inserted in the BGP table

• The sender’s AS number is prepended to the AS-path attribute when the routing update crosses AS boundary

• The receiver of BGP routing information can use the AS-path to determine through which AS the information has passed

• An AS that receives routing information with its own AS number in the AS-path silently ignores the information


AS-Path Example

AS 37AS 37

AS 21AS 21

AS 123AS 123

10.0.0.110.0.0.1

21.0.0.121.0.0.1

37.0.0.137.0.0.1

Network=10.0.0.0/8Network=10.0.0.0/8AS-Path=123AS-Path=123

Netw

ork=

10.0.0.0/8N

etwo

rk=10.0.0.0/8

AS

-Path

=21 123

AS

-Path

=21 123

Sender’s AS number is prepended to AS Sender’s AS number is prepended to AS path when the update crosses AS boundarypath when the update crosses AS boundary

Network=10.0.0.0/8Network=10.0.0.0/8AS-Path=37 21 AS-Path=37 21 123123

Loop detected, incoming Loop detected, incoming update is ignoredupdate is ignored


Next-Hop Attribute

• Next-hop attribute indicates the next-hop IP address used for packet forwarding

• Usually set to the IP address of the sending BGP router

• Can be set to a third-party IP address to optimize routing


Next-Hop Processing

AS 37AS 37

AS 21AS 21

AS 123AS 123

Rtr-ARtr-A

Rtr-BRtr-B

Rtr-CRtr-C

10.0.0.110.0.0.2

10.1.0.6

10.1.0.5

Network=Network=2121.0.0.0/8.0.0.0/8AS-Path=123 21AS-Path=123 21Next-hop = 10.1.0.5Next-hop = 10.1.0.5

Network=Network=2121.0.0.0/8.0.0.0/8AS-Path=21AS-Path=21Next-hop = 10.0.0.1Next-hop = 10.0.0.1

•Next-hop is usually set to the IP address of the sending router


AS 37AS 37

AS 21AS 21

AS 123AS 123

Rtr-ARtr-A

Rtr-BRtr-B

Rtr-CRtr-C

10.0.0.1

10.0.0.2

10.0.0.3

Next-Hop Processing on Shared Media

Network=Network=2121.0.0.0/8.0.0.0/8AS-Path=123 21AS-Path=123 21Next-hop = Next-hop = 10.0.0.110.0.0.1


• If the receiving BGP router is in the same subnet as the current next-hop, the next-hop address is not changed to optimize packet forwarding


FrameRelay

AS 37AS 37

AS 21AS 21

AS 123AS 123

Rtr-ARtr-A

Rtr-BRtr-B

Rtr-CRtr-C

10.0.0.110.0.0.2

10.0.0.3

Next-Hop Processing on NBMA Network

• BGP next-hop processing can break connectivity with improper networkdesigns over partially-meshed WAN networks


Network=Network=2121.0.0.0/8.0.0.0/8AS-Path=123 21AS-Path=123 21Next-hop = Next-hop = 10.0.0.110.0.0.1

Connectivity is broken, Rtr-C Connectivity is broken, Rtr-C cannot reach next-hop 10.0.0.1cannot reach next-hop 10.0.0.1


BGP Path Attribute Summary

Well-known mandatory attributes• Recognized by everone, always present• AS-Path, Next-Hop, Origin

Well-known discretionary• Recognized by everone, optional• Local Preference, Atomic Aggregate

Optional transitive• Might not be recognized, propagated if not• BGP Community, Aggregator

Optional non-transitive• Might not be recognized, dropped if not• Multi-exit-discriminator


Summary


• Describe the concept of BGP path attribute

• Explain the difference between well-known and optional BGP attributes

• List common mandatory and optional BGP attributes

• Describe the functionality of AS-Path, Origin and Next-Hop attributes


Review Questions

• What is a BGP Path Attribute?

• Which BGP Path Attributes must be carried with each update?

• List three well-known mandatory attributes?

• How are the optional transitive attributes propagated between BGP neighbors?

• How do the BGP routers detect routing loops?

• When is the next-hop attribute different from the IP address of the sending router?

BGP Neighbor Configuration and Session Establishment



Objectives

Upon completion of this section, you will be able to perform the following tasks: • Explain the need for static neighbor

configuration

• Describe the BGP session establishment procedures

• Describe the TCP MD5 signature protection and its benefits for BGP


BGP does not Auto-discover Neighbors

• BGP neighbors are not discovered - they must be configured manually

• Configuration must be done on both sides of the connection

• Both routers will attempt to connect to the other with a TCP session on port number 179

• Only one session will remain if both connection attempts succeed

• Source IP address of incoming connection attempts is verified against a list of configured neighbors


Small BGP Network Used in Following Examples

AS 37AS 3737.0.0.0/837.0.0.0/8

AS 21AS 2121.0.0.0/821.0.0.0/8

AS 123AS 1231.0.0.0/81.0.0.0/8

Rtr-ARtr-A

Rtr-BRtr-B

Rtr-CRtr-C

2.3.4.52.3.4.6

3.4.5.6

3.4.5.7


BGP Neighbors - Idle State

• Initially all BGP sessions to the neighbors are idle

Rtr-A#show ip bgp sumBGP table version is 1, main routing table version 1

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State2.3.4.5 4 21 0 0 0 0 0 never Idle3.4.5.6 4 37 0 0 0 0 0 never Idle

Rtr-A#show ip bgp sumBGP table version is 1, main routing table version 1

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State2.3.4.5 4 21 0 0 0 0 0 never Idle3.4.5.6 4 37 0 0 0 0 0 never Idle


Establishing Session

• TCP session is established when the neighbor becomes reachable

• BGP Open messages are exchanged

Rtr-A#debug ip tcp transactionsRtr-A#debug ip bgp events0:06:17: BGP: 2.3.4.5 went from Idle to Active0:06:22: TCB0012A910 created0:06:22: TCB0012A910 setting property 0 12A8B40:06:22: TCB0012A910 bound to 2.3.4.6.110030:06:22: TCP: sending SYN, seq 3142900499, ack 00:06:22: TCP0: Connection to 2.3.4.5:179, advertising MSS 14600:06:22: TCP0: state was CLOSED -> SYNSENT [11003 -> 2.3.4.5(179)]0:06:22: TCP0: state was SYNSENT -> ESTAB [11003 -> 2.3.4.5(179)]0:06:22: TCP0: Connection to 2.3.4.5:179, received MSS 1460, MSS is 14600:06:22: TCB0012A910 connected to 2.3.4.5.1790:06:22: BGP: 2.3.4.5 went from Active to OpenSent0:06:22: BGP: 2.3.4.5 went from OpenSent to OpenConfirm0:06:22: BGP: 2.3.4.5 went from OpenConfirm to Established

Rtr-A#debug ip tcp transactionsRtr-A#debug ip bgp events0:06:17: BGP: 2.3.4.5 went from Idle to Active0:06:22: TCB0012A910 created0:06:22: TCB0012A910 setting property 0 12A8B40:06:22: TCB0012A910 bound to 2.3.4.6.110030:06:22: TCP: sending SYN, seq 3142900499, ack 00:06:22: TCP0: Connection to 2.3.4.5:179, advertising MSS 14600:06:22: TCP0: state was CLOSED -> SYNSENT [11003 -> 2.3.4.5(179)]0:06:22: TCP0: state was SYNSENT -> ESTAB [11003 -> 2.3.4.5(179)]0:06:22: TCP0: Connection to 2.3.4.5:179, received MSS 1460, MSS is 14600:06:22: TCB0012A910 connected to 2.3.4.5.1790:06:22: BGP: 2.3.4.5 went from Active to OpenSent0:06:22: BGP: 2.3.4.5 went from OpenSent to OpenConfirm0:06:22: BGP: 2.3.4.5 went from OpenConfirm to Established


BGP Open

The BGP Open message contains:• BGP Version number

• My Autonomous System number

• Hold Time

• BGP Router Identifier

• Optional Parameters


BGP Neighbors - Steady State

Rtr-A#show ip bgp sumBGP table version is 10, main routing table version 103 network entries (3/6 paths) using 516 bytes of memory3 BGP path attribute entries using 284 bytes of memory0 BGP route-map cache entries using 0 bytes of memory0 BGP filter-list cache entries using 0 bytes of memory

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcvd2.3.4.5 4 21 17 22 10 0 0 0:01:47 273.4.5.6 4 37 11 17 10 0 0 0:07:07 35

Rtr-A#show ip bgp sumBGP table version is 10, main routing table version 103 network entries (3/6 paths) using 516 bytes of memory3 BGP path attribute entries using 284 bytes of memory0 BGP route-map cache entries using 0 bytes of memory0 BGP filter-list cache entries using 0 bytes of memory

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcvd2.3.4.5 4 21 17 22 10 0 0 0:01:47 273.4.5.6 4 37 11 17 10 0 0 0:07:07 35

• All neighbors shall be up (no state info)


MD5 Authentication

• BGP Peers may optionally use MD5 TCP authentication using shared secret

• Both routers must be configured with the same password (MD5 shared secret)

• Each TCP segment is verified


Summary


• Explain that BGP neighbors must be configured

• Describe the information exchanged using the BGP Open message

• Explain the basic principles of BGP session establishment

• Describe the TCP MD5 signature protection and its benefits for BGP


Review Questions

• How are BGP neighbors discovered?

• Which steps must be taken before a BGP session is established?

• Which information is carried in a BGP Open message?

• What are the benefits of using MD5?

BGP Routing Updates



Objectives

Upon completion of this section, you will be able to perform the following tasks: • Describe BGP data structures

• Explain how BGP announces networks local to an Autonomous System

• Describe the processing of incoming BGP updates

• Explain the BGP route selection process

• Describe the propagation of the best BGP routes to other BGP neighbors


BGP in Action

BGP routing information exchange contains the following steps:• Receiving BGP routing updates

• Building BGP table

• BGP Route selection

• BGP Route propagation

• Building IP routing table

• Advertising local networks


Receiving Routing Update

• Information from the BGP tables is exchanged after adjacency establishment

Rtr-A#debug ip bgp update1:24:11: BGP: 2.3.4.5 rcv UPDATE about 37.0.0.0 255.0.0.0, next hop 2.3.4.5, path 21 37 metric 01:24:11: BGP: 2.3.4.5 rcv UPDATE about 1.0.0.0 255.0.0.0 -- denied1:24:11: BGP: 2.3.4.5 rcv UPDATE about 21.0.0.0 255.0.0.0, next hop 2.3.4.5, path 21 metric 01:24:11: BGP: nettable_walker 21.0.0.0/255.0.0.0 calling revise_route1:24:11: BGP: revise route installing 21.0.0.0/255.0.0.0 -> 2.3.4.5

Rtr-A#debug ip bgp update1:24:11: BGP: 2.3.4.5 rcv UPDATE about 37.0.0.0 255.0.0.0, next hop 2.3.4.5, path 21 37 metric 01:24:11: BGP: 2.3.4.5 rcv UPDATE about 1.0.0.0 255.0.0.0 -- denied1:24:11: BGP: 2.3.4.5 rcv UPDATE about 21.0.0.0 255.0.0.0, next hop 2.3.4.5, path 21 metric 01:24:11: BGP: nettable_walker 21.0.0.0/255.0.0.0 calling revise_route1:24:11: BGP: revise route installing 21.0.0.0/255.0.0.0 -> 2.3.4.5


Building BGP Table

• All inbound updates are placed into the BGP table

Rtr-A#show ip bgpBGP table version is 16, local router ID is 1.2.3.4Status codes: s suppressed, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path*> 1.0.0.0 0.0.0.0 0 32768 i* 21.0.0.0 3.4.5.6 0 37 21 i*> 2.3.4.5 0 0 21 i*> 37.0.0.0 3.4.5.6 0 0 37 i* 2.3.4.5 0 21 37 i

Rtr-A#show ip bgpBGP table version is 16, local router ID is 1.2.3.4Status codes: s suppressed, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path*> 1.0.0.0 0.0.0.0 0 32768 i* 21.0.0.0 3.4.5.6 0 37 21 i*> 2.3.4.5 0 0 21 i*> 37.0.0.0 3.4.5.6 0 0 37 i* 2.3.4.5 0 21 37 i


BGP Route Selection Criteria

• Exclude routes with inaccessible next-hop

• Prefer highest weight (local to router)

• Prefer highest local-preference (global within AS)

• Prefer routes that the router originated

• Prefer shorter AS paths (only length is compared)

• Prefer lowest origin code (IGP < EGP < Unknown)

• Prefer lowest MED

• Prefer external (EBGP) paths over internal (IBGP)

• For IBGP paths, prefer path through closest IGP neighbor

• For EBGP paths, prefer oldest (most stable) path

• Prefer paths from router with the lower BGP router ID


BGP Route Selection

• Best routes to the destination networks are selected from the BGP table

as123#show ip bgpBGP table version is 4, local router ID is 1.2.3.4Status codes: s suppressed, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path*> 1.0.0.0 0.0.0.0 0 32768 i*> 21.0.0.0 3.4.5.6 100 37 21 i* 2.3.4.5 0 0 21 i*> 37.0.0.0 3.4.5.6 0 100 37 i* 2.3.4.5 0 21 37 i


Network Next Hop Metric LocPrf Weight Path*> 1.0.0.0 0.0.0.0 0 32768 i*> 21.0.0.0 3.4.5.6 100 37 21 i* 2.3.4.5 0 0 21 i*> 37.0.0.0 3.4.5.6 0 100 37 i* 2.3.4.5 0 21 37 i


BGP Route Propagation

• Best BGP routes are propagated to BGP neighbors

as123#debug ip bgp update1:24:16: BGP: 3.4.5.6 computing updates, neighbor version 15, table version 16, starting at 0.0.0.01:24:16: BGP: 3.4.5.6 send UPDATE 21.0.0.0 255.0.0.0, next 3.4.5.7, metric 0, path 123 211:24:16: BGP: 3.4.5.6 1 updates enqueued (average=45, maximum=45)1:24:16: BGP: 3.4.5.6 update run completed, ran for 4ms, neighbor version 15, start version 16, throttled to 16, check point net 0.0.0.0

as123#debug ip bgp update1:24:16: BGP: 3.4.5.6 computing updates, neighbor version 15, table version 16, starting at 0.0.0.01:24:16: BGP: 3.4.5.6 send UPDATE 21.0.0.0 255.0.0.0, next 3.4.5.7, metric 0, path 123 211:24:16: BGP: 3.4.5.6 1 updates enqueued (average=45, maximum=45)1:24:16: BGP: 3.4.5.6 update run completed, ran for 4ms, neighbor version 15, start version 16, throttled to 16, check point net 0.0.0.0


Building IP Routing Table

• Best BGP routes are copied into the IP routing table based on administrative distance

as123#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

C 1.0.0.0 is directly connected, Loopback0C 2.0.0.0 is directly connected, Serial1C 3.0.0.0 is directly connected, Serial0B 21.0.0.0 [20/0] via 3.4.5.6, 00:02:06B 37.0.0.0 [20/0] via 3.4.5.6, 00:02:06

as123#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

C 1.0.0.0 is directly connected, Loopback0C 2.0.0.0 is directly connected, Serial1C 3.0.0.0 is directly connected, Serial0B 21.0.0.0 [20/0] via 3.4.5.6, 00:02:06B 37.0.0.0 [20/0] via 3.4.5.6, 00:02:06


Advertising Local Networks

• BGP router process keeps a list of local networks (defined with network command or through redistribution)

• BGP process periodically scans the IP forwarding table and inserts or revokes routes from BGP routing table based on their presence in the forwarding table


Advertising Local Networks - Example 1/2

• BGP route is revoked after the network is removed from the forwarding table

as123# debug ip routingas123# debug ip bgp update%LINEPROTO-5-UPDOWN: Line protocol on Loopback0 changed state to down1:34:33: RT: interface Loopback0 removed from routing table1:34:33: RT: del 1.0.0.0 via 0.0.0.0, connected metric [0/0]1:34:33: RT: delete network route to 1.0.0.01:34:33: BGP: route down 1.0.0.0 255.0.0.01:34:33: BGP: no valid path for 1.0.0.0 255.0.0.01:34:33: BGP: nettable_walker 1.0.0.0/255.0.0.0 no best path selected1:34:34: BGP: 2.3.4.5 send UPDATE 1.0.0.0 255.0.0.0 -- unreachable1:34:34: BGP: 2.3.4.5 1 updates enqueued (average=25, maximum=25)1:34:34: BGP: 2.3.4.5 update run completed, ran for 4ms, neighbor version 4, start version 5, throttled to 5, check point net 0.0.0.01:34:34: BGP: 3.4.5.6 send UPDATE 1.0.0.0 255.0.0.0 -- unreachable

as123# debug ip routingas123# debug ip bgp update%LINEPROTO-5-UPDOWN: Line protocol on Loopback0 changed state to down1:34:33: RT: interface Loopback0 removed from routing table1:34:33: RT: del 1.0.0.0 via 0.0.0.0, connected metric [0/0]1:34:33: RT: delete network route to 1.0.0.01:34:33: BGP: route down 1.0.0.0 255.0.0.01:34:33: BGP: no valid path for 1.0.0.0 255.0.0.01:34:33: BGP: nettable_walker 1.0.0.0/255.0.0.0 no best path selected1:34:34: BGP: 2.3.4.5 send UPDATE 1.0.0.0 255.0.0.0 -- unreachable1:34:34: BGP: 2.3.4.5 1 updates enqueued (average=25, maximum=25)1:34:34: BGP: 2.3.4.5 update run completed, ran for 4ms, neighbor version 4, start version 5, throttled to 5, check point net 0.0.0.01:34:34: BGP: 3.4.5.6 send UPDATE 1.0.0.0 255.0.0.0 -- unreachable


Advertising Local Networks - Example 2/2

• BGP route is advertised after the network appears in the forwarding table

1:36:42: RT: add 1.0.0.0 255.0.0.0 via 0.0.0.0, connected metric [0/0]1:36:42: RT: interface Loopback0 added to routing table1:36:42: BGP: route up 1.0.0.0 255.0.0.01:36:42: BGP: nettable_walker 1.0.0.0/255.0.0.0 route sourced locally%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up1:36:43: BGP: 2.3.4.5 computing updates, neighbor version 5, table version 6, starting at 0.0.0.01:36:43: BGP: 2.3.4.5 send UPDATE 1.0.0.0 255.0.0.0, next 2.3.4.6, metric 0, path 1231:36:44: BGP: 2.3.4.5 1 updates enqueued (average=50, maximum=50)1:36:44: BGP: 2.3.4.5 update run completed, ran for 4ms, neighbor version 5, start version 6, throttled to 6, check point net 0.0.0.0

1:36:42: RT: add 1.0.0.0 255.0.0.0 via 0.0.0.0, connected metric [0/0]1:36:42: RT: interface Loopback0 added to routing table1:36:42: BGP: route up 1.0.0.0 255.0.0.01:36:42: BGP: nettable_walker 1.0.0.0/255.0.0.0 route sourced locally%LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to up1:36:43: BGP: 2.3.4.5 computing updates, neighbor version 5, table version 6, starting at 0.0.0.01:36:43: BGP: 2.3.4.5 send UPDATE 1.0.0.0 255.0.0.0, next 2.3.4.6, metric 0, path 1231:36:44: BGP: 2.3.4.5 1 updates enqueued (average=50, maximum=50)1:36:44: BGP: 2.3.4.5 update run completed, ran for 4ms, neighbor version 5, start version 6, throttled to 6, check point net 0.0.0.0


Summary

After completing this section, you should be able to perform the following tasks:• Describe the basic functions of BGP and its data

structures

• Explain how BGP announces networks local to an Autonomous System

• Describe the processing of incoming BGP updates

• Explain the BGP route selection process

• Describe the propagation of the best BGP routes to other BGP neighbors


Review Questions

• Which of the received routing updates are forwarded to other neighbors?

• Which incoming BGP updates are stored in the BGP table?

• Which BGP routes are announced to other BGP neighbors

• With no BGP attributes modified, which criteria will determine route selection?

• Describe methods that are available to announce local networks

Simple BGP Configuration



Objectives

Upon completion of this section, you will be able to perform the following tasks: • Initial BGP setup

• Configuring BGP neighbors

• Originating local networks in BGP

• Basic BGP route aggregation


Start the BGP Routing Process

router bgp as-number

router(config)#

• Starts BGP routing• Get your AS number from InterNIC (www.internic.net)

or RIPE (www.ripe.net)• Use private AS numbers (64512 - 65535) if you run

BGP in a private network• Only one BGP routing process per router is allowed


Configure External Neighbors

neighbor ip-address remote-as as-numberneighbor ip-address description neighbor description

router(config-router)#

• Defines an external neighbor and (optionally) assigns a description• External neighbor has to be reachable over directly

connected subnet


Temporary Disable BGP Neighbor

neighbor ip-address shutdown


• Disables communication with a BGP neighbor• Usage scenarios:

• Debugging and troubleshooting• Shutdown the neighbor during extensive

modification of routing policies to prevent inconsistent routing data


Configuring TCP MD5 Signature

neighbor ip-address password string


• Enables Message Digest 5 authentication on BGP session

• Use the same password string on both routers


Announcing Networks in BGP

Only administratively defined networks are announced in BGP

• Manually configure networks to be announced

• Use redistribution from IGP

• Use aggregation to announce summary prefixes


Manually Define Major Network

network major-network-number


• Allows advertising of major networks into BGP• At least one of the subnets must be present in the

routing table• The meaning of “network” command in BGP is

completely different from any other routing protocol


Redistributing Routes from IGP

• Easier than listing networks in BGP process in large networks

• Redistributed routes carry origin-attribute ‘incomplete’

• Always filter redistributed routes to prevent route leaking

• Avoid in Service Provider environments


Simple IGP to BGP Redistribution

• Configure redistribution in BGP process• Configure route-filter using distribute list• Caveat:

• BGP routes originated through redistribution have incomplete origin

router(config)#router bgp <AS>redistribute <IGP>distribute-list <ACL> out <IGP>!access-list <ACL> permit <network>

router(config)#router bgp <AS>redistribute <IGP>distribute-list <ACL> out <IGP>!access-list <ACL> permit <network>


Redistribution Using Route-Maps

• Origin can be set to ‘IGP’ with a route-map• Other BGP path path attributes can also be set

• Metric• Next-hop• Community

router(config)#router bgp <AS>redistribute <IGP> route-map intoBGP!route-map intoBGP permitmatch ip address <ACL>set origin igp!access-list <ACL> permit <network>

router(config)#router bgp <AS>redistribute <IGP> route-map intoBGP!route-map intoBGP permitmatch ip address <ACL>set origin igp!access-list <ACL> permit <network>


Classless BGP Operation

• BGP4 supports Classless Interdomain Routing (CIDR)• Any BGP router can advertise individual

networks or supernets (prefixes)• Prefix notation is used with BGP instead of

subnet masks• 192.168.0.0/16 = 192.168.0.0 255.255.0.0


Manually Announce Classless Prefix in BGP

network ip-prefix-address mask subnet-mask


• Configures a classless prefix to be advertised into BGP

• The prefix must exactly match an entry in the IP forwarding table

• Hint: use a static route to null 0 to create a matching prefix in the IP forwarding table


Advertising a Supernet Prefix Example

• Advertise prefix 192.168.0.0/16 assigned to the Internet Service Provider

router(config)#

router bgp 123network 192.168.0.0 mask 255.255.0.0!ip route 192.168.0.0 255.255.0.0 null 0

router(config)#

router bgp 123network 192.168.0.0 mask 255.255.0.0!ip route 192.168.0.0 255.255.0.0 null 0


Aggregating BGP Networks

Summarization is called aggregation in BGP

• Aggregation creates summary routes (called aggregates) from networks already in BGP table

• Individual networks could be announced or suppressed


Configuring Aggregation (#1)

router bgp as-numberaggregate-address address-prefix mask

router(config)#

• Specify aggregation range in BGP routing process• The aggregate will be announced if there is at least

one network in the specified range in the BGP table• Individual networks will still be announced in

outgoing BGP updates


Configuring Aggregation (#2)

router bgp as-numberaggregate-address address-prefix mask summary-only

router(config)#

• Configure aggregation of BGP routes• Advertise only the aggregate and not the individual

networks

Benefits:• Smaller BGP routing tables•More stable internetworks (less route flapping)

Drawbacks:• Problems with multi-homed customers


Multihomed Customer Problem

• Customer prefers Primary provider using Alternate only as backup

• Primary provider advertises the aggregate, Alternate provider advertises individual network

Alternate ProviderAlternate Provider

MultihomedMultihomedCustomerCustomer

Primary ProviderPrimary Provider(aggregating)(aggregating)

Rest of the InternetRest of the Internet

192.1.1.0/24192.1.1.0/24

192.1.1.0/24192.1.1.0/24

192.1.1.0/24192.1.1.0/24

192.1.0.0/16192.1.0.0/16


Multihomed Customer Problem

• Remote autonomous systems prefer longest-match prefix, traffic toward the customer flows through Alternate provider

• Solution: don’t use aggregation

Alternate ProviderAlternate Provider

MultihomedMultihomedCustomerCustomer

Primary ProviderPrimary Provider(aggregating)(aggregating)

Rest of the InternetRest of the Internet

192.1.1.0/24192.1.1.0/24

192.1.1.0/24192.1.1.0/24

192.1.1.0/24192.1.1.0/24

192.1.0.0/16192.1.0.0/16192.1.0.0/16192.1.1.0/24192.1.1.0/24


router(config)#

router bgp 123network 192.168.0.0 mask 255.255.240.0aggregate-address 192.168.16.0 255.255.240.0aggregate-address 192.168.32.0 255.255.240.0 summary-only!ip route 192.168.0.0 255.255.240.0 null 0

router(config)#

router bgp 123network 192.168.0.0 mask 255.255.240.0aggregate-address 192.168.16.0 255.255.240.0aggregate-address 192.168.32.0 255.255.240.0 summary-only!ip route 192.168.0.0 255.255.240.0 null 0

Classless BGP Examples

• Advertise prefix 192.168.0.0/20

• Aggregate networks in 192.168.16.0/20 and announce individual networks

• Aggregate networks in 192.168.32.0/20 suppressing individual network announcements


Aggregation Example - BGP Table


Network Next Hop Metric LocPrf Weight Path*> 1.0.0.0 0.0.0.0 0 32768 i* 21.0.0.0 3.4.5.6 0 37 21 i*> 2.3.4.5 0 0 21 i*> 37.0.0.0 3.4.5.6 0 0 37 i* 2.3.4.5 0 21 37 i*> 192.168.0.0/20 0.0.0.0 0 32768 i*> 192.168.16.0/20 0.0.0.0 0 32768 i*> 192.168.16.0 0.0.0.0 0 32768 ?*> 192.168.17.0 0.0.0.0 0 32768 ?*> 192.168.32.0/20 0.0.0.0 0 32768 is> 192.168.32.0 0.0.0.0 0 32768 ?s> 192.168.33.0 0.0.0.0 0 32768 ?


Network Next Hop Metric LocPrf Weight Path*> 1.0.0.0 0.0.0.0 0 32768 i* 21.0.0.0 3.4.5.6 0 37 21 i*> 2.3.4.5 0 0 21 i*> 37.0.0.0 3.4.5.6 0 0 37 i* 2.3.4.5 0 21 37 i*> 192.168.0.0/20 0.0.0.0 0 32768 i*> 192.168.16.0/20 0.0.0.0 0 32768 i*> 192.168.16.0 0.0.0.0 0 32768 ?*> 192.168.17.0 0.0.0.0 0 32768 ?*> 192.168.32.0/20 0.0.0.0 0 32768 is> 192.168.32.0 0.0.0.0 0 32768 ?s> 192.168.33.0 0.0.0.0 0 32768 ?


Aggregation Example - Outgoing BGP Update

Router#debug ip bgp updates1:36:43: BGP: 2.3.4.5 send UPDATE 192.168.0.0 255.255.240.0, next

2.3.4.6, metric 0, path 1231:36:43: BGP: 2.3.4.5 send UPDATE 192.168.16.0 255.255.255.0, next




2.3.4.6, metric 0, path 123

Router#debug ip bgp updates1:36:43: BGP: 2.3.4.5 send UPDATE 192.168.0.0 255.255.240.0, next





2.3.4.6, metric 0, path 123


Summary


• Explain that external peers are recommended to be directly connected

• Describe how to generate routing information about own networks using either a list of networks or redistribution from the IGP

• Explain the benefits of generating route summaries at once and avoid doing aggregation at a later stage


Review Questions

• How many BGP processes can be active in a router?

• Which parameters must be configured for a BGP neighbor?

• What potential problem could be caused by redistributing all routes from the IGP to BGP?

• Why are external BGP neighbors almost always directly connected?

• What are the benefits of BGP route summarization?

• When should you use BGP aggregation?

• What are the drawbacks of using BGP aggregation?

Monitoring and Troubleshooting BGP



Objectives

Upon completion of this section, you will be able to perform the following tasks:

• Monitor the overall BGP status

• Monitor BGP neighbors

• Monitor BGP table

• Inspect individual prefixes in the BGP table

• Perform basic BGP debugging

• Troubleshoot simple BGP-related problems


Monitoring Overall BGP Routing

Fred#show ip bgp summaryBGP table version is 8, main routing table version 84 network entries (8/12 paths) using 832 bytes of memory5 BGP path attribute entries using 576 bytes of memory0 BGP route-map cache entries using 0 bytes of memory0 BGP filter-list cache entries using 0 bytes of memory2 received paths for inbound soft reconfiguration

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd1.1.0.1 4 213 80 81 8 0 0 01:15:51 21.1.0.3 4 387 79 81 0 0 0 00:00:15 Active1.2.0.1 4 213 82 82 0 0 0 02:15:23 Idle

Fred#show ip bgp summaryBGP table version is 8, main routing table version 84 network entries (8/12 paths) using 832 bytes of memory5 BGP path attribute entries using 576 bytes of memory0 BGP route-map cache entries using 0 bytes of memory0 BGP filter-list cache entries using 0 bytes of memory2 received paths for inbound soft reconfiguration

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd1.1.0.1 4 213 80 81 8 0 0 01:15:51 21.1.0.3 4 387 79 81 0 0 0 00:00:15 Active1.2.0.1 4 213 82 82 0 0 0 02:15:23 Idle

show ip bgp summary

router>

• Displays BGP memory usage, BGP neighbors and the state of communication with them


Monitoring BGP Neighbors

Fred#show ip bgp neighbor 1.2.0.1BGP neighbor is 1.2.0.1, remote AS 213, external link Index 3, Offset 0, Mask 0x8 BGP version 4, remote router ID 10.1.1.1 BGP state = Established, table version = 11, up for 01:23:05 Last read 00:00:05, hold time is 180, keepalive interval is 60 seconds Minimum time between advertisement runs is 30 seconds Received 92 messages, 0 notifications, 0 in queue Sent 92 messages, 0 notifications, 0 in queue Connections established 1; dropped 0 Last reset never No. of prefix received 2

show ip bgp neighbor ip-address

router>

• Displays detailed neighbor information


Monitoring BGP Table

Fred#show ip bgpBGP table version is 11, local router ID is 12.1.2.3Status codes: s suppressed, d damped, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path*> 10.0.0.0 1.2.0.1 500 0 213 i* 1.1.0.1 1000 0 213 i*> 11.0.0.0 1.2.0.1 500 0 213 i* 1.1.0.1 1000 0 213 i*> 12.0.0.0 0.0.0.0 0 32768 i*> 14.0.0.0 1.1.0.3 0 0 387 i

Fred#show ip bgpBGP table version is 11, local router ID is 12.1.2.3Status codes: s suppressed, d damped, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path*> 10.0.0.0 1.2.0.1 500 0 213 i* 1.1.0.1 1000 0 213 i*> 11.0.0.0 1.2.0.1 500 0 213 i* 1.1.0.1 1000 0 213 i*> 12.0.0.0 0.0.0.0 0 32768 i*> 14.0.0.0 1.1.0.3 0 0 387 i

show ip bgp

router>

• Displays all routes in the BGP table in summary format


Displaying Entries in BGP Table

Fred#show ip bgp 11.0.0.0BGP routing table entry for 11.0.0.0/8, version 5Paths: (2 available, best #1, advertised over EBGP) 213 1.2.0.1 from 1.2.0.1 (10.1.1.1) Origin IGP, metric 500, localpref 100, valid, external, best 213 1.1.0.1 from 1.1.0.1 (11.0.0.1) Origin IGP, metric 1000, localpref 100, valid, external

Fred#show ip bgp 11.0.0.0BGP routing table entry for 11.0.0.0/8, version 5Paths: (2 available, best #1, advertised over EBGP) 213 1.2.0.1 from 1.2.0.1 (10.1.1.1) Origin IGP, metric 500, localpref 100, valid, external, best 213 1.1.0.1 from 1.1.0.1 (11.0.0.1) Origin IGP, metric 1000, localpref 100, valid, external

show ip bgp ip-prefix [mask subnet-mask]

router>

• Displays detailed information about all paths for a single prefix

AS-PathAS-Path

Next-hopNext-hop

Advertising router IP addressAdvertising router IP address

Advertising router router-IDAdvertising router router-ID

Other BGP attributesOther BGP attributes


Debugging BGP

debug ip tcp transaction

router#

• Displays all TCP transactions (start of session, session errors …)

debug ip bgp event

router#

• Displays significant BGP events (neighbor state transitions, update runs)


Debugging BGP

debug ip bgp keepalives

router#

• Debugs BGP keepalive packets

debug ip bgp updates

router#

• Displays all incoming or outgoing BGP updates (use with caution)


Debugging BGP

debug ip bgp updates acl

router#

• Displays all incoming or outgoing BGP updates for routes matching an IP ACL

debug ip bgp neighbor-ip updates [acl]

router#

• Displays all BGP updates received from or sent to a BGP neighbor (optionally matching an IP ACL)


BGP Session Startup Troubleshooting

Common BGP Session Startup Symptoms• BGP neighbors do not become active

• BGP neighbor is active, but the session is never established

• BGP neighbor oscillates between idle and active


BGP Session Startup Issues: 1/4

Symptom

• BGP neighbors do not become activeshow ip bgp neighbor display the neighbor state as idle for several minutes

Diagnose

• Neighbor is not directly connected

Verification

• Verify with show ip route



Symptom

• BGP neighbor is active, session is not establisheddebug ip tcp transaction display shows that the SYN TCP packet is not answered with a SYN+ACK packet

Diagnose

• Neighbor is not reachable

Verification

• Verify connectivity with ping

• Check for access list presence


BGP Session StartupNeighbor not Reachable

Router#debug ip tcp transaction

16:34:30: TCB82119C40 created16:34:30: TCB82119C40 setting property TCP_WINDOW_SIZE (0) 8223BDE816:34:30: TCB82119C40 setting property TCP_TOS (11) 8223BDEC16:34:30: TCB82119C40 bound to 192.168.4.13.1100716:34:30: TCP: sending SYN, seq 545426735, ack 016:34:30: TCP0: Connection to 192.168.4.14:179, advertising MSS 146016:34:30: TCP0: state was CLOSED -> SYNSENT [11007 -> 192.168.4.14(179)]

16:35:12: TCP0: state was SYNSENT -> CLOSED [11007 -> 192.168.4.14(179)]16:35:12: TCB 0x82119C40 destroyed


16:34:30: TCB82119C40 created16:34:30: TCB82119C40 setting property TCP_WINDOW_SIZE (0) 8223BDE816:34:30: TCB82119C40 setting property TCP_TOS (11) 8223BDEC16:34:30: TCB82119C40 bound to 192.168.4.13.1100716:34:30: TCP: sending SYN, seq 545426735, ack 016:34:30: TCP0: Connection to 192.168.4.14:179, advertising MSS 146016:34:30: TCP0: state was CLOSED -> SYNSENT [11007 -> 192.168.4.14(179)]

16:35:12: TCP0: state was SYNSENT -> CLOSED [11007 -> 192.168.4.14(179)]16:35:12: TCB 0x82119C40 destroyed

SYN packet is sentSYN packet is sent

SYN+ACK reply never came back, SYN+ACK reply never came back, TCP session is closedTCP session is closed



Symptom

• BGP neighbor is active, session is not establisheddebug ip tcp transaction display shows that the SYN TCP packet is answered with a RST packet

Diagnose

• This router is not configured as BGP neighbor on the neighboring router

Verification

• Check IP addresses of BGP neighbors with show ip bgp summary on the neighbor router


BGP Session StartupNeighbor not Configured


16:30:30: TCB82119C40 created16:30:30: TCB82119C40 setting property TCP_WINDOW_SIZE (0) 8223BDE816:30:30: TCB82119C40 setting property TCP_TOS (11) 8223BDEC16:30:30: TCB82119C40 bound to 192.168.4.13.1100516:30:30: TCP: sending SYN, seq 305377215, ack 016:30:30: TCP0: Connection to 192.168.4.14:179, advertising MSS 146016:30:30: TCP0: state was CLOSED -> SYNSENT [11005 -> 192.168.4.14(179)]16:30:30: TCP0: state was SYNSENT -> CLOSED [11005 -> 192.168.4.14(179)]16:30:30: TCP0: bad seg from 192.168.4.14 -- closing connection: seq 0 ack 305377216 rcvnxt 0 rcvwnd 0 len 016:30:30: TCP0: connection closed - remote sent RST16:30:30: TCB 0x82119C40 destroyed


16:30:30: TCB82119C40 created16:30:30: TCB82119C40 setting property TCP_WINDOW_SIZE (0) 8223BDE816:30:30: TCB82119C40 setting property TCP_TOS (11) 8223BDEC16:30:30: TCB82119C40 bound to 192.168.4.13.1100516:30:30: TCP: sending SYN, seq 305377215, ack 016:30:30: TCP0: Connection to 192.168.4.14:179, advertising MSS 146016:30:30: TCP0: state was CLOSED -> SYNSENT [11005 -> 192.168.4.14(179)]16:30:30: TCP0: state was SYNSENT -> CLOSED [11005 -> 192.168.4.14(179)]16:30:30: TCP0: bad seg from 192.168.4.14 -- closing connection: seq 0 ack 305377216 rcvnxt 0 rcvwnd 0 len 016:30:30: TCP0: connection closed - remote sent RST16:30:30: TCB 0x82119C40 destroyed

SYN packet is sentSYN packet is sent

Neighbor replies with RST packet, Neighbor replies with RST packet, TCP session is closedTCP session is closed



Symptom

• BGP neighbor oscillates between active and idledebug ip tcp transaction display the TCP session being established and torn down immediately

Diagnose

• AS-number mismatch between BGP neighbors

Verification

• Verify the AS-numbers configured for neighboring routers show ip bgp summary on both routes


BGP Session StartupAS-number Mismatch

Router#debug ip tcp transactionRouter#debug ip bgp event

16:40:43: TCB82119C40 created16:40:43: TCP0: state was LISTEN -> SYNRCVD [179 -> 192.168.4.14(11000)]16:40:43: TCP0: Connection to 192.168.4.14:11000, received MSS 146016:40:43: TCP: sending SYN, seq 918933898, ack 86282885316:40:43: TCP0: Connection to 192.168.4.14:11000, advertising MSS 146016:40:43: TCP0: state was SYNRCVD -> ESTAB [179 -> 192.168.4.14(11000)]16:40:43: TCB821197BC callback16:40:43: TCB821197BC accepting 82119C40 from 192.168.4.14.1100016:40:44: BGP: 192.168.4.14 reset due to BGP Notification sent16:40:44: TCP0: state was ESTAB -> FINWAIT1 [179 -> 192.168.4.14(11000)]16:40:44: TCP0: sending FIN

Router#debug ip tcp transactionRouter#debug ip bgp event

16:40:43: TCB82119C40 created16:40:43: TCP0: state was LISTEN -> SYNRCVD [179 -> 192.168.4.14(11000)]16:40:43: TCP0: Connection to 192.168.4.14:11000, received MSS 146016:40:43: TCP: sending SYN, seq 918933898, ack 86282885316:40:43: TCP0: Connection to 192.168.4.14:11000, advertising MSS 146016:40:43: TCP0: state was SYNRCVD -> ESTAB [179 -> 192.168.4.14(11000)]16:40:43: TCB821197BC callback16:40:43: TCB821197BC accepting 82119C40 from 192.168.4.14.1100016:40:44: BGP: 192.168.4.14 reset due to BGP Notification sent16:40:44: TCP0: state was ESTAB -> FINWAIT1 [179 -> 192.168.4.14(11000)]16:40:44: TCP0: sending FIN

TCP session is establishedTCP session is established

BGP notification is sent due to AS BGP notification is sent due to AS number mismatch in open messagenumber mismatch in open message


Summary


• Monitor the overall BGP function

• Monitor the neighbor sessions

• Monitor the BGP table

• Monitor specific entries in the BGP table

• Debug and troubleshoot simple BGP


Review Questions

• What information do you get from a show ip bgp summary?

• Which command is used to display detailed BGP neighbor information

• How does the output from show ip bgp tell you which route to a specific destination is selected as the best?

• Which is the most common reason for a BGP session not leaving the idle state?

• What happens when a BGP session is established, but terminated immediately?


Summary

After completing this lesson, you should be able to perform the following tasks:

• Explain the need for BGP and typical BGP usage scenarios

• Describe basic BGP technical characteristics

• Describe BGP path attributes

• Describe BGP session establishment and routing information exchange

• Configure basic BGP on Cisco router

• Monitor and troubleshoot basic BGP setup