Guest

IP Routing

Configuring the BGP Local-AS Feature

Cisco - Configuring the BGP Local-AS Feature

Document ID: 13761

Updated: Aug 10, 2005

   Print

Introduction

This document explains the Border Gateway Protocol (BGP) local-AS feature, which was first available in Cisco IOS® Software Release 12.0(5)S.

The local-AS feature allows a router to appear to be a member of a second autonomous system (AS), in addition to its real AS. This feature can only be used for true eBGP peers. You cannot use this feature for two peers that are members of different confederation sub-ASs.

Prerequisites

Requirements

This document requires an understanding of the BGP routing protocol and its operations. Refer to the BGP Case Studies.

Components Used

The information in this document applies to these software and hardware versions.

  • Cisco IOS Software Release 12.2(28)

  • Cisco 2500 Series Routers

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Command Syntax

This list shows the syntax of the commands that the configurations in this document use.

Local-AS cannot be customized for individual peers in a peer group.

Local-AS cannot have the local BGP protocol AS number or the AS number of the remote peer.

The local-as command is valid only if the peer is a true eBGP peer. It does not work for two peers in different sub-ASs in a confederation.

Refer to the BGP Command Reference Guide for additional information on BGP commands.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Background Information

The local-AS feature is useful if ISP-A purchases ISP-B, but ISP-B's customers do not want to modify any peering arrangements or configurations. The local-AS feature allows routers in ISP-B to become members of ISP-A's AS. At the same time, these routers appear to their customers to retain their ISP-B AS number.

In Figure 1, ISP-A has not yet purchased ISP-B. In Figure 2, ISP-A has purchased ISP-B, and ISP-B uses the local-AS feature.

In Figure 2, ISP-B belongs to AS 100, and ISP-C to AS 300. When peering with ISP-C, ISP-B uses AS 200 as its AS number with the use of the neighbor ISP-C local-as 200 command. In updates sent from ISP-B to ISP-C, the AS_SEQUENCE in the AS_PATH attribute contains "200 100". The "200" is prepended by ISP-B due to the local-as 200 command configured for ISP-C.

Normally a combined ISP-A/B renumbers the routers in ISP-B to be part of AS 100. What if ISP-C is unable to change its eBGP configurations with ISP-B? Prior to the local-AS feature, the combined ISP-A/B has to maintain two AS numbers. The local-as command allows ISP-A/B to physically be one AS while it appears to be two ASs to ISP-C.

Configure

This section presents you with the information to configure the features this document describes.

Note: In order to find additional information on the commands this document uses, use the Command Lookup Tool (registered customers only) .

Network Diagrams

This document uses these network setups.

Figure 1

39a.gif

Figure 2

39b.gif

Configurations

This document uses these configurations.

ISP-B (AS 100, local-as 200)
hostname ISP-B
! 
interface serial 0 
ip address 192.168.1.1 255.255.255.252 
! 
interface ethernet 0 
ip address 192.168.4.1 255.255.255.0 
! 
router bgp 100 

!--- Note the AS number 100. This is the AS number of ISP-A, which is now
!--- used by all routers in ISP-B after its acquisition by ISP-A.

 neighbor 192.168.1.2 remote-as 300 

!--- Defines the e-BGP connection to ISP-C.

 neighbor 192.168.1.2 local-as 200 

!--- This command makes the remote router in ISP-C to see this
!--- router as belonging to AS 200 instead of AS 100. 
!--- This also make this router to prepend AS 200 in 
!--- all updates to ISP-C.

network 192.168.4.0
! 
!

ISP-C (AS 300)
hostname ISP-C
! 
interface serial 1 
ip address 192.168.1.2 255.255.255.252 
! 
interface ethernet 0 
ip address 192.168.9.1 255.255.255.0 
! 
router bgp 300 
 neighbor 192.168.1.1 remote-as 200

!--- Defines the e-BGP connection to ISP-B.


!--- Note AS is 200 and not AS 100.
 
 network 192.168.9.0 
! 
!

Verify

This section provides information you can use to confirm your configuration properly works.

Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.

Refer to the BGP Command Reference Guide in order to learn more about the show commands used in this section.

View the BGP routing table to see how the local-as command changed the AS_PATH. What you observe is that ISP-B prepends AS 200 to updates that are sent to and received from ISP-C. In addition, note that ISP-B is in AS number 100.

ISP-B# 
show ip bgp summary

  BGP router identifier 192.168.4.1, local AS number 100 
  BGP table version is 3, main routing table version 3 
  2 network entries and 2 paths using 266 bytes of memory 
  2 BGP path attribute entries using 104 bytes of memory 
  1 BGP AS-PATH entries using 24 bytes of memory 
  0 BGP route-map cache entries using 0 bytes of memory 
  0 BGP filter-list cache entries using 0 bytes of memory 
  BGP activity 2/6 prefixes, 2/0 paths, scan interval 15  secs 

  Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd 
  192.168.1.2     4   300      29      29        3    0    0 00:25:19       1

In this output, note that ISP-C sees ISP-B as part of AS 200.

ISP-C# show ip bgp summary 
  BGP table version is 3, main routing table version 3 
  2 network entries (2/6 paths) using 480 bytes of memory 
  2 BGP path attribute entries using 192 bytes of memory 
  0 BGP route-map cache entries using 0 bytes of memory 
  0 BGP filter-list cache entries using 0 bytes of memory 

  Neighbor        V    AS MsgRcvd MsgSent  TblVer InQ OutQ   Up/Down  State/PfxRcd 
  192.168.1.1     4   200      34      34       3   0    0  00:30:19   1

Note in this output that ISP-B prepends "200" to the routes learned from ISP-C.

ISP-B# 
show ip bgp

  BGP table version is 3, local router ID is 192.168.4.1 
  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 
  *> 192.168.4.0      0.0.0.0               0          32768   i 
  *> 192.168.9.0      192.168.1.2           0              0   200 300 i

Note that ISP-C sees routes from ISP-B with an AS_PATH of "200 100".

ISP-C# show ip bgp 
  BGP table version is 3, local router ID is 192.168.1.2 
  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 
  *> 192.168.4.0      192.168.1.1         0              0 200 100 i 
  *> 192.168.9.0      0.0.0.0             0          32768 i

These commands show the configured local-as values in their output:

ISP-B# show ip bgp neighbors 192.168.1.2
BGP neighbor is 192.168.1.2,  remote AS 300,  local AS 200, external link
  BGP version 4, remote router ID 192.168.9.1
  BGP state = Established, up for 00:22:42
  Last read 00:00:42, hold time is 180, keepalive interval is 60 seconds
  Neighbor capabilities:
    Route refresh: advertised and received(old & new)
    Address family IPv4 Unicast: advertised and received
  Message statistics:
    InQ depth is 0
    OutQ depth is 0
                         Sent       Rcvd
    Opens:                  1          1
    Notifications:          0          0
    Updates:                2          1
    Keepalives:            25         25
    Route Refresh:          0          1
    Total:                 28         28
  Default minimum time between advertisement runs is 30 seconds

! Output Suppressed

Troubleshoot

The debug ip bgp updates command displays the received prefixes with its attributes from the neighbor. This output shows that the prefix 192.168.4.0/24 is received with AS PATH 200, 100.

ISP-C#
*May 10 12:45:14.947: BGP(0): 192.168.1.1 computing updates, afi 0, neighbor ver
sion 0, table version 5, starting at 0.0.0.0
*May 10 12:45:14.947: BGP(0): 192.168.1.1 send UPDATE (format) 192.168.9.0/24, n
ext 192.168.1.2, metric 0, path
*May 10 12:45:14.947: BGP(0): 192.168.1.1 1 updates enqueued (average=52, maximu
m=52)
*May 10 12:45:14.947: BGP(0): 192.168.1.1 update run completed, afi 0, ran for 0
ms, neighbor version 0, start version 5, throttled to 5
*May 10 12:45:14.947: BGP: 192.168.1.1 initial update completed
*May 10 12:45:15.259: BGP(0): 192.168.1.1 rcvd UPDATE w/ attr: nexthop 192.168.1
.1, origin i, metric 0, path 200 100
ISP-C#
*May 10 12:45:15.259: BGP(0): 192.168.1.1 rcvd 192.168.4.0/24
*May 10 12:45:15.279: BGP(0): Revise route installing 192.168.4.0/24 -> 192.168.
1.1 to main IP table
ISP-C#

Related Information

Updated: Aug 10, 2005
Document ID: 13761