BGP ipv4 as-path attributes

LAB2: BGP – IPv4

Disclaimer

This Configuration Guide is designed to assist members to enhance their skills in respective technology area.  While every effort has been made to ensure that all material is as complete and accurate as possible, the enclosed material is presented on an “as is” basis.  Neither the authors nor Forum assume any liability or responsibility to any person or entity with respect to loss or damages incurred from the information contained in this guide. This Lab Guide was developed by RSTForum.  Any similarities between material presented in this configuration guide and any other material is completely coincidental. 

LAB 2: Diagram

Note: This Lab was developed on Cisco IOS Version15.2(4) M1 ADVENTERPRISEK9-M. 

LAB 2: Configure BGP AS-PATH Attributes:

Task 1: Configure BGP AS-PATH Attributes for Autonomous

• In the configuration mode of router configure BGP AS-PATH Attributes by following command:

R1:

interface loopback 100  

ip address 100.0.0.1 255.255.255.0

exit

router bgp 65100

address-family ipv4

network 100.0.0.0 mask 255.255.255.0

exit

R4:

interface loopback 100

ip address 100.0.0.4 255.255.255.0

exit

router bgp 65200

address-family ipv4

network 100.0.0.0 mask 255.255.255.0

exit

• Verify IPv4 BGP routes by following command:

R2#show ip bgp

! (Shows router’s routing table and IPv4 routes entries)

BGP table version is 14, local router ID is 22.0.3.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

              x best-external, a additional-path, c RIB-compressed,

Origin codes: i - IGP, e - EGP, ? - incomplete

RPKI validation codes: V valid, I invalid, N Not found

Network            Next Hop        Metric   LocPrf    Weight      Path

 *>  11.0.1.0/24      11.0.1.1 0                          0             65100 i

 *>  11.0.2.0/24      11.0.1.1 0                             0              65100 i

 *>  11.0.3.0/24      11.0.1.1 0                             0              65100 i

 *>  22.0.1.0/24      0.0.0.0   0                          32768            i

 *>  22.0.2.0/24      0.0.0.0   0                          32768            i

 *>  22.0.3.0/24      0.0.0.0   0                          32768               i

 *>i 33.0.1.0/24      33.0.1.3 0              100       0                      i

 *>i 33.0.2.0/24      33.0.1.3 0              100       0                      i

 *>i 33.0.3.0/24      33.0.1.3 0              100       0                      i

 *>i 44.0.1.0/24      44.0.1.4 0              100       0                65200i

 *>i 44.0.2.0/24      44.0.1.4 0              100       0                65200i

 *>i 44.0.3.0/24      44.0.1.4 0              100       0                65200i

 * i 100.0.0.0/24     44.0.1.4  0                100           0                65200i

 *>                        11.0.1.1     0                                0                 65100i

(R2 can reach 100 network via next hop 11.0.1.1 through AS 65100 and via 44.0.1.4 through AS 65200. Best path for R2 to reach 100 network is via next hop 11.0.1.1 because 11.0.1.1 is EBGP neighbor with AD value 20 and 44.0.1.4 is received from IBGP neighbor with AD value 200.)

Task 2: Configure BGP AS-PATH Manipulation for Autonomous

• Prepend AS for as-path manipulation

! (Router R2 wants to make via 44.0.1.4 as best path instead of 11.0.1.1)

R2:

router bgp 65000

address-family ipv4

neighbor 11.0.1.1 route-map abc in

exit

route-map abc permit 10

set as-path prepend 222 22

exit 

(A policy is call on Router R2 that 100 network is prepended 222 22 via 11.0.1.1 through AS 65100 222 22 i.e. through 3 AS)

R1:

router bgp 65100

address-family ipv4

neighbor 22.0.1.2 route-map abc out

exit

route-map abc permit 10

set as-path prepend 111 11

exit

(A policy is call on Router R1 that 100 network is prepended 111 11 via 11.0.1.1 through AS 111 11 65100 i.e. through 3 AS)

• Verify IPv4 BGP routes by following command:

R2#show ip bgp

! (Shows router’s routing table and IPv4 routes entries)

BGP table version is 21, local router ID is 22.0.3.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

              x best-external, a additional-path, c RIB-compressed,

Origin codes: i - IGP, e - EGP, ? - incomplete

RPKI validation codes: V valid, I invalid, N Not found

          Network            Next Hop   Metric    LocPrf  Weight  Path

*>  11.0.1.0/24         11.0.1.1          0            0            22 22 65100 i

 *>  11.0.2.0/24        11.0.1.1          0            0          222 22 65100 i

 *>  11.0.3.0/24        11.0.1.1          0            0          222 22 65100 i

 *>  22.0.1.0/24         0.0.0.0        0           32768       i

 *>  22.0.2.0/24         0.0.0.0        0           32768       i

 *>  22.0.3.0/24         0.0.0.0        0           32768       i

 *>i 33.0.1.0/24         33.0.1.3         0 100      0          i

 *>i 33.0.2.0/24         33.0.1.3         0 100   0          i

 *>i 33.0.3.0/24         33.0.1.3         0 100   0          i

 *>i 44.0.1.0/24         44.0.1.4         0  100     0                  65200i

 *>i 44.0.2.0/24         44.0.1.4         0   100     0                  65200i

 *>i 44.0.3.0/24         44.0.1.4         0   100     0                  65200i

 *>i 100.0.0.0/24       44.0.1.4         0   100     0                  65200i

 *                              11.0.1.1          0                     0 222 22 65100 i

 (Router R2 now selects 44.0.1.4 as best path because it can reach 100 network through As 65200, where as to reach 100 network via 11.0.1.1 through AS 222 22  65100  111 11;Hence AS-PATH is manipulated.)