Challenge: BGP

I was hanging out in theLANtamer's Discord server and a friend showed me this challenge.

I later found out after I had completed the challenge, it was a BGP challenge from the Great Vinit Jain (https://twitter.com/vinugenie) from his Cisco Live presentation, at around 12 minutes in: ( https://www.ciscolive.com/global/on-demand-library/?search=BRKRST%203320#/video/1519328378542002IVEW )

The Challenge:

The Solution:

The Configs:

R1:

hostname R1 interface Loopback100  ip address 100.1.1.1 255.255.255.255 interface FastEthernet0/0  description "To R2"  ip address 10.1.12.1 255.255.255.0 router bgp 100  bgp log-neighbor-changes  network 100.1.1.1 mask 255.255.255.255  neighbor 10.1.12.2 remote-as 200

R2:

hostname R2 ip vrf 200  rd 200:200 interface FastEthernet0/0  description "To R1"  ip vrf forwarding 200  ip address 10.1.12.2 255.255.255.0 interface FastEthernet1/0  description "To R3"  ip vrf forwarding 200  ip address 10.1.23.2 255.255.255.0 router bgp 200  bgp log-neighbor-changes  !  address-family ipv4 vrf 200   neighbor 10.1.12.1 remote-as 100   neighbor 10.1.12.1 activate   neighbor 10.1.23.3 remote-as 100   neighbor 10.1.23.3 activate   neighbor 10.1.23.3 as-override  exit-address-family

R3:

hostname R3 interface FastEthernet0/0  description "To R4"  ip address 10.1.34.3 255.255.255.0 interface FastEthernet1/0  description "To R2"  ip address 10.1.23.3 255.255.255.0 router bgp 250  bgp log-neighbor-changes  neighbor 10.1.23.2 remote-as 200  neighbor 10.1.23.2 local-as 100 no-prepend replace-as  neighbor 10.1.34.4 remote-as 200  neighbor 10.1.34.4 local-as 200 no-prepend replace-as

R4:

hostname R4 interface FastEthernet0/0  description "To R3"  ip address 10.1.34.4 255.255.255.0 interface FastEthernet1/0  description "To R5"  ip address 10.1.45.4 255.255.255.0 router bgp 200  bgp log-neighbor-changes  neighbor 10.1.34.3 remote-as 200  neighbor 10.1.45.5 remote-as 300

R5:

hostname R5 interface FastEthernet1/0  description "To R4"  ip address 10.1.45.5 255.255.255.0 router bgp 300  bgp log-neighbor-changes  neighbor 10.1.45.4 remote-as 200

R5 - Verify:

R5#sho ip bgp BGP table version is 13, local router ID is 10.1.45.5 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  *>  100.1.1.1/32     10.1.45.4                              0 200 200 200 i R5#

TIL: as-path prepending

Today I learned: You can prepend any AS numbers in the prepended string.

They typical method of as-path prepending is to prepend or add your autonomous system number to the AS_PATH attribute to influence inbound traffic patterns.

You can technically add any autonomous system to the AS_PATH even AS's that don't belong to you.

NOTE: This is frowned upon in production. "Just because you can doesn't mean you should!"

See the example below:

Without context or a topology this seems a little bland but the results are there. You can see from the BGP table below we have prepended a bunch of AS's that do not belong to us.

Prepeding configured out-bound from R3 --> R1:

R3#sho run | s as-path|route-map|router bgp
router bgp 200

 neighbor 155.1.13.1 remote-as 100
 neighbor 155.1.13.1 route-map AS_254 out

ip as-path access-list 254 permit ^254$

route-map AS_254 permit 10
 match as-path 254
 set as-path prepend 254 250 123

route-map AS_254 permit 20

Showing the R1 partial BGP table:

R1#sho ip bgp neighbors 155.1.13.3 routes

[ ... OUTPUT OMITTED ... ]

     Network          Next Hop            Metric LocPrf Weight Path
 *>  28.119.16.0/24   155.1.13.3                             0 200 54 i
 *>  28.119.17.0/24   155.1.13.3                             0 200 54 i
 *   51.51.51.51/32   155.1.13.3                             0 200 254 250 123 254 ?
 *   205.90.31.0      155.1.13.3                             0 200 254 250 123 254 ?
 *   220.20.3.0       155.1.13.3                             0 200 254 250 123 254 ?
 *   222.22.2.0       155.1.13.3                             0 200 254 250 123 254 ?

Credit: This was influenced by a lab from the INE workbook.