NetworkTalk & BGP

A.7.a) Route-map and BGP Configuration with ISP-B

  1. ip as-path access-list 1 permit ^300_
    1. ISP-A will receive prefixes originated by ISP-B (as-number 300); all prefixes of the Internet; meaning 440k prefixes approximately. The routes received will begin obligatory with the as-number 300.
  2. ip as-path access-list 2 permit ^$
    1. This will allow ISP-A’s networks to be advertised to ISP-B . In other words ISP-B will receive prefixes concerning ISP-A’s networks.
  3. Two route-maps will be created and used, as we can see the sequence number is not important now, because we do not want to tag some prefixes differently. All prefixes from ISP-A have the same attribute preference.
    1. One route-map for controlling the BGP table routing into the BGP session from ISP-B to ISP-A:
      route-map AS-300-INCOMING permit 10
      – Same sequence number; the map-tag AS-300-INCOMING is used only to qualify incoming routes from ISP-B to ISP-A, but convention names can be different …
      match as-path 1
      ISP-A will receive the full Internet routing table from ISP-B; this will allow ISP-A to UPLOAD IP-packets to ISP-B (notice that LocalPref is 100, the default value unspecified)
    2. One route-map for controlling the Outgoing routing into the BGP session from ISP-A to ISP-B:
      route-map AS-300-OUTGOING permit 10
      match as-path 2
      – ISP-A will advertise its own networks to ISP-B. This means that ISP-A sends its routes to ISP-B, and so downloads IP-packets from ISP-B.
      – From ISP-B’s point of view, we can say that ISP-B will upload packets to ISP-A
  4. Then we apply the route-map to the ISP-B neighbor under the BGP router 1000’s session. We also declare ISP-A’s networks through the network statement.
    router bgp 1000
    neighbor 20.20.20.85 route-map AS-300-INCOMING in
    neighbor 20.20.20.85 route-map AS-300-OUTGOING out

Finally, we have the following template:

router bgp 1000
neighbor 20.20.20.85 remote-as 300
neighbor 20.20.20.85 description ISP-B-STM16
neighbor 20.20.20.85 version 4
address-family ipv4
neighbor 20.20.20.85 activate
neighbor 20.20.20.85 send-community both
neighbor 20.20.20.85 route-map AS-300-INCOMING in
neighbor 20.20.20.85 route-map AS-300-OUTGOING out

ip as-path access-list 2 permit ^$
ip as-path access-list 1 permit ^300_

route-map AS-300-INCOMINGS permit 10
match as-path 1

route-map AS-300-OUTGOING permit 10
match as-path 2

top

Come back to Tutorial Index”