BGP Lab Exercise 3
------------------

1.  This exercise will use AS Path filters on the BGP sessions.  These
   will ensure that we only announce our prefixes to our neighbours.
   Create an AS Path filter which permits only prefixes originated by
   your peer AS to enter your network

   RouterA(config)# ip as-path access-list 1 permit ^2$
                                     ! permit prefixes originated by AS2


2.  Create an AS Path filter which permits only prefixes originated by
   your AS to leave your network

   RouterA(config)# ip as-path access-list 2 permit ^$
                                     ! permit prefixes originated by local AS
   

3.  Enable soft reconfiguration for the BGP session.  This is only so
   that we can analyse the effect the applying the filter on our BGP
   session later on in this exercise.

   NOTE: The use of soft reconfiguration in general ISP operations
   is not recommended apart from during session debugging, as it
   requires extra memory and has been superceded by a BGP feature
   called "route refresh".  We use it in this exercise for debugging,
   but you would not use it in normal operations.

   RouterA(config)# router bgp 1
   RouterA(config-router)# address-family ipv4
   RouterA(config-router-af)# neighbor 196.200.220.xxx soft-reconfiguration in

   RouterA(config-router)# address-family ipv6
   RouterA(config-router-af)# neighbor 2001:4348:220:xx::x soft-reconfig in


4.  Now apply filter-list 1 inbound on the eBGP session you have with
   your peer AS.

   RouterA(config-router)# address-family ipv4
   RouterA(config-router-af)# neighbor 196.200.220.xxx filter-list 1 in
                                     ! apply as-path filter 1 inbound

   And apply filter-list 2 to outbound on the eBGP session you have with
   both AS100 and your peer AS.

   RouterA(config-router-af)#neighbor 196.200.220.xxx filter-list 2 out
   RouterA(config-router-af)#neighbor 196.200.222.1   filter-list 2 out
                                     ! apply as-path filter 2 outbound

   Do the same for the IPv6 peerings.  Remember you want to make the
   changes to the IPv6 address family and to the IPv6 neighbours.  The
   filter-list can be used for both IPv4 and IPv6 peerings.


5.  Now that you have applied the policy, you need to tell the BGP
   session to refresh the prefixes it has received from the neighbour.
   To do this, we use the route refresh capability of BGP.

   routerA#clear ip bgp 2 in         ! refresh the peering with AS2
   routerA#clear ip bgp 2 out        ! refresh the peering with AS2


6. Do the same for IPv6.

   routerA#clear bgp ipv6 unicast 2 in     ! refresh the peering with AS2
   routerA#clear bgp ipv6 unicast 2 out    ! refresh the peering with AS2


7.  Check the BGP process using "sh ip bgp" and "sh bgp ipv6" to see
   what prefixes you are now receiving from your neighbouring AS.  Use
   the more specific commands "sh ip bgp neighbor 196.200.220.xxx
   routes" and "sh ip bgp neighbor 196.200.220.xxx received-routes".
   What difference do you see?  Write your answer here:





   What about for IPv6?  Write the results of the IPv6 version of the
   above commands here: