This section demonstrates how MED can be used by one AS to influence routing decisions of another AS. 

 

                                                      Figure 1-18

Step 1: Configure BGP as TOPO displayed

Switch A:

admin@XorPlus#set protocols bgp bgp-id 6.6.6.6
admin@XorPlus#set protocols bgp local-as "6"
admin@XorPlus#set protocols bgp peer 1001::2 local-ip "1001::1"
admin@XorPlus#set protocols bgp peer 1001::2 as "9"
admin@XorPlus#set protocols bgp peer 1001::2 ipv6-unicast true
admin@XorPlus#set protocols bgp peer 3003::2 local-ip "3003::1"
admin@XorPlus#set protocols bgp peer 3003::2 as "9"
admin@XorPlus#set protocols bgp peer 3003::2 ipv6-unicast true

Switch B:

admin@XorPlus#set protocols bgp bgp-id 26.26.26.26
admin@XorPlus#set protocols bgp local-as "9"
admin@XorPlus#set protocols bgp peer 1001::1 local-ip "1001::2"
admin@XorPlus#set protocols bgp peer 1001::1 as "6"
admin@XorPlus#set protocols bgp peer 1001::1 ipv6-unicast true
admin@XorPlus#set protocols bgp peer 2002::2 local-ip "2002::1"
admin@XorPlus#set protocols bgp peer 2002::2 as "12"
admin@XorPlus#set protocols bgp peer 2002::2 ipv6-unicast true

Switch C:

admin@XorPlus#set protocols bgp bgp-id 9.9.9.9
admin@XorPlus#set protocols bgp local-as "9"
admin@XorPlus#set protocols bgp peer 3003::1 local-ip "3003::2"
admin@XorPlus#set protocols bgp peer 3003::1 as "6"
admin@XorPlus#set protocols bgp peer 3003::1 ipv6-unicast true
admin@XorPlus#set protocols bgp peer 5005::2 local-ip "5005::1"
admin@XorPlus#set protocols bgp peer 5005::2 as "10"
admin@XorPlus#set protocols bgp peer 5005::2 ipv6-unicast true

Switch D:

admin@XorPlus#set protocols bgp bgp-id 44.44.44.44
admin@XorPlus#set protocols bgp local-as 12
admin@XorPlus#set protocols bgp peer 2002::1 local-ip "2002::2"
admin@XorPlus#set protocols bgp peer 2002::1 as 9
admin@XorPlus#set protocols bgp peer 2002::1 ipv6-unicast true

Switch E:

admin@XorPlus#set protocols bgp bgp-id 33.33.33.33
admin@XorPlus#set protocols bgp local-as 10
admin@XorPlus#set protocols bgp peer 5005::1 local-ip "5005::2"
admin@XorPlus#set protocols bgp peer 5005::1 as 9
admin@XorPlus#set protocols bgp peer 5005::1 ipv6-unicast true

 

Step 2: Switch D and Switch E propagate BGP route entry 9999::/64 to Switch B andSwitch C

Note: check the BGP route table on Switch A.

Switch B:

admin@XorPlus# run show bgp routes ipv6
Status Codes: * valid route, > best route
Origin Codes: i IGP, e EGP, ? incomplete
 
   Prefix                        Nexthop                   Peer             AS Path
   ----------------------------  ------------------------  ---------------  ------------
*> 9999::/64                     2002::2                   44.44.44.44      12 i

Switch C:

admin@XorPlus# run show bgp routes ipv6
Status Codes: * valid route, > best route
Origin Codes: i IGP, e EGP, ? incomplete
 
   Prefix                        Nexthop                   Peer             AS Path
   ----------------------------  ------------------------  ---------------  ------------
*> 9999::/64                     5005::2                   33.33.33.33      10 i

Switch A:

admin@XorPlus# run show bgp routes ipv6
Status Codes: * valid route, > best route
Origin Codes: i IGP, e EGP, ? incomplete
 
   Prefix                        Nexthop                   Peer             AS Path
   ----------------------------  ------------------------  ---------------  ------------
*  9999::/64                     1001::2                   26.26.26.26      9 12 i
*> 9999::/64                     3003::2                   9.9.9.9          9 10 i
admin@XorPlus#
admin@XorPlus# run show bgp routes ipv6 de
Possible completions:
  <IPNet>                       Print BGP IPv6 routes of specified prefix
  detail                        Print detailed BGP IPv6 routes
admin@XorPlus# run show bgp routes ipv6 detail
9999::/64
        From peer: 9.9.9.9
        Route: Winner
        Origin: IGP
        AS Path: 9 10
        Nexthop: 3003::2
        Multiple Exit Discriminator: 0
        Local Preference: 100
9999::/64
        From peer: 26.26.26.26
        Route: Not Used
        Origin: IGP
        AS Path: 9 12
        Nexthop: 1001::2
        Multiple Exit Discriminator: 0
        Local Preference: 100
 

Note: The MED all is 0. The two BGP route entry.

 

Step 3: Modify the MED value on Switch C

Switch C:

admin@XorPlus# set protocols bgp med 100

Switch A:

admin@XorPlus# run show bgp routes ipv6
Status Codes: * valid route, > best route
Origin Codes: i IGP, e EGP, ? incomplete
 
   Prefix                        Nexthop                   Peer             AS Path
   ----------------------------  ------------------------  ---------------  ------------
*  9999::/64                     3003::2                   9.9.9.9          9 10 i
*> 9999::/64                     1001::2                   26.26.26.26      9 12 i
admin@XorPlus# run show bgp routes ipv6 detail
9999::/64
        From peer: 26.26.26.26
        Route: Winner
        Origin: IGP
        AS Path: 9 12
        Nexthop: 1001::2
        Multiple Exit Discriminator: 0
        Local Preference: 100
9999::/64
        From peer: 9.9.9.9
        Route: Not Used
        Origin: IGP
        AS Path: 9 10
        Nexthop: 3003::2
        Multiple Exit Discriminator: 100
        Local Preference: 100
admin@XorPlus#

Note: The best route is coming from Switch B, as the BGP route entry from Switch B has a smaller MED value. It will select the BGP route entry with smaller MED value if other attributes all have the same priority.

 

Step 4: Modify the MED larger than Switch C

Switch B:

admin@XorPlus# set protocols bgp med 200

Switch A:

admin@XorPlus# run show bgp routes ipv6
Status Codes: * valid route, > best route
Origin Codes: i IGP, e EGP, ? incomplete
 
   Prefix                        Nexthop                   Peer             AS Path
   ----------------------------  ------------------------  ---------------  ------------
*  9999::/64                     1001::2                   26.26.26.26      9 12 i
*> 9999::/64                     3003::2                   9.9.9.9          9 10 i
admin@XorPlus# run show bgp routes ipv6 detail
9999::/64
        From peer: 9.9.9.9
        Route: Winner
        Origin: IGP
        AS Path: 9 10
        Nexthop: 3003::2
        Multiple Exit Discriminator: 100
        Local Preference: 100
9999::/64
        From peer: 26.26.26.26
        Route: Not Used
        Origin: IGP
        AS Path: 9 12
        Nexthop: 1001::2
        Multiple Exit Discriminator: 200
        Local Preference: 100

Note: The BGP speaker selects the BGP route entry with the smaller MED.