BGP Path Attributes

BGP Path Attributes

“We Love Oranges AS Oranges Mean Pure Refreshment”

R1#sh ip bgp 40.40.40.0
*Mar  1 00:17:21.323: %SYS-5-CONFIG_I: Configured from console by console
R1#sh ip bgp 40.40.40.0
BGP routing table entry for 40.40.40.0/24, version 4
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1
  30
    3.3.3.2 from 3.3.3.2 (3.3.3.2)
      Origin IGP, metric 0, localpref 100, valid, external, best
  20
    2.2.2.2 from 2.2.2.2 (20.20.20.1)
      Origin IGP, metric 0, localpref 100, valid, external

R1#sh ip bgp
BGP table version is 4, local router ID is 3.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 20.20.20.0/24    2.2.2.2                  0             0 20 i
*> 30.30.30.0/24    3.3.3.2                  0             0 30 i
*> 40.40.40.0/24    3.3.3.2                  0             0 30 i
*                   2.2.2.2                  0             0 20 i

• BGP uses different path attributes to calculate the best path for a route.
• Each path attribute defines something different about the path.
• The most common BGP path attributes are:
  • Weight
  • Local Preference
  • AS_PATH
  • Origin
  • Multi Exit Discriminator

Weight

• Higher is preferred.
• Can be used to influcence outbound traffic.
• Cisco proprietary feature (not exactly a path attribute).
• Locally significant to a single router not advertised to any other BGP neighbors.
• Default weight is 0.
• Range from 0 to 65535 ( 2^16-1 )
• With a route-map use the set weight

R1(config)#router bgp 10
R1(config-router)#neighbor 2.2.2.2 weight 100

R1#sh ip bgp
BGP table version is 4, local router ID is 3.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 20.20.20.0/24    2.2.2.2                  0           100 20 i
*> 30.30.30.0/24    3.3.3.2                  0             0 30 i
*  40.40.40.0/24    3.3.3.2                  0             0 30 i
*>                  2.2.2.2                  0           100 20 i

ip prefix-list 20 seq 5 permit 40.40.40.0/24 le 32
route-map 30 permit 10
 match ip address prefix-list 20
 set weight 150
!
route-map 30 permit 20

R1#sh ip bgp
BGP table version is 4, local router ID is 3.3.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 20.20.20.0/24    2.2.2.2                  0           100 20 i
*> 30.30.30.0/24    3.3.3.2                  0             0 30 i
*> 40.40.40.0/24    3.3.3.2                  0           150 30 i
*                   2.2.2.2                  0           100 20 i

Local_Pref

• Can be used to influcence outbound traffic.
• This is a BGP path attribute.
• Defines the best exitpoint from the AS to a prefix.
• local_pref is advertised to all iBGP neighbors, but not to eBGP neighbors ( non transitive ).
• Higher value is preferred
• Range is 0 – 4,294,967, 295 ( 2^32-1)
• Default value is 100
• Use a route-map with set local-preference

access-list 1 permit 30.30.30.0 0.0.0.255

route-map 30 permit 10
 match ip address prefix-list 20
 set weight 150
!
route-map 30 permit 20
 match ip address 1
 set local-preference 120

R1# sh ip bgp
best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 20.20.20.0/24    2.2.2.2                  0           100 20 i
*> 30.30.30.0/24    3.3.3.2                  0    120      0 30 i
*> 40.40.40.0/24    3.3.3.2                  0           150 30 i
*                   2.2.2.2                  0           100 20 i

AS_Path Length

• Can be used to influcence inbound and outbound traffic
• AS_PATH is a BGP attribute
• Default PA to determine best path when no other PAs are modified
• Shortest AS_PATH is preferred
• With AS prepending you can create a longer path.

Multi Exit Distriminator

• Med is a BGP Path attribute.
• It can be modified to influence the inbound traffic.
• With dual links the incoming traffic can be influenced.
• It’s not advertised beyond the neighboring AS
• Range is 0 – 4,294,967, 295 ( 2^32-1)
• Default zero