Friday, December 14, 2007

Running BGP across parallel serial links

 

Whenever I'm describing the idea of running BGP across parallel serial links with duplicate IP addresses (like I did in the November IP Corner article, Load Balancing in BGP Networks, section External BGP Load Balancing), there's always someone asking “does it really work?” … so I'm enclosing a tested working configuration.

AS 11
AS 12

interface Serial1/1
ip address 10.0.1.9 255.255.255.252
encapsulation ppp
!
interface Serial1/2
ip address 10.0.1.9 255.255.255.252
encapsulation ppp
!
router bgp 11
bgp log-neighbor-changes
neighbor 10.0.1.10 remote-as 12
interface Serial1/1
ip address 10.0.1.10 255.255.255.252
encapsulation ppp
!
interface Serial1/2
ip address 10.0.1.10 255.255.255.252
encapsulation ppp
!
router bgp 12
bgp log-neighbor-changes
network 172.16.0.0
neighbor 10.0.1.9 remote-as 11
!
ip route 172.16.0.0 255.255.0.0 Null0

Here are a few printouts. First the BGP neighbors …

AS11#show ip bgp summary ¦ begin Neighbor
Neighbor   V  AS MsgRcvd MsgSent TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.1.10  4  12      13      12      2    0    0 00:09:02        1

… then the BGP routing table …

R2#show ip bgp | begin Network
   Network     Next Hop    Metric LocPrf Weight Path
*> 172.16.0.0  10.0.1.10        0             0 12 i

… and finally the internal details of the CEF entry (that's the only way to actually verify that the load balancing is taking place):

AS11#show ip cef 172.16.0.0 internal
172.16.0.0/16, version 35, epoch 0, per-destination sharing
0 packets, 0 bytes
  tag information from 10.0.1.10/32, shared
    local tag: 17
  via 10.0.1.10, 0 dependencies, recursive
    next hop 10.0.1.10, Serial1/1 via 10.0.1.10/32
    valid adjacency
    tag rewrite with Se1/1, point2point, tags imposed: {}
 
  Recursive load sharing using 10.0.1.10/32
  Load distribution: 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 (refcount 2)
 
  Hash  OK  Interface                 Address         Packets  Tags imposed
  1     Y   Serial1/1                 point2point           0    none
  2     Y   Serial1/2                 point2point           0    none
  3     Y   Serial1/1                 point2point           0    none
  4     Y   Serial1/2                 point2point           0    none
  5     Y   Serial1/1                 point2point           0    none
  6     Y   Serial1/2                 point2point           0    none
  7     Y   Serial1/1                 point2point           0    none
  8     Y   Serial1/2                 point2point           0    none
  9     Y   Serial1/1                 point2point           0    none
  10    Y   Serial1/2                 point2point           0    none
  11    Y   Serial1/1                 point2point           0    none
  12    Y   Serial1/2                 point2point           0    none
  13    Y   Serial1/1                 point2point           0    none
  14    Y   Serial1/2                 point2point           0    none
  15    Y   Serial1/1                 point2point           0    none
  16    Y   Serial1/2                 point2point           0    none

Posted to Cisco IOS Hints and Tricks by Ivan Pepelnjak
© Copyright Ivan Pepelnjak

Posted by at