WB1 7.25 BGP Bestpath Selection - Router-IDs

7.25 BGP Bestpath Selection - Router-IDs
 
• Modify the BGP router-ids in AS 100 as necessary so that traffic from R1 to AS 54 exits via R6.
 
Note
 
The next few steps after the MED attribute processing include:

a) Selecting external paths over internal, as the information should be more actual. Here external paths are prefixes learned via eBGP sessions on the router,while internal paths are learned via iBGP sessions.

b) Prefer the path with the minimum IGP metric to reach the NEXT_HOP IP address. This is a natural selection step, as it attempts to pick up to closes exit point based on the local AS IGP metrics. Keep in mind that exit point selection based on MED (adjacent AS metrics) is preferred over this step. Routing based on the closes exit point in terms of local IGP metric is called “hot potato” routing. On contrary, routing based on the MED values, i.e. based on the metric values advertised by the adjacent AS is called “cold potato” routing. If all prefixes have the same IGP cost to reach their NEXT_HOPs, the BGP process may consider inserting all of them into RIB, implementing equal cost multipath load-balancing. This feature is enabled by using the command maximum-paths [ibgp] under BGP configuration mode. Specify the ibgp keyword if you want to load balance among the paths learned via iBGP.

c) Among the prefixes learned from different eBGP peers, prefer the older one, to minimize route flapping.

d) Use BGP Router IDs of the advertising (peering) routers as tie-breakers for the best-path selection process. The path advertised by the peer with the lowest router ID is preferred.

In this scenario, the default BGP Router IDs for R4 and R6 are based on their Loopback0 IP address value. This makes R1 prefer R4 over R6 as the exit point, as all other criteria are the same. In order to change this, we configure R6 with an artificially lowers Router ID value. Remember that changing a router’s BGP router ID will hard reset all active BGP sessions.

Configuration

R6:
router bgp 100
bgp router-id 6.6.6.6

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變更前

Rack1R1#show ip bgp regexp _54$
BGP table version is 119, local router ID is 150.1.1.1
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
*>i28.119.16.0/24   204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i28.119.17.0/24   204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i114.0.0.0        204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i115.0.0.0        204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i116.0.0.0        204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i117.0.0.0        204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i118.0.0.0        204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
*>i119.0.0.0        204.12.1.254             0    100      0 54 i
* i                 54.1.1.254               0    100      0 54 i
Rack1R1#show ip bgp 119.0.0.0
BGP routing table entry for 119.0.0.0/8, version 87
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
        1    2
  54, (Received from a RR-client)    204.12.1.254 (metric 2560002816) from 155.1.146.4 (150.1.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal, best
  54, (Received from a RR-client)
    54.1.1.254 (metric 2560002816) from 155.1.146.6 (150.1.6.6)
      Origin IGP, metric 0, localpref 100, valid, internal
Rack1R1#

變更後

Rack1R1#show ip bgp regexp _54$
BGP table version is 130, local router ID is 150.1.1.1
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
*>i28.119.16.0/24   54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i28.119.17.0/24   54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i114.0.0.0        54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i115.0.0.0        54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i116.0.0.0        54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i117.0.0.0        54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i118.0.0.0        54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
*>i119.0.0.0        54.1.1.254               0    100      0 54 i
* i                 204.12.1.254             0    100      0 54 i
Rack1R1#
Rack1R1#show ip bgp 119.0.0.0
BGP routing table entry for 119.0.0.0/8, version 126
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x800
  Advertised to update-groups:
        1    2
  54, (Received from a RR-client)
    54.1.1.254 (metric 2560002816) from 155.1.146.6 (6.6.6.6)
      Origin IGP, metric 0, localpref 100, valid, internal, best
  54, (Received from a RR-client)
    204.12.1.254 (metric 2560002816) from 155.1.146.4 (150.1.4.4)
      Origin IGP, metric 0, localpref 100, valid, internal
Rack1R1#