4-1-1. Cisco dual BGP with Metric (Redundancy)

Cisco dual BGP with Metric (Redundancy)

ISPs : 2 WAN links : 2 CPE : 1 PE : 2 Device : Cisco router with LAN/WAN ports. WAN1 / Serial IP : 10.20.1.0/30 (mask 255.255.255.252) WAN2 / Serial IP : 10.30.1.0/30 (mask 255.255.255.252) LAN / Ethernet IP : 100.100.100.0/24 Receiving Route : Default route only from ISP Memory : Full route(recommend 512M), partial route (recommend 128-256M) LAN : ISP_A (AS20), ISP_B(AS30), Customer (AS10) and Other ISP(AS50)

[Answer]

Stand alone Metric attribute won’t be good idea as a solution for failover and load-sharing, when you have more than two ISP links.

why? simply shortest AS path is more prefer attribute than Metric value on BGP selection criteria. 

 

For example, if you put metric value on BGP announcements toward ISP_"B" link to make less prefer to pick path for the inbound traffic. All traffic which is out side of ISP_"B" will flow to ISP_"A", however, still traffic source which is connnected to ISP_"B", will flow to link with ISP_"B". BGP table for the destination IP / 100.100.100.2 on ISP_"B" side, two paths would be available for the destination. See below example.

TR_B#sh ip bgp 100.100.100.2
BGP routing table entry for 100.100.100.0/24, version 46
Paths: (3 available, best #3, table Default-IP-Routing-Table)
Flag: 0x820
  Advertised to update-groups:
     2
  50 20 10
    30.50.1.2 from 30.50.1.2 (30.50.1.2)
      Origin incomplete, localpref 100, valid, external
  20 10
    20.30.1.1 from 20.30.1.1 (20.50.1.1)
      Origin incomplete, localpref 100, valid, external
  10
    10.30.1.1 from 30.30.1.1 (30.30.1.1)
      Origin incomplete, metric 300, localpref 100, valid, internal, best
TR_B#

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