notes: Troubleshooting Load-Balancing Scenarios in Small BGP Networks

1.  Problem: Load Balancing and Managing Outbound Traffic from a Single Router When Dual Homed to Same ISP—Cause: BGP Installs Only One Best Path in the Routing Table

In multihomed scenarios, a common concern that enterprise network operators face is improperly utilizing the external links going to the ISP. Typically, enterprise customers dual-home to either the same or different ISPs to load-share outgoing and incoming traffic.

debugs and verification:

show ip bgp a.b.c.d

show ip route a.b.c.d

Solution:

Cisco IOS Software allows, by configuration, the installation of more than one route for the same prefix,  This does come with a tight check: Multiple paths that are candidates to go in the routing table have the exact same BGP attribute except for the router ID (RID). If two or more paths have identical attributes except for the RID, they can go in the routing table and load sharing can be achieved for traffic going to that prefix.

maximum-path  n

The maximum-path n command allows two equal BGP paths to be installed in the routing table. Cisco IOS Software allows a maximum of six equal paths. the BGP output, only one path has "best" in its output, but both have "multipath" and thus both will be installed in the routing table.

2. Problem: Load Balancing and Managing Outbound Traffic in an IBGP Network—Cause: By Default, IBGP in Cisco IOS Software Allows Only a Single Path to Get Installed in the Routing Table Even Though Multiple Equal BGP Paths Exist

If multiple paths are received from different IBGP neighbors for the same prefix, only one best path will be selected and installed in the routing table. This results in other alternate paths being unused.

debugs and verification:

show ip bgp a.b.c.d

show ip route a.b.c.d

Solution:

maximum-path ibgp n

For maximum-paths ibgp to work, the following conditions must be met:

In both paths, all BGP attributes—LOCAL_PREF, MED,ORIGIN, and AS_PATH (entire AS_PATH)—must be identical.

Both paths must be learned through IBGP.

Both paths must be synchronized.

Both paths must have a reachable next hop.

Both paths must have an EQUAL IGP cost to the next hop.