Technical Tip: How to modify BGP next hop for route reflector peering

Description

This article describes that BGP Route reflectors have the special ability to advertise routes learned from an iBGP peer to other iBGP peers. The route reflector will reduce the number of iBGP peering’s, but it does not change the next hop.

Scope

FortiGate.

Solution

In the following diagram, eBGP peering is configured over directly connected interfaces. The iBGP peerings are configured over Loopback addresses. An IGP is running to advertise Loopback addresses.

Each router advertises its Loopback address (/32) into BGP.

R4 is configured as a Route Reflector.

• R4 sees R3 (3.3.3.3) as the next hop for the eBGP route(1.1.1.1).
• R2 sees R4 (4.4.4.4) as the next hop for iBGP and eBGP routes.
• R3 sees R4 (4.4.4.4) as the next hop for iBGP routes and R1 (10.1.3.1) for eBGP routes.

Updating next hop for non-reflected routes:

By design, when R3 advertises the eBGP route for 1.1.1.1/32 received from R1, it will not change the next hop. This can be overridden if needed using 'set next-hop-self enable' on iBGP peering with R4.

R3:

config router bgp
    config neighbor
        edit "4.4.4.4"
            set next-hop-self enable
        next
    end
end

Verification on R4:

R4 # get router info bgp network   <--- Shows the BGP database.
VRF 0 BGP table version is 2, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight RouteTag Path
*>i1.1.1.1/32       3.3.3.3                  0    100      0        0 65002 i <-/1> <----- Originated in external AS.
*>i2.2.2.2/32       2.2.2.2                  0    100      0        0 i <-/1>
*>i3.3.3.3/32       3.3.3.3                  0    100      0        0 i <-/1>
*> 4.4.4.4/32       0.0.0.0                       100  32768        0 i <-/1>

Total number of prefixes 4

R4 installs an external route (1.1.1.1/32) in RIB with R3 as the next hop.

Updating next hop for reflected routes:

In this environment, R4 acts as a route reflector. Enabling 'next-hop-self' in the BGP neighbor configuration will not affect reflected routes; it modifies the next hop for local and eBGP learned routes only. 

Related article:

Technical Tip: Configuring BGP route reflector