EBGP Multihop

By default, EBGP neighbors exchange packets with a TTL (Time To Live) set to 1. If you attempt to establish an EBGP session between loopbacks, BGP packets will be dropped due to an expired TTL.

The ebgp-multihop keyword is a Cisco IOS option. It must be configured on each peer. The ebgp-multihop keyword is only used for EBGP sessions, not for IBGP. EBGP neighbors are usually directly connected (over a WAN connection, for example) to establish an EBGP session. However, sometimes one of the directly connected routers is unable to run BGP. The ebgp-multihop keyword allows for a logical connection to be made between peer routers, even if they are not directly connected. The ebgp-multihop keyword allows for an EBGP peer to be up to 255 hops away and still create an EBGP session.

If redundant links exist between two EBGP neighbors and loopback addresses are used, you must configure ebgp-multihop. Otherwise, the router decrements the TTL before giving the packet to the loopback interface, meaning that the normal IP forwarding logic discards the packet.

				
					//R1 
ip route 10.20.20.1 255.255.255.255 209.165.201.2
!
router bgp 64496
 neighbor 10.20.20.1 remote-as 64511
 neighbor 10.20.20.1 update-source loopback0
 /* Informs R1 to use the Loopback 0 IP address 
    as the source IP address for all BGP TCP packets sent to R2 */
 neighbor 10.20.20.1 ebgp-multihop 2
 /* Allows for two routers that are not directly connected
 to establish an EBGP session. A TTL value of 2 is defined */

// R2
ip route 10.10.10.1 255.255.255.255 209.165.201.1
!
router bgp 64511
 neighbor 10.10.10.1 remote-as 64496
 neighbor 10.10.10.1 update-source loopback0
 neighbor 10.10.10.1 ebgp-multihop 2
				
			
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