- Cisco Networking: EBGP load balancing with EBGP session between loopback interfaces

Multihop EBGP sessions are the traditional way to implement EBGP load balancing on parallel links. EBGP session is established between loopback interfaces of adjacent routers (see the next diagram; initial router configurations are included at the bottom of the article) and static routes (or an extra instance of a dynamic routing protocol) are used to achieve connectivity between loopback interfaces (BGP next-hops). The load balancing is an automatic result of the recursive route lookup of BGP next hops.

EBGP session between loopback interfaces is most appropriate in scenarios where all the links between the EBGP neighbors have identical bandwidth. To achieve proportional load balancing across links with different bandwidths, you could use EIGRP as the dynamic routing protocol or single-hop MPLS traffic engineering. However, it’s better to use parallel EBGP sessions in the unequal-bandwidth scenarios.

Multihop EBGP session with static routes

The following configuration steps are necessary when you decide to use static routes to provide connectivity between the loopback interfaces (endpoints of the EBGP session):

• Configure static routes toward the loopback address of EBGP neighbor. The static routes can point to the next-hop (see the configuration of R1) or the physical interface (as configured on E1).

Static routes pointing to the physical interface shall be used only on point-to-point interfaces.

Static route configuration on R1

ip route 10.0.0.23 255.255.255.255 10.0.1.1
ip route 10.0.0.23 255.255.255.255 10.0.1.9

Static route configuration on E1

ip route 10.0.0.1 255.255.255.255 Serial1/0
ip route 10.0.0.1 255.255.255.255 Serial1/3 

Configure EBGP neighbor. Use the neighbor’s loopback address and specify the source address of the EBGP session with the neighbor address update-source loopback x router configuration command.

If you don’t use the neighbor update-source router configuration command, the EBGP session will not be established. When the source interface of the BGP session is not specified, the router uses the IP address of the outgoing link as the source address of the BGP session and the EBGP peer will reset the incoming BGP session coming from an unknown IP address.

BGP neighbor configuration on R1

router bgp 64800
neighbor 10.0.0.23 remote-as 65000
neighbor 10.0.0.23 update-source Loopback0

BGP neighbor configuration on E1

router bgp 65000
neighbor 10.0.0.1 remote-as 64800
neighbor 10.0.0.1 update-source Loopback0

Configure multihop EBGP session with neighbor ebgp-multihop router configuration command.

Without the neighbor ebgp-multihop router configuration command,
the EBGP session will not be established as Cisco IOS expects a regular
EBGP neighbor to be directly connected.

EBGP multihop configuration on R1

router bgp 64800
neighbor 10.0.0.23 ebgp-multihop 2 

EBGP multihop configuration on E1

router bgp 65000
neighbor 10.0.0.1 ebgp-multihop 2

The neighbor ebgp-multihop command makes the EBGP session
vulnerable to TCP session hijacking. To minimize this risk, use the
lowest possible TTL value (2) as the maximum hop count parameter in this command.
The default TTL value setting (when the neighbor ebgp-multihop is used without
the maximum hop count parameter) is 255.
The effects of the maximum hop count parameter on the TTL field of outgoing IP
packets is shown in the following Wireshark capture.

After the neighbor ebgp-multihop has been configured, the EBGP session is established and E1 advertises a BGP prefix to R1:

R1#show ip bgp summary | begin Neighbor Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.0.0.23 4 65000 20 18 6 0 0 00:06:53 1

The show ip bgp neighbor indicates that the neighbor 10.0.0.23 is indeed an EBGP neighbor that is up to two hops away:

R1#show ip bgp neighbor 10.0.0.23 BGP neighbor is 10.0.0.23, remote AS 65000, external link BGP version 4, remote router ID 10.0.0.23 BGP state = Established, up for 00:07:50 … deleted … Address tracking is enabled, the RIB does have a route to 10.0.0.23 Connections established 2; dropped 1 Last reset 00:07:59, due to Peer closed the session External BGP neighbor may be up to 2 hops away. Transport(tcp) path-mtu-discovery is enabled … deleted …

After BGP has converged, the BGP routing table on E1 contains a single copy of the prefix 10.1.7.0/24 advertised by R1.

BGP routing table on E1

R1#show ip bgp | begin Network
  Network          Next Hop            Metric LocPrf Weight Path
*> 10.7.1.0/24      10.0.0.23                0             0 65000 i 

The IP routing table also contains a single copy of the prefix …

Route to 10.7.1.0/24 in the IP routing table

R1#show ip route 10.7.1.0
Routing entry for 10.7.1.0/24
 Known via "bgp 64800", distance 20, metric 0
 Tag 65000, type external
 Last update from 10.0.0.23 00:12:34 ago
 Routing Descriptor Blocks:
 * 10.0.0.23, from 10.0.0.23, 00:12:34 ago
     Route metric is 0, traffic share count is 1
     AS Hops 1
     Route tag 65000 

… but the CEF table (which contains the results of the recursive lookup) has two equal-cost paths to the prefix:

Multiple entries to 10.7.1.0/24 in the CEF table

R1#show ip cef 10.7.1.0
10.7.1.0/24
 nexthop 10.0.1.1 Serial1/0
 nexthop 10.0.1.9 Serial1/2 

Fast failover of multihop EBGP session

The fast external failover functionality that is enabled by default on all EBGP sessions does not work on multihop EBGP sessions as it’s tied to the changes in physical interface status and does not use the IP routing table to check EBGP neighbor reachability. Furthermore, Bidirectional Forwarding Detection (BFD) cannot be used with multihop EBGP neighbors.

For example, after both links from R1 to E1 fail, R1 detects the failure only after the BGP keepalive timer (with the default value of 3 minutes) expires.

BGP keepalive timer detects EBGP neighbor failure

16:45:41.127: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/0, changed state to down
16:45:47.619: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/2, changed state to down
16:48:17.091: %BGP-5-ADJCHANGE: neighbor 10.0.0.23 Down BGP Notification sent
16:48:17.095: %BGP-3-NOTIFICATION: sent to neighbor 10.0.0.23 4/0 (hold time expired) 0 bytes 

To improve the detection of an EBGP session failure when the EBGP session uses the neighbor ebgp-multihop option, you have to configure fast BGP neighbor loss detection with the neighbor fall-over command. It’s recommended to use a route-map matching only host routes with the neighbor fall-over command to ensure that the router does not interpret a summary route or a default route as a viable path toward the EBGP neighbor. The fast BGP neighbor loss configuration on E1 is shown in the following printout:

Fast BGP neighbor loss detection configuration on E1

router bgp 65000
neighbor 10.0.0.1 fall-over route-map EBGP_multihop
!
ip prefix-list EBGP_multihop seq 5 permit 0.0.0.0/0 ge 32
!
route-map EBGP_multihop permit 10
match ip address prefix-list EBGP_multihop 

After the neighbor fall-over has been configure on E1, the EBGP session failure detection improves dramatically (the session failure is detected within a few seconds after the last link toward the neighbor fails).

Fast BGP neighbor loss detection on E1

16:57:11.391: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/0, changed state to down
16:57:13.915: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/3, changed state to down
16:57:15.231: %BGP-5-ADJCHANGE: neighbor 10.0.0.1 Down Route to peer lost 

Final router configurations

Final configuration of E1

service timestamps debug uptime
service timestamps log datetime msec
no service password-encryption
!
hostname E1
!
no aaa new-model
no ip source-route
ip cef
!
archive
log config
 hidekeys
!
interface Loopback0
ip address 10.0.0.23 255.255.255.255
!
interface Serial1/0
ip address 10.0.1.1 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
!
interface Serial1/3
ip address 10.0.1.9 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
!
router bgp 65000
no synchronization
bgp log-neighbor-changes
network 10.7.1.0 mask 255.255.255.0
neighbor 10.0.0.1 remote-as 64800
neighbor 10.0.0.1 ebgp-multihop 2
neighbor 10.0.0.1 update-source Loopback0
neighbor 10.0.0.1 fall-over route-map EBGP_multihop
no auto-summary
!
ip forward-protocol nd
ip route 10.0.0.1 255.255.255.255 Serial1/0
ip route 10.0.0.1 255.255.255.255 Serial1/3
ip route 10.7.1.0 255.255.255.0 Null0
no ip http server
no ip http secure-server
!
ip prefix-list EBGP_multihop seq 5 permit 0.0.0.0/0 ge 32
!
route-map EBGP_multihop permit 10
match ip address prefix-list EBGP_multihop
!
control-plane
!
line con 0
stopbits 1
line aux 0
stopbits 1
line vty 0 4
login
!
end 

Final configuration of R1

service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
no aaa new-model
no ip source-route
ip cef
!
archive
log config
 hidekeys
!
interface Loopback0
ip address 10.0.0.1 255.255.255.255
ip ospf 1 area 0
!
interface FastEthernet0/0
ip address 10.2.0.1 255.255.255.0
ip ospf 1 area 0
duplex half
!
interface Serial1/0
ip address 10.0.1.2 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
serial restart-delay 0
!
interface Serial1/2
ip address 10.0.1.10 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
serial restart-delay 0
!
!
router ospf 1
log-adjacency-changes
passive-interface default
no passive-interface FastEthernet0/0
!
router bgp 64800
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.23 remote-as 65000
neighbor 10.0.0.23 ebgp-multihop 2
neighbor 10.0.0.23 update-source Loopback0
neighbor 10.0.0.23 fall-over route-map EBGP_multihop
no auto-summary
!
ip forward-protocol nd
ip route 10.0.0.23 255.255.255.255 10.0.1.1
ip route 10.0.0.23 255.255.255.255 10.0.1.9
no ip http server
no ip http secure-server
!
ip prefix-list EBGP_multihop seq 5 permit 0.0.0.0/0 ge 32
!
route-map EBGP_multihop permit 10
match ip address prefix-list EBGP_multihop
!
control-plane
!
line con 0
stopbits 1
line aux 0
stopbits 1
line vty 0 4
login
!
end 

Single-hop EBGP session between loopback interfaces

IOS releases prior to 12.2(13)T and 12.0(22)S required a multihop EBGP session between loopback interfaces of adjacent routers. Adjacent routers are by definition not using true multihop session, but the neighbor ebgp-multihop router configuration command was still needed to bypass the “directly connected EBGP neighbor” check built into Cisco IOS. The neighbor disable-connected-check router configuration command disables that check without changing the TTL value in outgoing TCP packets. The fast failover behavior or BFD support remains unchanged: since the neighbor’s IP address is not directly connected the default failover detection and BFD do not work; you have to use the neighbor fall-over router configuration command.

BGP configurations of E1 and R1 where the neighbor ebgp-multihop has been replaced with the neighbor disable-connected-check command are included below:

Configuration of E1

router bgp 65000
no synchronization
bgp log-neighbor-changes
network 10.7.1.0 mask 255.255.255.0
neighbor 10.0.0.1 remote-as 64800
neighbor 10.0.0.1 disable-connected-check
neighbor 10.0.0.1 update-source Loopback0
neighbor 10.0.0.1 fall-over route-map EBGP_multihop
no auto-summary
!
ip route 10.0.0.1 255.255.255.255 Serial1/0
ip route 10.0.0.1 255.255.255.255 Serial1/3
ip route 10.7.1.0 255.255.255.0 Null0
!
ip prefix-list EBGP_multihop seq 5 permit 0.0.0.0/0 ge 32
!
route-map EBGP_multihop permit 10
match ip address prefix-list EBGP_multihop 

Configuration of R1

router bgp 64800
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.23 remote-as 65000
neighbor 10.0.0.23 disable-connected-check
neighbor 10.0.0.23 update-source Loopback0
neighbor 10.0.0.23 fall-over route-map EBGP_multihop
no auto-summary
!
ip route 10.0.0.23 255.255.255.255 10.0.1.1
ip route 10.0.0.23 255.255.255.255 10.0.1.9
!
ip prefix-list EBGP_multihop seq 5 permit 0.0.0.0/0 ge 32
!
route-map EBGP_multihop permit 10
match ip address prefix-list EBGP_multihop 

After the BGP session between E1 and R1 is established, you can use the show ip bgp neighbor command to inspect the details of the EBGP session. The output of this command on R1 confirms that the E1 is an EBGP neighbor that is not directly connected, but R1 nonetheless uses TTL=1 in outgoing TCP packets.

R1#show ip bgp neighbor 10.0.0.23 BGP neighbor is 10.0.0.23, remote AS 65000, external link BGP version 4, remote router ID 10.0.0.23 BGP state = Established, up for 00:33:08 … deleted … Address tracking is enabled, the RIB does have a route to 10.0.0.23 Connections established 1; dropped 0 Last reset never External BGP neighbor not directly connected.Outgoing TTL 1 Local host: 10.0.0.1, Local port: 39201 Foreign host: 10.0.0.23, Foreign port: 179 Connection tableid (VRF): 0 … deleted … Transport(tcp) path-mtu-discovery is enabled Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Connection is ECN Disabled, Mininum incoming TTL 0,

Initial router configurations

Initial configuration of E1

service timestamps debug uptime
service timestamps log datetime msec
no service password-encryption
!
hostname E1
!
no aaa new-model
no ip source-route
ip cef
!
archive
log config
 hidekeys
!
interface Loopback0
ip address 10.0.0.23 255.255.255.255
!
interface Serial1/0
ip address 10.0.1.1 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
!
interface Serial1/3
ip address 10.0.1.9 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
!
router bgp 65000
no synchronization
bgp log-neighbor-changes
network 10.7.1.0 mask 255.255.255.0
!
ip forward-protocol nd
ip route 10.7.1.0 255.255.255.0 Null0
no ip http server
no ip http secure-server
!
control-plane
!
line con 0
stopbits 1
line aux 0
stopbits 1
line vty 0 4
login
!
end 

Initial configuration of R1

service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
no aaa new-model
no ip source-route
ip cef
!
archive
log config
 hidekeys
!
interface Loopback0
ip address 10.0.0.1 255.255.255.255
ip ospf 1 area 0
!
interface FastEthernet0/0
ip address 10.2.0.1 255.255.255.0
ip ospf 1 area 0
duplex half
!
interface Serial1/0
ip address 10.0.1.2 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
serial restart-delay 0
!
interface Serial1/2
ip address 10.0.1.10 255.255.255.252
encapsulation ppp
no peer neighbor-route
keepalive 3
serial restart-delay 0
!
!
router ospf 1
log-adjacency-changes
passive-interface default
no passive-interface FastEthernet0/0
!
!
ip forward-protocol nd
no ip http server
no ip http secure-server
!
control-plane
!
line con 0
stopbits 1
line aux 0
stopbits 1
line vty 0 4
login
!
end