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EBGP Multihop

EBGP Multihop

Network topology:

An eBGP session will be setup between R0 & R1 routers using their Loopback interfaces. Static routes are configured between R0 and R1 routers to reach the Loopback interfaces. The primary path to reach 2.2.2.2 from R0 is through SW0. A floating static route (AD=254) is also configured through SW2.

The multihop eBGP connection is configured using¬†neighbor ebgp-multihop <value>¬†command under BGP. The “value” indicates the number of hops. The range of “value” is 1 to 255. Since BGP session is to be formed between Loopback interfaces, technically, that is one hop away from the routers.

R0 configuration

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface FastEthernet0/0
 ip address 10.1.1.1 255.255.255.0
!
interface FastEthernet0/1
 ip address 10.2.2.1 255.255.255.0
!
router bgp 100
 no synchronization
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 200
 neighbor 2.2.2.2 ebgp-multihop 2
 neighbor 2.2.2.2 update-source Loopback0
 no auto-summary
!
ip route 2.2.2.2 255.255.255.255 10.1.1.2 track 10
ip route 2.2.2.2 255.255.255.255 10.2.2.2 254
!
ip sla 1
 icmp-echo 2.2.2.2 source-interface FastEthernet0/0
 timeout 3000
 threshold 2
 frequency 3
ip sla schedule 1 life forever start-time now
!
track 10 rtr 1 reachability
 delay down 10 up 15
!

As seen below, R0 uses static route (default AD of 1) to reach 2.2.2.2.

Static route for 2.2.2.2

R0# show ip route 2.2.2.2
Routing entry for 2.2.2.2/32
  Known via "static", distance 1, metric 0
  Routing Descriptor Blocks:
  * 10.1.1.2
      Route metric is 0, traffic share count is 1

Prefix 192.168.2.0 is advertised by R1 router via BGP. When R0 receives this prefix via eBGP connection, it installs in its BGP table first and the next-hop for 192.168.2.0 prefix will be the BGP Router ID of R1 i.e. 2.2.2.2. BGP performs recursive routing to find out the outgoing interface and checks if it can reach the next-hop before declaring the route as “best route” and installing in the routing table.

Recursive routing

R0# show ip cef 192.168.2.0
192.168.2.0/24, version 18, epoch 0, cached adjacency 10.1.1.2
0 packets, 0 bytes
  via 2.2.2.2, 0 dependencies, recursive
    next hop 10.1.1.2, FastEthernet0/0 via 2.2.2.2/32
    valid cached adjacency

R0# show ip cef 2.2.2.2

2.2.2.2/32, version 17, epoch 0, cached adjacency 10.1.1.2
0 packets, 0 bytes
  via 10.1.1.2, 1 dependency, recursive
    next hop 10.1.1.2, FastEthernet0/0 via 10.1.1.2/32
    valid cached adjacency

BGP Session

R0# show ip bgp neighbors | include BGP state|Connections established|Last reset
  BGP state = Established, up for 00:29:55
  Connections established 1; dropped 0
  Last reset never


Failover- When FastEthernet 0/0 interface of R1 is shutdown manually

When Fa 0/0 interface of R1 is shutdown, the state of tracking object goes down and the floating static route becomes active. Hence, 2.2.2.2 is now reachable via SW2.

Floating static route

00:37:35.883: %TRACKING-5-STATE: 10 rtr 1 reachability Up->Down

R0# show ip route 2.2.2.2

Routing entry for 2.2.2.2/32
  Known via "static", distance 254, metric 0
  Routing Descriptor Blocks:
  * 10.2.2.2
      Route metric is 0, traffic share count is 1

The outgoing interface changes to Fa 0/1 for all routes advertised by R1 to R0.

After Failover: BGP entries

R0# show ip cef 192.168.2.0
192.168.2.0/24, version 18, epoch 0, cached adjacency 10.2.2.2
0 packets, 0 bytes
  via 2.2.2.2, 0 dependencies, recursive
    next hop 10.2.2.2, FastEthernet0/1 via 2.2.2.2/32
    valid cached adjacency

R0# show ip cef 2.2.2.2

2.2.2.2/32, version 19, epoch 0, cached adjacency 10.2.2.2
0 packets, 0 bytes
  via 10.2.2.2, 1 dependency, recursive
    next hop 10.2.2.2, FastEthernet0/1 via 10.2.2.2/32
    valid cached adjacency

The following output clearly shows that the BGP session stays “established” during failover.

BGP session after failover

R0# show ip bgp neighbors | include BGP state|Connections established|Last reset
  BGP state = Established, up for 00:30:55
  Connections established 1; dropped 0
  Last reset never

BGP Next-hop reachability using a default route 0.0.0.0/0

A default-route 0.0.0.0/0 is only configured on R0 for reachability to 2.2.2.2. It shows that a BGP session is still established without a more specific route to 2.2.2.2.

Default-route with BGP on R0

R0(config)# ip route 0.0.0.0 0.0.0.0 10.1.1.2
R0(config)# no ip route 2.2.2.2 255.255.255.255 10.1.1.2 track 10
R0(config)# no ip route 2.2.2.2 255.255.255.255 10.2.2.2 254
R0(config)# end
R0# clear ip bgp *
R0# 
02:18:30.743: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Down User reset

02:19:04.435: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Up

R0# show ip cef 2.2.2.2
0.0.0.0/0, version 20, epoch 0, cached adjacency 10.1.1.2
0 packets, 0 bytes
  via 10.1.1.2, 1 dependency, recursive
    next hop 10.1.1.2, FastEthernet0/0 via 10.1.1.2/32
    valid cached adjacency

R0# show ip bgp | begin Network

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.2.0      2.2.2.2                  0             0 200 i

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