eBGP as CE-PE Routing Protocol in MPLS

We can use any of the below protocols as CE-PR routing protocol in MPLS domain.

  • Connected
  • Static
  • RIPv2
  • OSPF
  • EIGRP
  • ISIS
  • eBGP
Note: we cannot have iBGP session.
Advantages of using eBGP as CE_PE routing protocol:

1)NO need to specifically do redistribute on PE router. Customer routes are automatically redistributed into configured VRF.
2)All prefix properties are carried over single BGP protocol from end-end.

R0(customer1:AS2)—– R2(PE)——–MPLS—–R5(PE)——R6(customer1:AS2)

In R2;

!
router bgp 1
no synchronization
bgp log-neighbor-changes
neighbor 5.5.5.5 remote-as 1
neighbor 5.5.5.5 update-source Loopback0
no auto-summary
!
address-family vpnv4
neighbor 5.5.5.5 activate
neighbor 5.5.5.5 send-community extended
exit-address-family
!
address-family ipv4 vrf two
redistribute rip
no synchronization
exit-address-family
!
address-family ipv4 vrf one
neighbor 12.1.1.2 remote-as 2
neighbor 12.1.1.2 activate
no synchronization
exit-address-family
!
R0 routes are available in PE-R5;
R5#show ip bgp vpnv4 vrf one
BGP table version is 13, local router ID is 5.5.5.5
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete
Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 1:1 (default for vrf one)
*>i100.100.100.100/32
2.2.2.2                  0    100      0 2 i
R5#
After establishing eBGP connection between R5 and R6, these routes will not be available to R6 as R6 ignores the update as it sees its own AS number in the AS-path attribute;
Two ways to override;
1. AS-override (On PE router)
2.Allowas-in (On CE router. In hub-spoke scenario this command is used in PE too)

R6#show ip route 100.100.100.100
% Network not in table
R6#

so, configure as-override on the PE router-R5 which will replace the neighbor AS-2 with its own AS number:1

R5(config-router-af)#neighbor 13.1.2.2 as-override
R5(config-router-af)#
*Mar  1 00:28:06.683: %BGP-5-ADJCHANGE: neighbor 13.1.2.2 vpn vrf one Down AS-override change
*Mar  1 00:28:07.195: %BGP-5-ADJCHANGE: neighbor 13.1.2.2 vpn vrf one Up
R5(config-router-af)#
R6#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 2
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
1 1
13.1.2.1 from 13.1.2.1 (5.5.5.5)
Origin IGP, localpref 100, valid, external, best
R6#

Now, with allowas-in on CE router;

R6(config-router)#neighbor 13.1.2.1 allowas-in 3
R6#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 4
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
1 2
13.1.2.1 from 13.1.2.1 (5.5.5.5)
Origin IGP, localpref 100, valid, external, best
R6#
R6#

HTH,


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