Introduction
Loop prevention in BGP is done by verifying the AS number in the AS Path. If the receiving router sees its own AS number in the AS Path of the received BGP packet, the packet is dropped. The receiving Router assumes that the packet was originated from its own AS and has reached the same place from where it originated initially.
The feature could be a disaster if customers are using same AS number along the various sites and disallows customer sites having identical AS numbers to be linked by another AS number. In such a scenario, routing updates from one site will be dropped when the other site receives them.
To override this feature, AS-Override function causes to replace the AS number of originating router with the AS number of the sending BGP router. The command is neighbor ip-address as-override and can only be executed under the VPNv4 address-family.
Here are the steps to illustrate the As-Override process
- Router TAURUS_Site-A advertises route 10.3.3.3 with AS100.
- Router PE-1 propagates this as an internal route to PE2 as AS100.
- PE2 prepends 10.3.3.3 with AS 121 and replaces 100 in the AS-Path to 121 and propagates the prefix.
- Router TAURUS_Site-B accepts 10.3.3.3 update.
Prerequisite
- Understanding of MPLS Technology
- Understanding of MBGP
- Understanding of OSPF routing protocol
Topology Diagram
Background
In this Topology, router PE-1 and PE-2 forms the Service Provider MPLS Cloud. The two routers are connected via fast Ethernet interface 0/0 and are running OSPF (Area 0) routing protocol. MPLS is configured on physical links (Fast Ethernet 0/0) of the SP network. Tagging is done via LDP and the labels are assigned in the range 100-199 on PE1 and 200-299 on PE2.
TAURUS and CINDY are the two customers with multiple sites (Site-A and Site-B). Customer TAURUS is operating under AS 100 and customer CINDY is operating under AS 200.
VPNv4 neighbor relationship is configured between (vrf TAURUS and vrf CINDY)
- PE-1 & TAURUS_Site-A
- PE-1 & CINDY_Site-A
- PE-2 & TAURUS_Site-B
- PE-2 & CINDY_Site-A
Routes from each site are advertised to PE routers within EBGP session. These routes are further propagated to next PE routers which are then forwarded to respective Customer-Site.
Note: All configurations are tested on Cisco 3700 series router with IOS 12.4.
Configuration
1. PE Configuration
PE-1PE-2
hostname PE-1 ip cef ip vrf CINDY rd 1:200 route-target export 1:200 route-target import 1:200 ip vrf TAURUS rd 1:100 route-target export 1:100 route-target import 1:100 no ip domain lookup mpls label range 100 199 mpls label protocol ldp interface Loopback0 ip address 1.1.1.1 255.255.255.255 interface FastEthernet0/0 ip address 10.12.12.1 255.255.255.0 duplex auto speed auto mpls ip interface Serial0/0 ip vrf forwarding TAURUS ip address 192.13.13.1 255.255.255.252 clock rate 2000000 interface Serial0/1 ip vrf forwarding CINDY ip address 192.14.14.1 255.255.255.252 clock rate 2000000 router ospf 10 router-id 1.1.1.1 log-adjacency-changes network 1.1.1.1 0.0.0.0 area 0 network 10.12.12.1 0.0.0.0 area 0 router bgp 121 no synchronization bgp log-neighbor-changes network 11.11.11.11 mask 255.255.255.255 neighbor 2.2.2.2 remote-as 121 neighbor 2.2.2.2 update-source Loopback0 neighbor 2.2.2.2 next-hop-self no auto-summary address-family vpnv4 neighbor 2.2.2.2 activate neighbor 2.2.2.2 send-community both exit-address-family address-family ipv4 vrf TAURUS redistribute connected neighbor 192.13.13.2 remote-as 100 neighbor 192.13.13.2 activate neighbor 192.13.13.2 as-override no synchronization exit-address-family address-family ipv4 vrf CINDY redistribute connected neighbor 192.14.14.2 remote-as 200 neighbor 192.14.14.2 activate neighbor 192.14.14.2 as-override no synchronization exit-address-family mpls ldp router-id Loopback0 exit | hostname PE-2 ip cef ip vrf CINDY rd 1:200 route-target export 1:200 route-target import 1:200 ip vrf TAURUS rd 1:100 route-target export 1:100 route-target import 1:100 no ip domain lookup mpls label range 200 299 mpls label protocol ldp interface Loopback0 ip address 2.2.2.2 255.255.255.255 interface Loopback1 ip address 22.22.22.22 255.255.255.255 interface FastEthernet0/0 ip address 10.12.12.2 255.255.255.0 duplex auto speed auto mpls ip interface Serial0/0 ip vrf forwarding TAURUS ip address 192.23.23.1 255.255.255.252 clock rate 2000000 interface Serial0/1 ip vrf forwarding CINDY ip address 192.26.26.1 255.255.255.252 clock rate 2000000 router ospf 10 router-id 2.2.2.2 log-adjacency-changes network 2.2.2.2 0.0.0.0 area 0 network 10.12.12.2 0.0.0.0 area 0 router bgp 121 no synchronization bgp log-neighbor-changes network 22.22.22.22 mask 255.255.255.255 neighbor 1.1.1.1 remote-as 121 neighbor 1.1.1.1 update-source Loopback0 neighbor 1.1.1.1 next-hop-self no auto-summary address-family vpnv4 neighbor 1.1.1.1 activate neighbor 1.1.1.1 send-community both exit-address-family address-family ipv4 vrf TAURUS redistribute connected neighbor 192.23.23.2 remote-as 100 neighbor 192.23.23.2 activate neighbor 192.23.23.2 as-override no synchronization exit-address-family address-family ipv4 vrf CINDY redistribute connected neighbor 192.26.26.2 remote-as 200 neighbor 192.26.26.2 activate neighbor 192.26.26.2 as-override no synchronization exit-address-family mpls ldp router-id Loopback0 exit |
2. CE Configuration
TAURUS_Site-ACINDY_Site-ATAURUS_Site-BCINDY_Site-B
hostname TAURUS-Site_A ip cef no ip domain lookup interface Loopback0 ip address 10.3.3.3 255.255.255.255 interface Serial0/0 ip address 192.13.13.2 255.255.255.252 clock rate 2000000 router bgp 100 no synchronization bgp log-neighbor-changes network 10.3.3.3 mask 255.255.255.255 neighbor 192.13.13.1 remote-as 121 no auto-summary exit | hostname CINDY-SITE_A ip cef no ip domain lookup interface Loopback0 ip address 10.4.4.4 255.255.255.255 interface Serial0/0 ip address 192.14.14.2 255.255.255.252 clock rate 2000000 router bgp 200 no synchronization bgp log-neighbor-changes network 10.4.4.4 mask 255.255.255.255 neighbor 192.14.14.1 remote-as 121 no auto-summary exit | hostname TAURUS-Site_B ip cef no ip domain lookup interface Loopback0 ip address 10.5.5.5 255.255.255.255 interface Serial0/0 ip address 192.23.23.2 255.255.255.252 clock rate 2000000 router bgp 100 no synchronization bgp log-neighbor-changes network 10.5.5.5 mask 255.255.255.255 neighbor 192.23.23.1 remote-as 121 no auto-summary exit | hostname CINDY-SITE_B ip cef no ip domain lookup interface Loopback0 ip address 10.6.6.6 255.255.255.255 interface Serial0/0 ip address 192.26.26.2 255.255.255.252 clock rate 2000000 router bgp 200 no synchronization bgp log-neighbor-changes network 10.6.6.6 mask 255.255.255.255 neighbor 192.26.26.1 remote-as 121 no auto-summary exit |
Verification
PE-1#show ip bgp vpnv4 all summary
< output truncated >
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2 4 121 115 116 15 0 0 01:09:16 4
192.13.13.2 4 100 70 74 15 0 0 00:46:42 1
192.14.14.2 4 200 41 44 15 0 0 00:36:14 1
PE-2#show ip bgp vpnv4 all summary
< output truncated >
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
1.1.1.1 4 121 119 118 15 0 0 01:11:22 4
192.23.23.2 4 100 53 56 15 0 0 00:48:07 1
192.26.26.2 4 200 41 44 15 0 0 00:36:46 1
PE-1#sh ip route vrf TAURUS bgp
192.23.23.0/30 is subnetted, 1 subnets
B 192.23.23.0 [200/0] via 2.2.2.2, 00:25:32
10.0.0.0/32 is subnetted, 2 subnets
B 10.3.3.3 [20/0] via 192.13.13.2, 00:35:30
B 10.5.5.5 [200/0] via 2.2.2.2, 00:33:03
PE-1#show ip route vrf CINDY bgp
192.26.26.0/30 is subnetted, 1 subnets
B 192.26.26.0 [200/0] via 2.2.2.2, 00:27:05
10.0.0.0/32 is subnetted, 2 subnets
B 10.6.6.6 [200/0] via 2.2.2.2, 00:32:36
B 10.4.4.4 [20/0] via 192.14.14.2, 00:34:51
PE-1#show ip bgp vpnv4 all
BGP table version is 15, local router ID is 11.11.11.11
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: 1:100 (default for vrf TAURUS)
*> 10.3.3.3/32 192.13.13.2 0 0 100 i
*>i10.5.5.5/32 2.2.2.2 0 100 0 100 i
*> 192.13.13.0/30 0.0.0.0 0 32768 ?
*>i192.23.23.0/30 2.2.2.2 0 100 0 ?
Route Distinguisher: 1:200 (default for vrf CINDY)
*> 10.4.4.4/32 192.14.14.2 0 0 200 i
*>i10.6.6.6/32 2.2.2.2 0 100 0 200 i
*> 192.14.14.0/30 0.0.0.0 0 32768 ?
*>i192.26.26.0/30 2.2.2.2 0 100 0 ?
TAURUS-Site_A#ping 10.5.5.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.5.5.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 456/696/972 ms
The prefix 10.5.5.5 (from Router TAURUS-Site_B) is receieved and pinged successfully from Router TAURUS-Site_A.
References
Cisco IOS IP and IP Routing Command Reference
