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Replies

Dumb BGP question #2241

rtjensen4
Level 4
Level 4

Hi!

I'm trying to "mimic" my MPLS network in a completely seperated lab environment. My "PE" router (3750) is participating in my BGP routing. The "PE" is seeing all the routes from the two peers, but it's not passing routes from CE1 to CE 2. Im guessing it's because I'm missing a network statment on the PE or somthing, but I'm not sure. Overall setup is pretty simple. 2 2801s peer to my "PE" via BGP. Any assitance?

PE:

router bgp 12345

no synchronization

bgp log-neighbor-changes

neighbor <IP1> remote-as 64519

neighbor <IP2> remote-as 64519

no auto-summary

BGP router identifier 192.168.250.13, local AS number 12345

BGP table version is 19, main routing table version 19

7 network entries using 819 bytes of memory

7 path entries using 364 bytes of memory

4/3 BGP path/bestpath attribute entries using 560 bytes of memory

2 BGP AS-PATH entries using 48 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 1791 total bytes of memory

BGP activity 10/3 prefixes, 11/4 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

<CE1> 4 64519 169 167 19 0 0 02:42:37 5

<CE2> 4 64519 98 100 19 0 0 01:35:02 2

CE1:

router bgp 64519

no synchronization

bgp log-neighbor-changes

redistribute connected

neighbor <PEER1> remote-as 12345

neighbor <PEER1> default-originate

no auto-summary

BGP router identifier 192.168.250.100, local AS number 64519

BGP table version is 18, main routing table version 18

4 network entries using 528 bytes of memory

4 path entries using 208 bytes of memory

2/1 BGP path/bestpath attribute entries using 336 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

Bitfield cache entries: current 1 (at peak 2) using 32 bytes of memory

BGP using 1104 total bytes of memory

BGP activity 8/4 prefixes, 10/6 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

<PEER1> 4 12345 169 171 18 0 0 02:44:06 0

CE2:

router bgp 64519

no synchronization

bgp log-neighbor-changes

redistribute connected

redistribute static

neighbor <PEER2> remote-as 12345

no auto-summary

BGP router identifier 192.168.250.20, local AS number 64519

BGP table version is 11, main routing table version 11

2 network entries using 234 bytes of memory

2 path entries using 104 bytes of memory

2/1 BGP path/bestpath attribute entries using 280 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 618 total bytes of memory

BGP activity 6/4 prefixes, 6/4 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

<PEER2> 4 12345 105 103 11 0 0 01:39:25 0

2 Accepted Solutions

Accepted Solutions

Nagendra Kumar Nainar
Cisco Employee
Cisco Employee

Hi,

Your topology is like below,

CE1-----------PE-------------CE2

AS64519 AS12345 AS64519

When a BGP router receives BGP update from neighbor, it will not accept if it has it own AS number in AS-PATH. In our when CE2 receives BGP update from PE, it will have 64519 in its AS-PATH and so will ignore it.

Try the below command on both CEs,

neighbor allowas-in

or change the AS of any one CE.

HTH,

Nagendra

View solution in original post

Hello Ryan,

yes the PE has faculty to use

neigh .. as-override

as a result of this each site sees other sites BGP routes with a modified AS path like

PE_ASN PE_ASN

the private ASN is replaced with PE ASN

other possible options exist.

Hope to help

Giuseppe

View solution in original post

6 Replies 6

Nagendra Kumar Nainar
Cisco Employee
Cisco Employee

Hi,

Your topology is like below,

CE1-----------PE-------------CE2

AS64519 AS12345 AS64519

When a BGP router receives BGP update from neighbor, it will not accept if it has it own AS number in AS-PATH. In our when CE2 receives BGP update from PE, it will have 64519 in its AS-PATH and so will ignore it.

Try the below command on both CEs,

neighbor allowas-in

or change the AS of any one CE.

HTH,

Nagendra

OK, I will give it a try. I wonder why it works this way in my production environment. I wonder if it's somthing configured on the PE?

Hello Ryan,

yes the PE has faculty to use

neigh .. as-override

as a result of this each site sees other sites BGP routes with a modified AS path like

PE_ASN PE_ASN

the private ASN is replaced with PE ASN

other possible options exist.

Hope to help

Giuseppe

Ok, that worked. Thanks!

Hi,

Normally in PE router, Service providers will have "neighbor as-overide" command configured when MPLS is used to connnect 2 CE which are in same AS and run BGP as PE-CE protocol.

This command will check if the first AS number in AS-PATH and the AS number of neighbor to which it is going to advertise are same?. Is yes, the AS number will be replaced by its own AS number.

HTH,

Nagendra

Thanks Nagendra and Giuseppe for your help. My 3750 can't do as-overide, but the allow-as option achieves the same solution. Thanks!

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