Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Community Member

BGP routing- as path prepend config question

I have 2 cpe routers R3  and R4 running BGP between them like so:

Cust CORE >>>>Customer FW#1 >>>>R3>>>>>>>>>>T3>>>>>> Juniper PE >>>>>MPLS cloud

Cust CORE >>>>Customer FW#1  >>>R4  >>>>>>>>>>T3>>>>>>.Juniper PE>>>>>MPLS Cloud

...and also running BGP with PE routers.

3 customr subnets

10.10.162.0/24

10.10.166.0/24

10.10.161.0 /24

R3 was primary and R4 was the Backup.  R3 is the Perfered route since  R4 advertises a less perfered route configured with the as path prepend command.

My question :  i need to make only the .161 subnet more perfered for inbound traffic over the R4 circuit and have inbound traffic for the other 2 subnet perfer R3  

 

Still new to BGP, any suggestions/steps for cutover are  appreciated.

thx

2 REPLIES
Hall of Fame Super Silver

Re: BGP routing- as path prepend config question

Hello Fkleininger,

you  just need to use route-maps to perform selective AS path prepending

R3:

prepend only net 10.10.161.0/24

ip prefix-list prefix_prepended_r3 seq 5 permit 10.10.161.0/24

route-map select-prepend-r3 permit 10

match ip address prefix prefix_prepended_r3

set as-path prepend yourASN your ASN yourASN

route-map select-prepend-r3 permit 20

! empty  route-map  block to advertise the other IP prefixes without any prepending

router bgp yourASN

neigh PE1-address route-map select-prepend-r3 out

R4:

mirrored configuration

ip prefix-list prefix_prepended_r4 seq 5 permit 10.10.162.0/24

ip prefix-list prefix_prepended_r4 seq 10 permit 10.10.166.0/24

route-map select-prepend-r4 permit 10

match ip address prefix prefix_prepended_r4

set as-path prepend yourASN your ASN yourASN

route-map select-prepend-r4 permit 20

! empty route-map  block to advertise the other IP prefixes without any prepending

router bgp yourASN

neigh PE2-address route-map select-prepend-r4 out

note: another access-list or prefix-list can be used if you want to control what CE nodes advertise to PE nodes in the eBGP sessions so you need simply a match ip address or match ip address prefix statement in second route-map block

Hope to  help

Giuseppe

Community Member

Re: BGP routing- as path prepend config question

Aloha fkleininger,

You say you are already advertising these subnets from R3 and R4. Can I ask what is your current method for advertising these?  While ip prefix-lists will work they may be unneccessary depending on your current configuration.

Example, lets say I have two connections as you do R1 and R2 and I am assigned the following IP address block 47.47.160.0/19.  I can advertise this block through both routers in such a way that I never use as-path prepending or ip prefix-lists.

On R1 I advertise the following: (/19, /20 lower half, all even /24s)

network 47.47.160.0 mask 255.255.224.0     (advertises the whole /19)

network 47.47.160.0 mask 255.255.240.0     (advertise the lower /20)

network 47.47.160.0 mask 255.255.255.0     (advertises an even /24)

network 47.47.162.0 mask 255.255.255.0   

"     "     "     "     "     "     "     "     "     "

network 47.47.190.0 mask 255.255.255.0

On R2 I advertise as follows: (/19, /20 upper half, all odd /24s)

network 47.47.160.0 mask 255.255.224.0     (advertises the whole /19)

network 47.47.176.0 mask 255.255.240.0     (advertises the upper /20)

network 47.47.161.0 mask 255.255.255.0     (advertises an odd /24)

network 47.47.163.0 mask 255.255.255.0

"     "     "     "     "     "     "     "     "     "

network 47.47.191.0 mask 255.255.255.0

This is just an example and shows how you can influence which router will be used on the return path to your network.  All even /24s go through R1 and all odd /24s go through R2.  In case of a router failure the other router takes over due to the "shorter" prefix through the redundant router.  I used no ip prefix-lists or as-path prepending to influence my preferred path.

Again, knowing how you are currently doing your advertisements will help in determining the best way to accomplish your new goal.

HTH

4194
Views
5
Helpful
2
Replies
CreatePlease to create content