BGP Local Preference

 
BGP Weight
In the above topology R3 will generate these 2 prefixes 3.3.0.0/24 and 3.3.1.0/24 and R4 will generate these 2 prefixes 4.4.0.0/24 and 4.4.1.0/24 into BGP
Our goal is to use local preference to force R1 to route to 3.3.0.0/24 through R4
Now let’s check the BGP routing table of R1 and R2 before doing any changes
image


 image
As we can see both  R1 and R2 are using R3 to route to 3.3.0.0/24
now to apply local preference we need to match the prefix with a prefix list and then use a route-map to apply the route-map inbound to R4 on R1 (we want to influence outbound traffic so route-map should be applied inbound)
R1(config)#ip prefix-list R3_NET permit 3.3.0.0/24
R1(config)#route-map LOCAL_PREF permit 10
R1(config-route-map)#match ip address prefix-list R3_NET
R1(config-route-map)#set local-preference 200
R1(config-route-map)#route-map LOCAL_PREF permit 100    
R1(config)#router bgp 100
R1(config-router)#neighbor 10.1.14.4 route-map LOCAL_PREF in
Remember to clear the BGP neighbor inbound
Now let’s check the BGP on R1
image
As expected we find that R1 is now using R4 to route to 3.3.0.0/24 because the local preference is now 200
R1#sh ip bgp 3.3.0.0
BGP routing table entry for 3.3.0.0/24, version 6
Paths: (3 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
        1    2
  200
    10.1.14.4 from 10.1.14.4 (10.1.34.4)
      Origin IGP, localpref 200, valid, external, best
  200
    10.1.23.3 (inaccessible) from 10.1.12.2 (10.1.12.2)
      Origin IGP, metric 0, localpref 100, valid, internal
  200
    10.1.13.3 from 10.1.13.3 (10.1.34.3)
      Origin IGP, metric 0, localpref 100, valid, external
what about R2, when  we were doing the same setup with weight attribute we saw how the weight attribute is only local to the router it is configured on
let’s see BGP on R2
image
As we can see that now R2 is using R1 to reach 3.3.0.0/24 because the local preference of 200 was advertised from R1
Remember that local preference is confined through the same AS i.e it doesn’t get advertised to eBGP neighbors









Comments

Post a Comment

Popular posts from this blog

BPDU Filter vs BPDU Guard

        Today we will try to explore the difference between these 2 options, BPDU guard and BPDU filter We know that BPDU are used to communicate between switches in L2 networks  to ensure loop free topology. BPDU Guard is used to protect our access (or trunks incase of servers) ports from receiving undesired BPDU packets, this feature is helpful in enforcing your network boundary in access layer BPDU Guard can be enabled on global configuration as well as under the interface configuration BPDU Filter on the other hand is used to filter BPDU packets outbound when configured  in global configuration (after sending 11 BPDU out ) and will filter BPDU inbound/outbound when configured under the interface level To better understand these two features, let’s use this simple topology R1 is connected to SW1 through F0/0 to have R1 send and receive BPDU we will simple configure bridging group 1 on the router and we will join the bridge group when desired to test our configuration BPDU
Read more

BGP Weight Attribute

Image
Before we start the lab let's see the BGP path selection process If the path specifies a next hop that is inaccessible, drop the update. •Prefer the path with the highest weight. •If the weights are the same, prefer the path with the largest local preference. •If the local preferences are the same, prefer the path that was originated by BGP running on this router. •If no route was originated, prefer the route that has the shortest AS_path. •If all paths have the same AS_path length, prefer the path with the lowest origin code (where IGP is lower than EGP, and EGP is lower than incomplete). •If the origin codes are the same, prefer the path with the lowest MED attribute. •If the paths have the same MED, prefer the external path over the internal path. •If the paths are still the same, prefer the path through the closest IGP neighbor. •Prefer the path with the lowest IP address, as specified by the BGP router ID. I read this on CCIE Pursuit blog We Love Oranges As O
Read more

OSPF–Point To Multipoint Non-Broadcast

Image
Using same setup that we have been using in the last 3 posts, we will change the network type to point-to-multipoint Non-Broadcast we will also add a Ethernet link between R2 and R3 and enable OSPF on it Our final configuration should be something like this R1 interface Serial0/0   no ip address   encapsulation frame-relay   no frame-relay inverse-arp ! interface Serial0/0.123 multipoint   ip address 10.1.123.1 255.255.255.0   ip ospf network point-to-multipoint non-broadcast   ip ospf 1 area 0   frame-relay map ip 10.1.123.2 102   frame-relay map ip 10.1.123.3 103 !          router ospf 1   router-id 1.1.1.1   log-adjacency-changes   neighbor 10.1.123.2   neighbor 10.1.123.3 R2 interface Serial0/0   no ip address   encapsulation frame-relay   no frame-relay inverse-arp ! interface Serial0/0.123 multipoint   ip address 10.1.123.2 255.255.255.0   ip ospf network point-to-multipoint non-broadcast   ip ospf 1 area 0   frame-relay map ip 10.1.123.1 201 R3 interface
Read more